Fix issues in typesafe interpreter article.

This one uses line highlights!

code/time-traveling/TakeMax.hs

@@ -0,0 +1,21 @@
+takeUntilMax :: [Int] -> Int -> (Int, [Int])
+takeUntilMax [] m = (m, [])
+takeUntilMax [x] _ = (x, [x])
+takeUntilMax (x:xs) m
+    | x == m = (x, [x])
+    | otherwise =
+        let (m', xs') = takeUntilMax xs m
+        in (max m' x, x:xs')
+
+doTakeUntilMax :: [Int] -> [Int]
+doTakeUntilMax l = l'
+    where (m, l') = takeUntilMax l m
+
+takeUntilMax' :: [Int] -> Int -> (Int, [Int])
+takeUntilMax' [] m = (m, [])
+takeUntilMax' [x] _ = (x, [x])
+takeUntilMax' (x:xs) m
+    | x == m = (maximum (x:xs), [x])
+    | otherwise =
+        let (m', xs') = takeUntilMax' xs m
+        in (max m' x, x:xs')

code/time-traveling/ValueScore.hs

@@ -0,0 +1,28 @@
+import Data.Map as Map
+import Data.Maybe
+import Control.Applicative
+
+data Element = A | B | C | D
+    deriving (Eq, Ord, Show)
+
+addElement :: Element -> Map Element Int -> Map Element Int
+addElement = alter ((<|> Just 1) . fmap (+1))
+
+getScore :: Element -> Map Element Int -> Float
+getScore e m = fromMaybe 1.0 $ ((1.0/) . fromIntegral) <$> Map.lookup e m
+
+data BinaryTree a = Empty | Node a (BinaryTree a) (BinaryTree a) deriving Show
+type ElementTree = BinaryTree Element
+type ScoredElementTree = BinaryTree (Element, Float)
+
+assignScores :: ElementTree -> Map Element Int -> (Map Element Int, ScoredElementTree)
+assignScores Empty m = (Map.empty, Empty)
+assignScores (Node e t1 t2) m = (m', Node (e, getScore e m) t1' t2')
+    where
+        (m1, t1') = assignScores t1 m
+        (m2, t2') = assignScores t2 m
+        m' = addElement e $ unionWith (+) m1 m2
+
+doAssignScores :: ElementTree -> ScoredElementTree
+doAssignScores t = t'
+    where (m, t') = assignScores t m

code/typesafe-interpreter/TypesafeIntrV2.idr

@@ -0,0 +1,99 @@
+data ExprType
+  = IntType
+  | BoolType
+  | StringType
+
+repr : ExprType -> Type
+repr IntType = Int
+repr BoolType = Bool
+repr StringType = String
+
+intBoolImpossible : IntType = BoolType -> Void
+intBoolImpossible Refl impossible
+
+intStringImpossible : IntType = StringType -> Void
+intStringImpossible Refl impossible
+
+boolStringImpossible : BoolType = StringType -> Void
+boolStringImpossible Refl impossible
+
+decEq : (a : ExprType) -> (b : ExprType) -> Dec (a = b)
+decEq IntType IntType = Yes Refl
+decEq BoolType BoolType = Yes Refl
+decEq StringType StringType = Yes Refl
+decEq IntType BoolType = No intBoolImpossible
+decEq BoolType IntType = No $ intBoolImpossible . sym
+decEq IntType StringType = No intStringImpossible
+decEq StringType IntType = No $ intStringImpossible . sym
+decEq BoolType StringType = No boolStringImpossible 
+decEq StringType BoolType = No $ boolStringImpossible . sym
+
+data Op
+  = Add
+  | Subtract
+  | Multiply
+  | Divide
+
+data Expr
+  = IntLit Int
+  | BoolLit Bool
+  | StringLit String
+  | BinOp Op Expr Expr
+  | IfElse Expr Expr Expr
+
+data SafeExpr : ExprType -> Type where
+  IntLiteral : Int -> SafeExpr IntType
+  BoolLiteral : Bool -> SafeExpr BoolType
+  StringLiteral : String -> SafeExpr StringType
+  BinOperation : (repr a -> repr b -> repr c) -> SafeExpr a -> SafeExpr b -> SafeExpr c
+  IfThenElse : SafeExpr BoolType -> SafeExpr t -> SafeExpr t -> SafeExpr t
+
+typecheckOp : Op -> (a : ExprType) -> (b : ExprType) -> Either String (c : ExprType ** repr a -> repr b -> repr c) 
+typecheckOp Add IntType IntType = Right (IntType ** (+))
+typecheckOp Subtract IntType IntType = Right (IntType ** (-))
+typecheckOp Multiply IntType IntType = Right (IntType ** (*))
+typecheckOp Divide IntType IntType = Right (IntType ** div)
+typecheckOp _ _ _ = Left "Invalid binary operator application"
+
+requireBool : (n : ExprType ** SafeExpr n) -> Either String (SafeExpr BoolType)
+requireBool (BoolType ** e) = Right e
+requireBool _ = Left "Not a boolean."
+
+typecheck : Expr -> Either String (n : ExprType ** SafeExpr n)
+typecheck (IntLit i) = Right (_ ** IntLiteral i)
+typecheck (BoolLit b) = Right (_ ** BoolLiteral b)
+typecheck (StringLit s) = Right (_ ** StringLiteral s)
+typecheck (BinOp o l r) = do
+  (lt ** le) <- typecheck l
+  (rt ** re) <- typecheck r
+  (ot ** f) <- typecheckOp o lt rt
+  pure (_ ** BinOperation f le re)
+typecheck (IfElse c t e) =
+  do
+    ce <- typecheck c >>= requireBool
+    (tt ** te) <- typecheck t
+    (et ** ee) <- typecheck e
+    case decEq tt et of
+      Yes p => pure (_ ** IfThenElse ce (replace p te) ee)
+      No _ => Left "Incompatible branch types."
+
+eval : SafeExpr t -> repr t
+eval (IntLiteral i) = i
+eval (BoolLiteral b) = b
+eval (StringLiteral s) = s
+eval (BinOperation f l r) = f (eval l) (eval r)
+eval (IfThenElse c t e) = if (eval c) then (eval t) else (eval e)
+
+resultStr : {t : ExprType} -> repr t -> String
+resultStr {t=IntType} i = show i
+resultStr {t=BoolType} b = show b
+resultStr {t=StringType} s = show s
+
+tryEval : Expr -> String
+tryEval ex =
+  case typecheck ex of
+    Left err => "Type error: " ++ err
+    Right (t ** e) => resultStr $ eval {t} e
+
+main : IO ()
+main = putStrLn $ tryEval $ BinOp Add (IfElse (BoolLit True) (IntLit 6) (IntLit 7)) (BinOp Multiply (IntLit 160) (IntLit 2))

code/typesafe-interpreter/TypesafeIntrV3.idr

@@ -0,0 +1,120 @@
+data ExprType
+  = IntType
+  | BoolType
+  | StringType
+  | PairType ExprType ExprType
+
+repr : ExprType -> Type
+repr IntType = Int
+repr BoolType = Bool
+repr StringType = String
+repr (PairType t1 t2) = Pair (repr t1) (repr t2)
+
+decEq : (a : ExprType) -> (b : ExprType) -> Maybe (a = b)
+decEq IntType IntType = Just Refl
+decEq BoolType BoolType = Just Refl
+decEq StringType StringType = Just Refl
+decEq (PairType lt1 lt2) (PairType rt1 rt2) = do
+  subEq1 <- decEq lt1 rt1
+  subEq2 <- decEq lt2 rt2
+  let firstEqual = replace {P = \t1 => PairType lt1 lt2 = PairType t1 lt2} subEq1 Refl
+  let secondEqual = replace {P = \t2 => PairType lt1 lt2 = PairType rt1 t2} subEq2 firstEqual
+  pure secondEqual
+decEq _ _ = Nothing
+
+data Op
+  = Add
+  | Subtract
+  | Multiply
+  | Divide
+
+data Expr
+  = IntLit Int
+  | BoolLit Bool
+  | StringLit String
+  | BinOp Op Expr Expr
+  | IfElse Expr Expr Expr
+  | Pair Expr Expr
+  | Fst Expr
+  | Snd Expr
+
+data SafeExpr : ExprType -> Type where
+  IntLiteral : Int -> SafeExpr IntType
+  BoolLiteral : Bool -> SafeExpr BoolType
+  StringLiteral : String -> SafeExpr StringType
+  BinOperation : (repr a -> repr b -> repr c) -> SafeExpr a -> SafeExpr b -> SafeExpr c
+  IfThenElse : SafeExpr BoolType -> SafeExpr t -> SafeExpr t -> SafeExpr t
+  NewPair : SafeExpr t1 -> SafeExpr t2 -> SafeExpr (PairType t1 t2)
+  First : SafeExpr (PairType t1 t2) -> SafeExpr t1
+  Second : SafeExpr (PairType t1 t2) -> SafeExpr t2
+
+typecheckOp : Op -> (a : ExprType) -> (b : ExprType) -> Either String (c : ExprType ** repr a -> repr b -> repr c) 
+typecheckOp Add IntType IntType = Right (IntType ** (+))
+typecheckOp Subtract IntType IntType = Right (IntType ** (-))
+typecheckOp Multiply IntType IntType = Right (IntType ** (*))
+typecheckOp Divide IntType IntType = Right (IntType ** div)
+typecheckOp _ _ _ = Left "Invalid binary operator application"
+
+requireBool : (n : ExprType ** SafeExpr n) -> Either String (SafeExpr BoolType)
+requireBool (BoolType ** e) = Right e
+requireBool _ = Left "Not a boolean."
+
+typecheck : Expr -> Either String (n : ExprType ** SafeExpr n)
+typecheck (IntLit i) = Right (_ ** IntLiteral i)
+typecheck (BoolLit b) = Right (_ ** BoolLiteral b)
+typecheck (StringLit s) = Right (_ ** StringLiteral s)
+typecheck (BinOp o l r) = do
+  (lt ** le) <- typecheck l
+  (rt ** re) <- typecheck r
+  (ot ** f) <- typecheckOp o lt rt
+  pure (_ ** BinOperation f le re)
+typecheck (IfElse c t e) =
+  do
+    ce <- typecheck c >>= requireBool
+    (tt ** te) <- typecheck t
+    (et ** ee) <- typecheck e
+    case decEq tt et of
+      Just p => pure (_ ** IfThenElse ce (replace p te) ee)
+      Nothing => Left "Incompatible branch types."
+typecheck (Pair l r) =
+  do
+    (lt ** le) <- typecheck l
+    (rt ** re) <- typecheck r
+    pure (_ ** NewPair le re)
+typecheck (Fst p) =
+  do
+    (pt ** pe) <- typecheck p
+    case pt of
+      PairType _ _ => pure $ (_ ** First pe)
+      _ => Left "Applying fst to non-pair."
+typecheck (Snd p) =
+  do
+    (pt ** pe) <- typecheck p
+    case pt of
+      PairType _ _ => pure $ (_ ** Second pe)
+      _ => Left "Applying snd to non-pair."
+
+eval : SafeExpr t -> repr t
+eval (IntLiteral i) = i
+eval (BoolLiteral b) = b
+eval (StringLiteral s) = s
+eval (BinOperation f l r) = f (eval l) (eval r)
+eval (IfThenElse c t e) = if (eval c) then (eval t) else (eval e)
+eval (NewPair l r) = (eval l, eval r)
+eval (First p) = fst (eval p)
+eval (Second p) = snd (eval p)
+
+resultStr : {t : ExprType} -> repr t -> String
+resultStr {t=IntType} i = show i
+resultStr {t=BoolType} b = show b
+resultStr {t=StringType} s = show s
+resultStr {t=PairType t1 t2} (l,r) = "(" ++ resultStr l ++ ", " ++ resultStr r ++ ")"
+
+tryEval : Expr -> String
+tryEval ex =
+  case typecheck ex of
+    Left err => "Type error: " ++ err
+    Right (t ** e) => resultStr $ eval {t} e
+
+main : IO ()
+main = putStrLn $ tryEval $ BinOp Add (Fst (IfElse (BoolLit True) (Pair (IntLit 6) (BoolLit True)) (Pair (IntLit 7) (BoolLit False)))) (BinOp Multiply (IntLit 160) (IntLit 2))

config.toml

@@ -3,5 +3,11 @@ languageCode = "en-us"
 title = "Daniel's Blog"
 theme = "vanilla"
 pygmentsCodeFences = true
-pygmentsStyle = "github"
+pygmentsUseClasses = true
 summaryLength = 20
+
+[markup]
+  [markup.tableOfContents]
+    endLevel = 4
+    ordered = false
+    startLevel = 3

content/about.md

@@ -1,8 +1,8 @@
 ---
 title: About
 ---
-I'm Daniel, a Computer Science student currently in my third (and final) undergraduate year at Oregon State University.
-Due my initial interest in calculators and compilers, I got involved in the Programming Language Theory research
+I'm Daniel, a Computer Science student currently working towards my Master's Degree at Oregon State University.
+Due to my initial interest in calculators and compilers, I got involved in the Programming Language Theory research
 group, gaining same experience in formal verification, domain specific language, and explainable computing.
 
 For work, school, and hobby projects, I use a variety of programming languages, most commonly C/C++,

content/blog/backend_math_rendering.md

@@ -0,0 +1,304 @@
+---
+title: Rendering Mathematics On The Back End
+date: 2020-07-21T14:54:26-07:00
+tags: ["Website", "Nix", "Ruby", "KaTeX"]
+---
+
+Due to something of a streak of bad luck when it came to computers, I spent a
+significant amount of time using a Linux-based Chromebook, and then a
+Pinebook Pro. It was, in some way, enlightening. The things that I used to take
+for granted with a 'powerful' machine now became a rare luxury: StackOverflow,
+and other relatively static websites, took upwards of ten seconds to finish
+loading. On Slack, each of my keypresses could take longer than 500ms to
+appear on the screen, and sometimes, it would take several seconds. Some
+websites would present me with a white screen, and remain that way for much
+longer than I had time to wait. It was awful.
+
+At one point, I installed uMatrix, and made it the default policy to block
+all JavaScript. For the most part, this worked well. Of course, I had to
+enable JavaScript for applications that needed to be interactive, like
+Slack, and Discord. But for the most part, I was able to browse the majority
+of the websites I normally browse. This went on until I started working
+on the [compiler series]({{< relref "00_compiler_intro.md" >}}) again,
+and discovered that the LaTeX math on my page, which was required
+for displaying things like inference rules, didn't work without
+JavaScript. I was left with two options:
+
+* Allow JavaScript, and continue using MathJax to render my math.
+* Make it so that the mathematics are rendered on the back end.
+
+I've [previously written about math rendering]({{< relref "math_rendering_is_wrong.md" >}}),
+and made the observation that MathJax's output for LaTeX is __identical__
+on every computer. From the MathJax 2.6 change log:
+
+> _Improved CommonHTML output_. The CommonHTML output now provides the same layout quality and MathML support as the HTML-CSS and SVG output. It is on average 40% faster than the other outputs and the markup it produces are identical on all browsers and thus can also be pre-generated on the server via MathJax-node.
+
+It seems absurd, then, to offload this kind of work into the users, to
+be done over and over again. As should be clear from the title of
+this post, this made me settle for the second option: it was
+__obviously within reach__, especially for a statically-generated website
+like mine, to render math on the backend.
+
+I settled on the following architecture:
+
+* As before, I would generate my pages using Hugo.
+* I would use the KaTeX NPM package to render math.
+* To build the website no matter what system I was on, I would use Nix.
+
+It so happens that Nix isn't really required for using my approach in general.
+I will give my setup here, but feel free to skip ahead.
+
+### Setting Up A Nix Build
+My `default.nix` file looks like this:
+
+```Nix {linenos=table}
+    { stdenv, hugo, fetchgit, pkgs, nodejs, ruby }:
+
+    let
+        url = "https://dev.danilafe.com/Web-Projects/blog-static.git";
+        rev = "<commit>";
+        sha256 = "<hash>";
+        requiredPackages = import ./required-packages.nix {
+            inherit pkgs nodejs;
+        };
+    in
+        stdenv.mkDerivation {
+            name = "blog-static";
+            version = rev;
+            src = fetchgit {
+                inherit url rev sha256;
+            };
+            builder = ./builder.sh;
+            converter = ./convert.rb;
+            buildInputs = [
+                hugo
+                requiredPackages.katex
+                (ruby.withPackages (ps: [ ps.nokogiri ]))
+            ];
+        }
+```
+
+I'm using `node2nix` to generate the `required-packages.nix` file, which allows me,
+even from a sandboxed Nix build, to download and install `npm` packages. This is needed
+so that I have access to the `katex` binary at build time. I fed the following JSON file
+to `node2nix`:
+
+```JSON {linenos=table}
+[
+    "katex"
+]
+```
+
+The Ruby script I wrote for this (more on that soon) required the `nokogiri` gem, which
+I used for traversing the HTML generated for my site. Hugo was obviously required to
+generate the HTML.
+
+### Converting LaTeX To HTML
+After my first post complaining about the state of mathematics on the web, I received
+the following email (which the author allowed me to share):
+
+> Sorry for having a random stranger email you, but in your blog post
+[(link)](https://danilafe.com/blog/math_rendering_is_wrong) you seem to focus on MathJax's
+difficulty in rendering things server-side, while quietly ignoring that KaTeX's front
+page advertises server-side rendering. Their documentation [(link)](https://katex.org/docs/options.html)
+even shows (at least as of the time this email was sent) that it renders both HTML
+(to be arranged nicely with their CSS) for visuals and MathML for accessibility.
+
+The author of the email then kindly provided a link to a page they generated using KaTeX and
+some Bash scripts. The math on this page was rendered at the time it was generated.
+
+This is a great point, and KaTeX is indeed usable for server-side rendering. But I've
+seen few people who do actually use it. Unfortunately, as I pointed out in my previous post on the subject,
+few tools actually take your HTML page and replace LaTeX with rendered math.
+Here's what I wrote about this last time:
+
+> [In MathJax,] The bigger issue, though, was that the `page2html`
+program, which rendered all the mathematics in a single HTML page,
+was gone. I found `tex2html` and `text2htmlcss`, which could only
+render equations without the surrounding HTML. I also found `mjpage`,
+which replaced mathematical expressions in a page with their SVG forms.
+
+This is still the case, in both MathJax and KaTeX. The ability
+to render math in one step is the main selling point of front-end LaTeX renderers:
+all you have to do is drop in a file from a CDN, and voila, you have your
+math. There are no such easy answers for back-end rendering. In fact,
+as we will soon see, it's not possible to just search-and-replace occurences
+of mathematics on your page, either. To actually get KaTeX working
+on the backend, you need access to tools that handle the potential variety
+of edge cases associated with HTML. Such tools, to my knowledge, do not
+currently exist.
+
+I decided to write my own Ruby script to get the job done. From this script, I
+would call the `katex` command-line program, which would perform
+the heavy lifting of rendering the mathematics.
+
+There are two types of math on my website: inline math and display math.
+On the command line ([here are the docs](https://katex.org/docs/cli.html)),
+the distinction is made using the `--display-mode` argument. So, the general algorithm
+is to replace the code inside the `$$...$$` with their display-rendered version,
+and the code inside the `\(...\)` with the inline-rendered version. I came up with
+the following Ruby function:
+
+```Ruby {linenos=table}
+def render_cached(cache, command, string, render_comment = nil)
+  cache.fetch(string) do |new|
+    puts "  Rendering #{render_comment || new}"
+    cache[string] = Open3.popen3(command) do |i, o, e, t|
+      i.write new
+      i.close
+      o.read.force_encoding(Encoding::UTF_8).strip
+    end
+  end
+end
+```
+
+Here, the `cache` argument is used to prevent re-running the `katex` command
+on an equation that was already rendered before (the output is the same, after all).
+The `command` is the specific shell command that we want to invoke; this would
+be either `katex` or `katex -d`. The `string` is the math equation to render,
+and the `render_comment` is the string to print to the console instead of the equation
+(so that long, display math equations are not printed out to standard out).
+
+Then, given a substring of the HTML file, we use regular expressions
+to find the `\(...\)` and `$$...$$`s, and use the `render_cached` method
+on the LaTeX code inside.
+
+```Ruby {linenos=table}
+def perform_katex_sub(inline_cache, display_cache, content)
+  rendered = content.gsub /\\\(((?:[^\\]|\\[^\)])*)\\\)/ do |match|
+    render_cached(inline_cache, "katex", $~[1])
+  end
+  rendered = rendered.gsub /\$\$((?:[^\$]|$[^\$])*)\$\$/ do |match|
+    render_cached(display_cache, "katex -d", $~[1], "display")
+  end
+  return rendered
+end
+```
+
+There's a bit of a trick to the final layer of this script. We want to be
+really careful about where we replace LaTeX, and where we don't. In
+particular, we _don't_ want to go into the `code` tags. Otherwise,
+it wouldn't be possible to talk about LaTeX code! I also suspect that
+some captions, alt texts, and similar elements should also be left alone.
+However, I don't have those on my website (yet), and I won't worry about
+them now. Either way, because of the code tags,
+we can't just search-and-replace over the entire page; we need to be context
+aware. This is where `nokogiri` comes in. We parse the HTML, and iterate
+over all of the 'text' nodes, calling `perform_katex_sub` on all
+of those that _aren't_ inside code tags.
+
+Fortunately, this kind of iteration is pretty easy to specify thanks to something called XPath.
+This was my first time encountering it, but it seems extremely useful: it's
+a sort of language for selecting XML nodes. First, you provide an 'axis',
+which is used to specify the positions of the nodes you want to look at
+relative to the root node. The axis `/` looks at the immediate children
+(this would be the `html` tag in a properly formatted document, I would imagine).
+The axis `//` looks at all the transitive children. That is, it will look at the
+children of the root, then its children, and so on. There's also the `self` axis,
+which looks at the node itself.
+
+After you provide an axis, you need to specify the type of node that you want to
+select. We can write `code`, for instance, to pick only the `<code>....</code>` tags
+from the axis we've chosen. We can also use `*` to select any node, and we can
+use `text()` to select text nodes, such as the `Hello` inside of `<b>Hello</b>`.
+
+We can also apply some more conditions to the nodes we pick using `[]`.
+For us, the relevant feature here is `not(...)`, which allows us to
+select nodes that do __not__ match a particular condition. This is all
+we need to know.
+
+We write:
+
+* `//`, starting to search for nodes everywhere, not just the root of the document.
+* `*`, to match _any_ node. We want to replace math inside of `div`s, `span`s, `nav`s,
+all of the `h`s, and so on.
+* `[not(self::code)]`, cutting out all the `code` tags.
+* `/`, now selecting the nodes that are immediate descendants of the nodes we've selected.
+* `text()`, giving us the text contents of all the nodes we've selected.
+
+All in all:
+
+```
+//*[not(self::code)]/text()
+```
+
+Finally, we use this XPath from `nokogiri`:
+
+```Ruby {linenos=table}
+files = ARGV[0..-1]
+inline_cache, display_cache = {}, {}
+
+files.each do |file|
+  puts "Rendering file: #{file}"
+  document = Nokogiri::HTML.parse(File.open(file))
+  document.search('//*[not(self::code)]/text()').each do |t|
+    t.replace(perform_katex_sub(inline_cache, display_cache, t.content))
+  end
+  File.write(file, document.to_html)
+end
+```
+
+I named this script `convert.rb`; it's used from inside of the Nix expression
+and its builder, which we will cover below.
+
+### Tying it All Together
+Finally, I wanted an end-to-end script to generate HTML pages and render the LaTeX in them.
+I used Nix for this, but the below script will largely be compatible with a non-Nix system.
+I came up with the following, commenting on Nix-specific commands:
+
+```Bash {linenos=table}
+# Nix-specific; set up paths.
+source $stdenv/setup
+
+# Build site with Hugo
+# The cp is Nix-specific; it copies the blog source into the current directory.
+cp -r $src/* .
+hugo --baseUrl="https://danilafe.com"
+
+# Render math in HTML and XML files.
+# $converter is Nix-specific; you can just use convert.rb.
+find public/ -regex "public/.*\.html" | xargs ruby $converter
+
+# Output result
+# $out is Nix-specific; you can replace it with your destination folder.
+mkdir $out
+cp -r public/* $out/
+```
+
+This is it! Using the two scripts, `convert.rb` and `builder.sh`, I
+was able to generate my blog with the math rendered on the back-end.
+Please note, though, that I had to add the KaTeX CSS to my website's
+`<head>`.
+
+### Caveats
+The main caveat of my approach is performance. For every piece of
+mathematics that I render, I invoke the `katex` command. This incurs
+the penalty of Node's startup time, every time, and makes my approach
+take a few dozen seconds to run on my relatively small site. The
+better approach would be to use a NodeJS script, rather than a Ruby one,
+to perform the conversion. KaTeX also provides an API, so such a NodeJS
+script can find the files, parse the HTML, and perform the substitutions.
+I did quite like using `nokogiri` here, though, and I hope that an equivalently
+pleasant solution exists in JavaScript.
+
+Re-rendering the whole website is also pretty wasteful. I rarely change the
+mathematics on more than one page at a time, but every time I do so, I have
+to re-run the script, and therefore re-render every page. This makes sense
+for me, since I use Nix, and my builds are pretty much always performed
+from scratch. On the other hand, for others, this may not be the best solution.
+
+### Alternatives
+The same person who sent me the original email above also pointed out
+[this `pandoc` filter for KaTeX](https://github.com/Zaharid/pandoc_static_katex).
+I do not use Pandoc, but from what I can see, this fitler relies on
+Pandoc's `Math` AST nodes, and applies KaTeX to each of those. This
+should work, but wasn't applicable in my case, since Hugo's shrotcodes
+don't mix well with Pandoc. However, it certainly seems like a workable
+solution.
+
+### Conclusion
+With the removal of MathJax from my site, it is now completely JavaScript free,
+and contains virtually the same HTML that it did beforehand. This, I hope,
+makes it work better on devices where computational power is more limited.
+I also hope that it illustrates a general principle - it's very possible,
+and plausible, to render LaTeX on the back-end for a static site.

content/blog/dell_is_horrible/index.md

@@ -0,0 +1,381 @@
+---
+title: DELL Is A Horrible Company And You Should Avoid Them At All Costs
+date: 2020-07-23T13:40:05-07:00
+tags: ["Electronics"]
+---
+
+I really do not want this to be a consumer electronics blog. Such things
+aren't interesting to me, and nor do I have much knowledge
+about them. However, sometimes, ripples from these areas make their way
+into my life, and this is one such instance. Let me tell you
+{{< sidenote "right" "source-note" "a story" >}}
+I originally wrote about this in
+<a href="https://www.dell.com/community/XPS/Ridiculously-Bad-Support-Experience/td-p/7554383">a thread on DELL's support website</a>. Some of this post is
+going to be adapted from the support website, but some things have happened
+since. You will probably notice the change between the terse language I used
+in the original post and the fresh text that I'm writing now.
+{{< /sidenote >}} of
+my experience with DELL and their XPS 2-in-1 laptop, which has gone on since
+around January of 2020, and is still going at the time of writing, in July
+2020, half a year later.
+
+I was, until recently, an undergraduate student in Computer Science. I will
+soon be starting my Masters in Computer Science, too. I say this to make one
+thing clear: I need a computer. Not only is it a necessity for my major,
+but the majority of my hobbies -- including this blog -- are digital, too.
+Since my university is a couple of hours from my home, I travel back and forth
+a lot. I also have a cozy little spot in the
+{{< sidenote "right" "offices-note" "graduate student offices" >}}
+They're a bunch of cubicles in a keycard-protected room, really. Nothing fancy.
+{{< /sidenote >}}at my university, but travel by bus, so I find myself spending
+roughly equal portions of my work time at home and 'elsewhere'. A laptop
+as my primary machine, I thought, made sense. But it had to be a decent one.
+Persuaded by one of my instructors, who stressed the importance of vision and
+a decent screen, I settled on a DELL XPS, which at the time came with a 4k
+display.
+
+As is commonplace, things went great at first. The screen _was_ really nice,
+all of my code compiled swiftly, and even the games I occasionally played ran
+at a solid 60fps. I was happy with my purchase.
+
+There was one hiccup before things went really downhill, a sort of
+foreshadowing of things to come. My trackpad didn't work at peculiar times.
+
+### Prologue: Trackpad Hiccups
+While working, booted into Linux, I noticed that my trackpad was having some
+trouble. It was stuttering, and occasionally wouldn't work at all for seconds
+at a time. I assumed that this was a problem with the trackpad drivers on
+Linux, or perhaps the whole system was freezing up. I rebooted, and the
+problem went away.
+
+Until it came back.
+
+A few days later, my trackpad was freezing virtually every minute.
+It was strange, but fortunately, I'm used to a keyboard-based workflow, and
+the malfunctions did not affect me too much. It was just a little troubling.
+What soon made it more troubling, was that I noticed this exact same issue
+occurring on Windows. To me, this meant one dreadful thing: it was a hardware
+issue.
+
+I poked and prodded for a little bit, and finally discovered the cause:
+whenever I put my hand on the left palmrest, the trackpad would reliably stop
+working. Knowing what the issue was, I called DELL. I spoke to a guy on the
+other end, who had me run through diagnostics, driver updates, and BIOS
+settings (I imagined this was procedure, so I didn't mind doing the extra
+work to make the other guy's job easier). Finally, he scheduled a repair
+appointment. A technician came into my house, took off the laptop cover,
+and said something along the lines of:
+
+> Now look. They gave me a whole new motherboard and case to replace yours,
+but in my personal opinion, this is a bad idea. Things are bound to break
+when you do this. See how the replacement case has an insulating piece
+of fabric under the left palmrest, and yours doesn't? Why don't we rip
+the fabric off the replacement case, and tape it in place on your machine,
+without any reassembly?
+
+This man was wiser than any of the other DELL technicians, I now understand.
+The repair went without a hitch. He grilled me for going to college instead of
+just picking up a trade, which was cheaper and offered more job security.
+In the end, I felt a little weird about having a piece of fabric duct taped
+inside my computer, but the trackpad had no more issues ever since. All was
+well.
+
+### Service Request 1: Broken D Key
+All was well, that is, until the middle of winter term. I was typing up an
+assignment for a university class. I was working as usual, when I suddenly
+noticed that the "d" key stopped working - it had to be pressed rather weird
+to register on the computer. I looked down, and discovered that the key had
+snapped in half. The top part of the key fell off shortly thereafter.
+
+{{< figure src="brokenkey.jpg" caption="The broken D key shortly after the above events." >}}
+
+At that point, I was more surprised than anything. I hadn't heard of something
+like this ever happening, especially under circumstances as normal as typing.
+Regardless, I contacted support, and set up a repair appointment. Things only
+went downhill from there.
+
+Again, the appointment was scheduled, and only a few days later, another
+technician arrived at my house. The only way to repair the key, he said,
+was to replace the whole keyboard. They keyboard happens to be located
+underneath all the other hardware, and so, the entire laptop had to be
+disassembled and reassembled from scratch. He worked for about an hour, and
+eventually, he put the machine together. The words of the previous
+technician, who wanted to avoid doing exactly what had just been done, echoed
+in my head:
+
+> Things are bound to break when you do this.
+
+I asked him to test it, just to make sure everything works. Sure enough,
+not everything did work: the machine no longer had sound!
+
+### Service Request 2: No sound
+During diagnostics, the laptop did not emit the "beep" it usually does. This
+was the first sign. Booting into Windows, the sound icon was crossed out in
+red, and no sound was present. Booting into Linux led to similar results.
+The microphone on the machine did not seem to work either. The service
+technician said that he didn't have the parts to repair it, told me he'd call
+it in, and left. Soon after, I got an email asking for times I'm available to
+call: I said "any time except for 1-4 pacific time". DELL support proceeded
+to call me at 3pm pacific time, when I had no service. Unable to reach me,
+they promptly notified me that they are archiving my service request.
+
+This all occurred near finals week at my university, so I had to put the issue
+on hold. I had to maintain my grades, and I had to grade heaps of assignments
+from other students. Though the lack of sound was annoying, it wasn't as
+pressing as preparing for exams, so it was during spring break that I finally
+called again, and scheduled the service appointment. By then,
+{{< sidenote "right" "pandemic-note" "the pandemic was in full swing," >}}
+Just for posterity, in 2020, there had been an outbreak of COVID-19,
+a Coronavirus. Many states in the U.S., including my own, issued
+the orders for lockdown and social distancing, which meant the closing
+of schools, restaurants, and, apparently, the cessation of in-person
+repairs.
+{{< /sidenote >}}and DELL told me they'd mail me a box to put my laptop in, and
+I'd have to mail it off to their service center. Sure, I thought, that's
+fine. If it's at the service center, they won't ever "not have the required
+parts". I told the tech support person my address, he read it back to me, and
+so it was settled.
+
+Until, that is, the box arrived at the wrong address.
+
+I had received the machine as a gift from my family, who purchased the
+computer to arrive at their address. The box arrived at that address too,
+despite my explicit instructions to have it deliver to my current residence.
+Since my family and I live 2 hours apart, it took 4 total hours to get the box
+to me (a drive that couldn't be made right away!), and by the time I had it,
+DELL was already threatening me again with closing the service request.
+Eventually, I was able to mail the machine off, and about 5 business days
+later (business days during which I did not have a working machine, which is
+very necessary for my school and job) I received it back. I was excited to
+have the machine back, but that didn't last very long. As I was using the
+computer with Wolfram Mathematica (a rather heavy piece of software running
+under Linux), I noticed that it was discharging even while plugged in. I
+booted into Windows, and was greeted with a warning, something along the
+lines of: "you are using a slow charger. Please use the official adapter".
+But I was using the official adapter! I also tried to plug my mouse into the
+relevant USB-C port, only to discover that it did not work. I had to make
+another service requests.
+
+### Service Request 3: Broken Charging Port
+This time, I made sure to tell the person on the other end of the support
+call to please send it to my address. I asked if there was anything I can do,
+or anyone I can contact, and was told "no, just mail the computer in again."
+I obliged. The box arrived at the right address this time, so I was able to
+ship it off.
+
+In the "describe your issue" field on the provided form, I begged the
+technicians to send me a working machine. "Please", I wrote "Last time I got
+a machine back from support, it was still broken. I really need it for school
+and work!". 5 business days later, I received the machine back. I plugged it
+in to make sure it worked, only to find out . . . that the very same charging
+port that I requested be repaired, is still broken! It would've been funny,
+if it wasn't infuriating. How is it possible for me to receive a machine from
+repairs, without the thing I asked to repair being as much as improved?!
+
+Worse, a day after I received the machine back (I was able to keep using it
+thanks to it having two USB-C ports capable of charging), the LCD suddenly
+flashed, and started flickering. Thinking it was a software glitch, I
+restarted the machine, only to discover the same flickering during the boot
+animation and menu. Not only was the charging port not repaired, but now my
+LCD was broken! (in the below picture, the screen is meant to be blue, but
+the bottom part of the display is purple and flickering).
+
+{{< figure src="brokenlcd.jpg" caption="The broken LCD." >}}
+
+### Service Request 4: Broken LCD
+I called in to support again, and they once again told me to ship the machine 
+off. What's worse, they accused me of breaking the port myself, and told me
+this was no longer covered under basic warranty. I had to explain all over
+again that the port worked fine before the fateful day the D-key snapped. They
+told me they'd "look into it". Eventually, I received a box in the mail. I
+wasn't told I would be receiving a box, but that wasn't a big deal. I mailed
+off the machine.
+
+The UPS shipping was always the most streamlined part of the process. A day
+later, I was told my machine was received intact. Another day, and I was
+informed that the technicians are starting to work on it. And then,
+a few hours later:
+
+> __Current Status:__
+> The part(s) needed to repair your system are not currently in stock.
+> __What's Next:__
+> In most cases the parts are available is less than five days.
+
+A few days is no big deal, and it made sense that DELL wouldn't just
+have screens lying around. So I waited. And waited. And waited. Two weeks
+later, I got a little tired of waiting, and called the repair center.
+An automated message told me:
+
+> We're currently experiencing heavy call volumes. Please try again later. Goodbye.
+
+And the call was dropped. This happened every time I tried to call, no matter
+the hour. The original status update -- the one that notified me about the
+part shortage -- came on May 8th, but the machine finally arrived to me
+(without prior warning) on June 2nd, almost a month later.
+
+The charging port worked. Sound
+worked. The screen wasn't flickering. I was happy for the brief moments that
+my computer was loading. As soon as I started vim, though, I noticed something
+was off: the fonts looked more pixelated. The DPI settings I'd painstakingly
+tweaked were wrong. Now that I thought about it, even the GRUB menu was
+larger. My suspicion growing, I booted into Windows, and looked at the display
+settings. Noticeably fewer resolutions were listed in the drop-down menu;
+worse, the highest resolution was 1080p. After almost a month of waiting,
+DELL replaced my 4k laptop display with a 1080p one.
+
+### System Replacement: Worse LCD Screen
+
+I admit, I was angry. At the same time, the absurdity of it all was also
+unbearable. Was this constant loop of hardware damage, the endless number of
+support calls filled with hoarse jazz music, part of some kind of Kafkaesque
+dream? I didn't know. I was at the end of my wits as to what to do. As a last
+resort, I made [a tweet](https://twitter.com/DanilaTheWalrus/status/1268056637383692289)
+from my almost-abandoned account. DELL Support's Twitter
+account [quickly responded](https://twitter.com/DellCares/status/1268064691416334344), eager as always to destroy any semblance of
+transparency by switching to private messages. I let them know my thoughts on the matter. I wanted a new machine.
+
+{{< figure src="dm_1.png" caption="The first real exchange between me and DELL support." >}}
+
+Of course we can proceed further. I wanted to know what kind of machine I was getting,
+though. As long as it was the same model that I originally bought,
+{{< sidenote "right" "replacement-note" "it would be better than what I have." >}}
+At least in principle, it would be. Perhaps the wear and tear on the replacement
+parts would be greater, but at least I would have, presumably, a machine
+in good condition that had the 4k screen that made me buy it in the first place.
+{{< /sidenote >}}
+Despite this, I knew that the machine I was getting was likely refurbished.
+This _had_ to mean that some of the parts would come from other, used, machines.
+This irked me, because, well, I payed for a new machine.
+
+{{< figure src="dm_2.png" caption="Ah, the classic use of canned responses." >}}
+
+Their use of the canned response, and their unwillingness to answer this simple
+question, was transparent. Indeed, the machine would be made of used
+parts. I still wanted to proceed. DELL requested that I sent an image of
+my machine which included its service tag, together with a piece of
+paper which included my name and email address. I obliged, and quickly got a response:
+
+{{< figure src="dm_3.png" caption="If it was me who was silent for 4 days, my request would've long been cancelled. " >}}
+
+Thanks, Kalpana. You will never hear this name again, not in this post.
+Only one or two messages from DELL support are ever from the same person.
+About a week later, I get the following beauty:
+
+{{< figure src="dm_4.png" caption="Excuse me? What's going on?" >}}
+
+My initial request was cancelled? Why wasn't I told? What was the reason?
+What the heck was going on at DELL Support? Should I be worried?
+My question of "Why" was answered with the apt response of "Yes",
+and a message meant to pacify me. While this was going on, I ordered
+a
+{{< sidenote "right" "pinebook-note" "Pinebook Pro." >}}
+The Pinebook – a $200 machine – has, thus far, worked more reliably than any DELL product
+I've had the misfortune of owning.
+{{< /sidenote >}} It was not a replacement for the DELL machine, but rather
+the first step towards migrating my setup to a stationary computer,
+and a small, lightweight SSH device. At this point,
+there was no more faith in DELL left in my mind.
+
+Soon, DELL required my attention, only to tell me that they put in
+a request to see that status of my request. How bureaucratic. Two
+more names -- Kareem and JKC -- flickered through the chats,
+also never to be seen again.
+
+{{< figure src="dm_5.png" caption="Not much of a conversation, really." >}}
+
+Finally, on July 9th (a month and six days after my first real message to DELL
+support), I was notified by my roommates that FedEx tried to deliver a package
+to our house, but gave up when no one came to sign for it. On one hand, this
+is great: FedEx didn't just leave my laptop on the porch. On the other hand,
+though, this was the first time I heard about receiving the machine. I got
+to the house the next day, unpacked the new computer, and tested all the things
+that had, at one point, failed. Everything seemed to work. I transfered all my
+files, wiped the old computer clean, and mailed it off. I also spent some
+time dealing with the fallout of DELL PremierColor starting on its own,
+and permanently altering the color profile of my display. I don't have the
+special, physical calibration device, and therefore still suspect that my
+screen is somewhat green.
+
+Today, I discovered that the microphone of the replacement machine didn't work.
+
+### Am I The Problem?
+When the mysterious FedEx package arrived at my door on July 9th, I did some
+digging to verify my suspicion that it was from DELL. I discovered their
+HQ in Lebanon, TN. This gave me an opportunity to
+{{< sidenote "right" "reviews-note" "see" >}}
+See, of course, modulo whatever bias arises when only those who feel strongly leave reviews.
+{{< /sidenote >}} whether or not I was alone in this situation. I was genuinely
+worried that I was suffering from the technical variant of
+[Munchausen Syndrome](https://www.webmd.com/mental-health/munchausen-syndrome#1),
+and that I was compulsively breaking my electronics. These worries were
+dispelled by the reviews on Google:
+
+{{< figure src="reviews_1.png" caption="Most of the reviews are pretty terse, but the ratings convey the general idea." >}}
+
+There were even some that were shockingly similar in terms of the apparent
+incompetence of the DELL technicians:
+
+{{< figure src="reviews_2.png" caption="Now, now, Maggie, I wouldn't go as far as recommending Apple." >}}
+
+So, this is not uncommon. This is how DELL deals with customers now. It's
+awfully tiring, really; I've been in and out of repairs continuously for
+almost half a year, now. That's 2.5% of my life at the time of writing,
+all non-stop since the D-key. And these people probably have spent considerable
+amounts of time, too.
+
+### It's About the Principle
+The microphone on my machine is rather inconsequential to me. I can, and regularly do,
+teleconference from my phone (a habit that I developed thanks to DELL, since
+my computer was so often unavailable). I don't need to dictate anything. Most
+of my communication is via chat.
+
+Really, compared to the other issues (keyboard, sound, charging, USB ports, the broken or low-resolution screen)
+the microphone is a benign problem. As I have now learned, things could be worse.
+
+But why should the thought, _"It could be worse"_, even cross my mind
+when dealing with such a matter? Virtually every issue that has
+occurred with my computer thus far could -- should! -- have been diagnosed
+at the repair center. The 'slow charger' warning shows up in BIOS,
+so just turning the computer on while plugged in should make it obvious something
+is wrong; doubly so when the very reason that the laptop was in repairs
+in the first place was because of the faulty charger. I refuse to believe
+that screens with different resolutions have the same part identifier,
+either. How have the standards of service from DELL fallen so low?
+How come this absurd scenario plays out not just for me, but
+for others as well? It would be comforting, in a way, to think
+that I was just the 'exceptional case'. But apparently, I'm not.
+This is standard practice.
+
+### Tl;DR
+Here are he problems I've had with DELL:
+
+* The machine shipped, apparently, with a missing piece of insulation.
+* The "D" key on the keyboard snapped after only a few months of use.
+* While repairing the "D" key, the DELL technician broke the computer's sound and microphone.
+* While repairing the sound and microphone, the DELL technicians broke a charging port.
+* The DELL technicians failed to repair the charging port, mailing me back a machine
+exhibiting the same issues, in addition to a broken LCD screen.
+* The repair of the LCD screen took almost a month, and concluded
+with me receiving a worse quality screen than I originally had.
+* The system replacement that followed the botched LCD repair took
+over a month to go through.
+* The replaced system was made partially of used parts, which
+DELL refused to admit.
+* The microphone on the replacement system was broken.
+
+### Closing Thoughts
+I will not be sending my system in again. It doesn't make sense to do so -
+after mailing my system in for repairs three times, I've measured empirically that
+the chance of failure is 100%. Every service request is a lottery, dutifully
+giving out a random prize of another broken part. I no longer wish to play;
+as any person who gambles should, I will quit while I'm ahead, and cut my losses.
+However, I hope for this story, which may be unusual in its level of detail,
+but not its content, to be seen by seen by someone. I hope to prevent
+someone out there from feeling the frustration, and anger, and peculiar amusement
+that I felt during this process. I hope for someone else to purchase a computer
+with money, and not with their sanity. A guy can hope.
+
+If you're reading this, please take this as a warning. __DELL is a horrible
+company. They have the lowest standards of customer support of any
+U.S. company that I've encountered. Their technicians are largely incompetent.
+Their quality assurance is non-existent. Stay away from them.__

content/blog/haskell_lazy_evaluation.md

@@ -1,95 +0,0 @@
----
-title: "Clairvoyance for Good: Using Lazy Evaluation in Haskell"
-date: 2020-05-03T20:05:29-07:00
-tags: ["Haskell"]
-draft: true
----
-
-While tackling a project for work, I ran across a rather unpleasant problem.
-I don't think it's valuable to go into the specifics here (it's rather
-large and convoluted); however, the outcome of this experience led me to
-discover a very interesting technique for lazy functional languages,
-and I want to share what I learned.
-
-### Time Traveling
-Some time ago, I read [this post](https://kcsongor.github.io/time-travel-in-haskell-for-dummies/) by Csongor Kiss about time traveling in Haskell. It's
-really cool, and makes a lot of sense if you have wrapped your head around
-lazy evaluation. I'm going to present my take on it here, but please check out
-Csongor's original post if you are interested.
-
-Say that you have a list of integers, like `[3,2,6]`. Next, suppose that
-you want to find the maximum value in the list. You can implement such
-behavior quite simply with pattern matching:
-
-```Haskell
-myMax :: [Int] -> Int
-myMax [] = error "Being total sucks"
-myMax (x:xs) = max x $ myMax xs
-```
-
-You could even get fancy with a `fold`:
-
-```Haskell
-myMax :: [Int] -> Int
-myMax = foldr1 max
-```
-
-All is well, and this is rather elementary Haskell. But now let's look at
-something that Csongor calls the `repMax` problem:
-
-> Imagine you had a list, and you wanted to replace all the elements of the
-> list with the largest element, by only passing the list once.
-
-How can we possibly do this in one pass? First, we need to find the maximum
-element, and only then can we have something to replace the other numbers
-with! It turns out, though, that we can just expect to have the future
-value, and all will be well. Csongor provides the following example:
-
-```Haskell {linenos=table}
-repMax :: [Int] -> Int -> (Int, [Int])
-repMax [] rep = (rep, [])
-repMax [x] rep = (x, [rep])
-repMax (l : ls) rep = (m', rep : ls')
-  where (m, ls') = repMax ls rep
-        m' = max m l
-
-doRepMax :: [Int] -> [Int]
-doRepMax xs = xs'
-  where (largest, xs') = repMax xs largest
-```
-
-In the above snippet, `repMax` expects to receive the maximum value of
-its input list. At the same time, it also computes that maximum value,
-returning it and the newly created list. `doRepMax` is where the magic happens:
-the `where` clauses receives the maximum number from `repMax`, while at the
-same time using that maximum number to call `repMax`!
-
-This works because Haskell's evaluation model is, effectively,
-[lazy graph reduction](https://en.wikipedia.org/wiki/Graph_reduction). That is,
-you can think of Haskell as manipulating your code as
-{{< sidenote "right" "tree-note" "a syntax tree," >}}
-Why is it called graph reduction, you may be wondering, if the runtime is
-manipulating syntax trees? To save on work, if a program refers to the
-same value twice, Haskell has both of those references point to the
-exact same graph. This violates the tree's property of having only one path
-from the root to any node, and makes our program a graph. Graphs that
-refer to themselves also violate the properties of a tree.
-{{< /sidenote >}} performing
-substitutions and simplifications as necessary until it reaches a final answer.
-What the lazy part means is that parts of the syntax tree that are not yet
-needed to compute the final answer can exist, unsimplied, in the tree. This is
-what allows us to write the code above: the graph of `repMax xs largest`
-effectively refers to itself. While traversing the list, it places references
-to itself in place of each of the elements, and thanks to laziness, these
-references are not evaluated.
-
-Let's try a more complicated example. How about instead of creating a new list,
-we return a `Map` containing the number of times each number occured, but only
-when those numbers were a factor of the maximum numbers. Our expected output
-will be:
-
-```
->>> countMaxFactors [1,3,3,9]
-
-fromList [(1, 1), (3, 2), (9, 1)]
-```

content/blog/haskell_lazy_evaluation/index.md

@@ -0,0 +1,564 @@
+---
+title: "Time Traveling In Haskell: How It Works And How To Use It"
+date: 2020-07-30T00:58:10-07:00
+tags: ["Haskell"]
+---
+
+I recently got to use a very curious Haskell technique
+{{< sidenote "right" "production-note" "in production:" >}}
+As production as research code gets, anyway!
+{{< /sidenote >}} time traveling. I say this with
+the utmost seriousness. This technique worked like
+magic for the problem I was trying to solve, and so
+I thought I'd share what I learned. In addition
+to the technique and its workings, I will also explain how 
+time traveling can be misused, yielding computations that
+never terminate.
+
+### Time Traveling
+Some time ago, I read [this post](https://kcsongor.github.io/time-travel-in-haskell-for-dummies/) by Csongor Kiss about time traveling in Haskell. It's
+really cool, and makes a lot of sense if you have wrapped your head around
+lazy evaluation. I'm going to present my take on it here, but please check out
+Csongor's original post if you are interested.
+
+Say that you have a list of integers, like `[3,2,6]`. Next, suppose that
+you want to find the maximum value in the list. You can implement such
+behavior quite simply with pattern matching:
+
+```Haskell
+myMax :: [Int] -> Int
+myMax [] = error "Being total sucks"
+myMax (x:xs) = max x $ myMax xs
+```
+
+You could even get fancy with a `fold`:
+
+```Haskell
+myMax :: [Int] -> Int
+myMax = foldr1 max
+```
+
+All is well, and this is rather elementary Haskell. But now let's look at
+something that Csongor calls the `repMax` problem:
+
+> Imagine you had a list, and you wanted to replace all the elements of the
+> list with the largest element, by only passing the list once.
+
+How can we possibly do this in one pass? First, we need to find the maximum
+element, and only then can we have something to replace the other numbers
+with! It turns out, though, that we can just expect to have the future
+value, and all will be well. Csongor provides the following example:
+
+```Haskell
+repMax :: [Int] -> Int -> (Int, [Int])
+repMax [] rep = (rep, [])
+repMax [x] rep = (x, [rep])
+repMax (l : ls) rep = (m', rep : ls')
+  where (m, ls') = repMax ls rep
+        m' = max m l
+```
+In this example, `repMax` takes the list of integers,
+each of which it must replace with their maximum element.
+It also takes __as an argument__ the maximum element,
+as if it had already been computed. It does, however,
+still compute the intermediate maximum element,
+in the form of `m'`. Otherwise, where would the future
+value even come from?
+
+Thus far, nothing too magical has happened. It's a little
+strange to expect the result of the computation to be
+given to us; it just looks like wishful
+thinking. The real magic happens in Csongor's `doRepMax`
+function:
+
+```Haskell
+doRepMax :: [Int] -> [Int]
+doRepMax xs = xs'
+  where (largest, xs') = repMax xs largest
+```
+
+Look, in particular, on the line with the `where` clause.
+We see that `repMax` returns the maximum element of the
+list, `largest`, and the resulting list `xs'` consisting
+only of `largest` repeated as many times as `xs` had elements.
+But what's curious is the call to `repMax` itself. It takes
+as input `xs`, the list we're supposed to process... and
+`largest`, the value that _it itself returns_.
+
+This works because Haskell's evaluation model is, effectively,
+[lazy graph reduction](https://en.wikipedia.org/wiki/Graph_reduction). That is,
+you can think of Haskell as manipulating your code as
+{{< sidenote "right" "tree-note" "a syntax tree," >}}
+Why is it called graph reduction, you may be wondering, if the runtime is
+manipulating syntax trees? To save on work, if a program refers to the
+same value twice, Haskell has both of those references point to the
+exact same graph. This violates the tree's property of having only one path
+from the root to any node, and makes our program a DAG (at least). Graph nodes that
+refer to themselves (which are also possible in the model) also violate the properties of a
+a DAG, and thus, in general, we are working with graphs.
+{{< /sidenote >}} performing
+substitutions and simplifications as necessary until it reaches a final answer.
+What the lazy part means is that parts of the syntax tree that are not yet
+needed to compute the final answer can exist, unsimplified, in the tree.
+Why don't we draw a few graphs to get familiar with the idea?
+
+### Visualizing Graphs and Their Reduction
+Let's start with something that doesn't have anything fancy. We can
+take a look at the graph of the expression:
+
+```Haskell
+length [1]
+```
+
+Stripping away Haskell's syntax sugar for lists, we can write this expression as:
+
+```Haskell
+length (1:[])
+```
+
+Then, recalling that `(:)`, or 'cons', is just a binary function, we rewrite:
+
+```Haskell
+length ((:) 1 [])
+```
+
+We're now ready to draw the graph; in this case, it's pretty much identical
+to the syntax tree of the last form of our expression:
+
+{{< figure src="length_1.png" caption="The initial graph of `length [1]`." class="small" >}}
+
+In this image, the `@` nodes represent function application. The
+root node is an application of the function `length` to the graph that
+represents the list `[1]`. The list itself is represented using two
+application nodes: `(:)` takes two arguments, the head and tail of the
+list, and function applications in Haskell are
+[curried](https://en.wikipedia.org/wiki/Currying). Eventually,
+in the process of evaluation, the body of `length` will be reached,
+and leave us with the following graph:
+
+{{< figure src="length_2.png" caption="The graph of `length [1]` after the body of `length` is expanded." class="small" >}}
+
+Conceptually, we only took one reduction step, and thus, we haven't yet gotten
+to evaluating the recursive call to `length`. Since `(+)`
+is also a binary function, `1+length xs` is represented in this
+new graph as two applications of `(+)`, first to `1`, and then
+to `length []`.
+
+But what is that box at the root? This box _used to be_ the root of the
+first graph, which was an application node. However, it is now a
+an _indirection_. Conceptually, reducing
+this indirection amounts to reducing the graph
+it points to. But why have we {{< sidenote "right" "altered-note" "altered the graph" >}}
+This is a key aspect of implementing functional languages.
+The language itself may be pure, while the runtime
+can be, and usually is, impure and stateful. After all,
+computers are impure and stateful, too!
+{{< /sidenote >}} in this manner? Because Haskell is a pure language,
+of course! If we know that a particular graph reduces to some value,
+there's no reason to reduce it again. However, as we will
+soon see, it may be _used_ again, so we want to preserve its value.
+Thus, when we're done reducing a graph, we replace its root node with
+an indirection that points to its result. 
+
+When can a graph be used more than once? Well, how about this:
+
+```Haskell
+let x = square 5 in x + x
+```
+
+Here, the initial graph looks as follows:
+
+{{< figure src="square_1.png" caption="The initial graph of `let x = square 5 in x + x`." class="small" >}}
+
+As you can see, this _is_ a graph, but not a tree! Since both
+variables `x` refer to the same expression, `square 5`, they
+are represented by the same subgraph. Then, when we evaluate `square 5`
+for the first time, and replace its root node with an indirection,
+we end up with the following:
+
+{{< figure src="square_2.png" caption="The graph of `let x = square 5 in x + x` after `square 5` is reduced." class="small" >}}
+
+There are two `25`s in the graph, and no more `square`s! We only
+had to evaluate `square 5` exactly once, even though `(+)`
+will use it twice (once for the left argument, and once for the right).
+
+Our graphs can also include cycles.
+A simple, perhaps _the most_ simple example of this in practice is Haskell's
+`fix` function. It computes a function's fixed point,
+{{< sidenote "right" "fixpoint-note" "and can be used to write recursive functions." >}}
+In fact, in the lambda calculus, <code>fix</code> is pretty much <em>the only</em>
+way to write recursive functions. In the untyped lambda calculus, it can
+be written as: $$\lambda f . (\lambda x . f (x \ x)) \  (\lambda x . f (x \ x))$$
+In the simply typed lambda calculus, it cannot be written in any way, and
+needs to be added as an extension, typically written as \(\textbf{fix}\).
+{{< /sidenote >}}
+It's implemented as follows:
+
+```Haskell
+fix f = let x = f x in x
+```
+
+See how the definition of `x` refers to itself? This is what
+it looks like in graph form:
+
+{{< figure src="fixpoint_1.png" caption="The initial graph of `let x = f x in x`." class="tiny" >}}
+
+I think it's useful to take a look at how this graph is processed. Let's
+pick `f = (1:)`. That is, `f` is a function that takes a list,
+and prepends `1` to it. Then, after constructing the graph of `f x`,
+we end up with the following:
+
+{{< figure src="fixpoint_2.png" caption="The graph of `fix (1:)` after it's been reduced." class="small" >}}
+
+We see the body of `f`, which is the application of `(:)` first to the
+constant `1`, and then to `f`'s argument (`x`, in this case). As
+before, once we evaluated `f x`, we replaced the application with
+an indirection; in the image, this indirection is the top box. But the
+argument, `x`, is itself an indirection which points to the root of `f x`,
+thereby creating a cycle in our graph. Traversing this graph looks like
+traversing an infinite list of `1`s.
+
+Almost there! A node can refer to itself, and, when evaluated, it
+is replaced with its own value. Thus, a node can effectively reference
+its own value! The last piece of the puzzle is how a node can access
+_parts_ of its own value: recall that `doRepMax` calls `repMax`
+with only `largest`, while `repMax` returns `(largest, xs')`.
+I have to admit, I don't know the internals of GHC, but I suspect
+this is done by translating the code into something like:
+
+```Haskell
+doRepMax :: [Int] -> [Int]
+doRepMax xs = snd t
+  where t = repMax xs (fst t)
+```
+
+#### Detailed Example: Reducing `doRepMax`
+
+If the above examples haven't elucidated how `doRepMax` works,
+stick around in this section and we will go through it step-by-step.
+This is a rather long and detailed example, so feel free to skip
+this section to read more about actually using time traveling.
+
+If you're sticking around, why don't we watch how the graph of `doRepMax [1, 2]` unfolds.
+This example will be more complex than the ones we've seen
+so far; to avoid overwhelming ourselves with notation,
+let's adopt a different convention of writing functions. Instead
+of using application nodes `@`, let's draw an application of a
+function `f` to arguments `x1` through `xn` as a subgraph with root `f`
+and children `x`s. The below figure demonstrates what I mean:
+
+{{< figure src="notation.png" caption="The new visual notation used in this section." class="small" >}}
+
+Now, let's write the initial graph for `doRepMax [1,2]`:
+
+{{< figure src="repmax_1.png" caption="The initial graph of `doRepMax [1,2]`." class="small" >}}
+
+Other than our new notation, there's nothing too surprising here.
+The first step of our hypothetical reduction would replace the application of `doRepMax` with its
+body, and create our graph's first cycle. At a high level, all we want is the second element of the tuple
+returned by `repMax`, which contains the output list. To get
+the tuple, we apply `repMax` to the list `[1,2]` and the first element
+of its result. The list `[1,2]` itself
+consists of two uses of the `(:)` function.
+
+{{< figure src="repmax_2.png" caption="The first step of reducing `doRepMax [1,2]`." class="small" >}}
+
+Next, we would also expand the body of `repMax`. In
+the following diagram, to avoid drawing a noisy amount of
+crossing lines, I marked the application of `fst` with
+a star, and replaced the two edges to `fst` with
+edges to similar looking stars. This is merely
+a visual trick; an edge leading to a little star is
+actually an edge leading to `fst`. Take a look:
+
+{{< figure src="repmax_3.png" caption="The second step of reducing `doRepMax [1,2]`." class="medium" >}}
+
+Since `(,)` is a constructor, let's say that it doesn't
+need to be evaluated, and that its
+{{< sidenote "right" "normal-note" "graph cannot be reduced further" >}}
+A graph that can't be reduced further is said to be in <em>normal form</em>,
+by the way.
+{{< /sidenote >}} (in practice, other things like
+packing may occur here, but they are irrelevant to us).
+If `(,)` can't be reduced, we can move on to evaluating `snd`. Given a pair, `snd`
+simply returns the second element, which in our
+case is the subgraph starting at `(:)`. We
+thus replace the application of `snd` with an
+indirection to this subgraph. This leaves us
+with the following:
+
+{{< figure src="repmax_4.png" caption="The third step of reducing `doRepMax [1,2]`." class="medium" >}}
+
+Since it's becoming hard to keep track of what part of the graph
+is actually being evaluated, I marked the former root of `doRepMax [1,2]` with
+a blue star. If our original expression occured at the top level,
+the graph reduction would probably stop here.  After all,
+we're evaluating our graphs using call-by-need, and there
+doesn't seem to be a need for knowing the what the arguments of `(:)` are.
+However, stopping at `(:)` wouldn't be very interesting,
+and we wouldn't learn much from doing so. So instead, let's assume
+that _something_ is trying to read the elements of our list;
+perhaps we are trying to print this list to the screen in GHCi.
+
+In this case, our mysterious external force starts unpacking and
+inspecting the arguments to `(:)`. The first argument to `(:)` is
+the list's head, which is the subgraph starting with the starred application
+of `fst`. We evaluate it in a similar manner to `snd`. That is,
+we replace this `fst` with an indirection to the first element
+of the argument tuple, which happens to be the subgraph starting with `max`:
+
+{{< figure src="repmax_5.png" caption="The fourth step of reducing `doRepMax [1,2]`." class="medium" >}}
+
+Phew! Next, we need to evaluate the body of `max`. Let's make one more
+simplification here: rather than substitututing `max` for its body
+here, let's just reason about what evaluating `max` would entail.
+We would need to evaluate its two arguments, compare them,
+and return the larger one. The argument `1` can't be reduced
+any more (it's just a number!), but the second argument,
+a call to `fst`, needs to be processed. To do so, we need to
+evaluate the call to `repMax`. We thus replace `repMax`
+with its body:
+
+{{< figure src="repmax_6.png" caption="The fifth step of reducing `doRepMax [1,2]`." class="medium" >}}
+
+We've reached one of the base cases here, and there
+are no more calls to `max` or `repMax`. The whole reason
+we're here is to evaluate the call to `fst` that's one
+of the arguments to `max`. Given this graph, doing so is easy.
+We can clearly see that `2` is the first element of the tuple
+returned by `repMax [2]`. We thus replace `fst` with
+an indirection to this node:
+
+{{< figure src="repmax_7.png" caption="The sixth step of reducing `doRepMax [1,2]`." class="medium" >}}
+
+This concludes our task of evaluating the arguments to `max`.
+Comparing them, we see that `2` is greater than `1`, and thus,
+we replace `max` with an indirection to `2`:
+
+{{< figure src="repmax_8.png" caption="The seventh step of reducing `doRepMax [1,2]`." class="medium" >}}
+
+The node that we starred in our graph is now an indirection (the
+one that used to be the call to `fst`) which points to
+another indirection (formerly the call to `max`), which
+points to `2`. Thus, any edge pointing to a star now
+points to the value 2. 
+
+By finding the value of the starred node, we have found the first
+argument of `(:)`, and returned it to our mysterious external force.
+If we were printing to GHCi, the number `2` would appear on the screen
+right about now. The force then moves on to the second argument of `(:)`,
+which is the call to `snd`. This `snd` is applied to an instance of `(,)`, which
+can't be reduced any further. Thus, all we have to do is take the second
+element of the tuple, and replace `snd` with an indirection to it:
+
+{{< figure src="repmax_9.png" caption="The eighth step of reducing `doRepMax [1,2]`." class="medium" >}}
+
+The second element of the tuple was a call to `(:)`, and that's what the mysterious
+force is processing now. Just like it did before, it starts by looking at the
+first argument of this list, which is the list's head. This argument is a reference to
+the starred node, which, as we've established, eventually points to `2`.
+Another `2` pops up on the console.
+
+Finally, the mysterious force reaches the second argument of the `(:)`,
+which is the empty list. The empty list also cannot be evaluated any
+further, so that's what the mysterious force receives. Just like that,
+there's nothing left to print to the console. The mysterious force ceases.
+After removing the unused nodes, we are left with the following graph:
+
+{{< figure src="repmax_10.png" caption="The result of reducing `doRepMax [1,2]`." class="small" >}}
+
+As we would have expected, two `2`s were printed to the console, and our
+final graph represents the list `[2,2]`.
+
+### Using Time Traveling
+Is time tarveling even useful? I would argue yes, especially
+in cases where Haskell's purity can make certain things
+difficult.
+
+As a first example, Csongor provides an assembler that works
+in a single pass. The challenge in this case is to resolve
+jumps to code segments occuring _after_ the jump itself;
+in essence, the address of the target code segment needs to be
+known before the segment itself is processed. Csongor's
+code uses the [Tardis monad](https://hackage.haskell.org/package/tardis-0.4.1.0/docs/Control-Monad-Tardis.html),
+which combines regular state, to which you can write and then
+later read from, and future state, from which you can
+read values before your write them. Check out
+[his complete example](https://kcsongor.github.io/time-travel-in-haskell-for-dummies/#a-single-pass-assembler-an-example) here.
+
+Alternatively, here's an example from my research, which my
+coworker and coauthor Kai helped me formulate. I'll be fairly
+vague, since all of this is still in progress. The gist is that
+we have some kind of data structure (say, a list or a tree),
+and we want to associate with each element in this data
+structure a 'score' of how useful it is. There are many possible
+heuristics of picking 'scores'; a very simple one is 
+to make it inversely propertional to the number of times
+an element occurs. To be more concrete, suppose
+we have some element type `Element`:
+
+{{< codelines "Haskell" "time-traveling/ValueScore.hs" 5 6 >}}
+
+Suppose also that our data structure is a binary tree:
+
+{{< codelines "Haskell" "time-traveling/ValueScore.hs" 14 16 >}}
+
+We then want to transform an input `ElementTree`, such as:
+
+```Haskell
+Node A (Node A Empty Empty) Empty
+```
+
+Into a scored tree, like:
+
+```Haskell
+Node (A,0.5) (Node (A,0.5) Empty Empty) Empty
+```
+
+Since `A` occured twice, its score is `1/2 = 0.5`. 
+
+Let's define some utility functions before we get to the
+meat of the implementation:
+
+{{< codelines "Haskell" "time-traveling/ValueScore.hs" 8 12 >}}
+
+The `addElement` function simply increments the counter for a particular
+element in the map, adding the number `1` if it doesn't exist. The `getScore`
+function computes the score of a particular element, defaulting to `1.0` if
+it's not found in the map.
+
+Just as before -- noticing that passing around the future values is getting awfully
+bothersome -- we write our scoring function as though we have
+a 'future value'.
+
+{{< codelines "Haskell" "time-traveling/ValueScore.hs" 18 24 >}}
+
+The actual `doAssignScores` function is pretty much identical to
+`doRepMax`:
+
+{{< codelines "Haskell" "time-traveling/ValueScore.hs" 26 28 >}}
+
+There's quite a bit of repetition here, especially in the handling
+of future values - all of our functions now accept an extra
+future argument, and return a work-in-progress future value.
+This is what the `Tardis` monad, and its corresponding
+`TardisT` monad transformer, aim to address. Just like the
+`State` monad helps us avoid writing plumbing code for
+forward-traveling values, `Tardis` helps us do the same
+for backward-traveling ones.
+
+#### Cycles in Monadic Bind
+
+We've seen that we're able to write code like the following:
+
+```Haskell
+(a, b) = f a c
+```
+
+That is, we were able to write function calls that referenced
+their own return values. What if we try doing this inside
+a `do` block? Say, for example, we want to sprinkle some time
+traveling into our program, but don't want to add a whole new
+transformer into our monad stack. We could write code as follows:
+
+```Haskell
+do
+    (a, b) <- f a c
+    return b
+```
+
+Unfortunately, this doesn't work. However, it's entirely
+possible to enable this using the `RecursiveDo` language
+extension:
+
+```Haskell
+{-# LANGUAGE RecursiveDo #-}
+```
+
+Then, we can write the above as follows:
+
+```Haskell
+do
+    rec (a, b) <- f a c
+    return b
+``` 
+
+This power, however, comes at a price. It's not as straightforward
+to build graphs from recursive monadic computations; in fact,
+it's not possible in general. The translation of the above
+code uses `MonadFix`. A monad that satisfies `MonadFix` has
+an operation `mfix`, which is the monadic version of the `fix`
+function we saw earlier:
+
+```Haskell
+mfix :: Monad m => (a -> m a) -> m a
+-- Regular fix, for comparison
+fix :: (a -> a) -> a
+```
+
+To really understand how the translation works, check out the
+[paper on recursive do notation](http://leventerkok.github.io/papers/recdo.pdf).
+
+### Beware The Strictness
+Though Csongor points out other problems with the
+time traveling approach, I think he doesn't mention
+an important idea: you have to be _very_ careful about introducing
+strictness into your programs when running time-traveling code.
+For example, suppose we wanted to write a function,
+`takeUntilMax`, which would return the input list,
+cut off after the first occurence of the maximum number.
+Following the same strategy, we come up with:
+
+{{< codelines "Haskell" "time-traveling/TakeMax.hs" 1 12 >}}
+
+In short, if we encounter our maximum number, we just return
+a list of that maximum number, since we do not want to recurse
+further. On the other hand, if we encounter a number that's
+_not_ the maximum, we continue our recursion.
+
+Unfortunately, this doesn't work; our program never terminates.
+You may be thinking:
+
+> Well, obviously this doesn't work! We didn't actually
+compute the maximum number properly, since we stopped
+recursing too early. We need to traverse the whole list,
+and not just the part before the maximum number.
+
+To address this, we can reformulate our `takeUntilMax`
+function as follows:
+
+{{< codelines "Haskell" "time-traveling/TakeMax.hs" 14 21 >}}
+
+Now we definitely compute the maximum correctly! Alas,
+this doesn't work either. The issue lies on lines 5 and 18,
+more specifically in the comparison `x == m`. Here, we 
+are trying to base the decision of what branch to take
+on a future value. This is simply impossible; to compute
+the value, we need to know the value!
+
+This is no 'silly mistake', either! In complicated programs
+that use time traveling, strictness lurks behind every corner.
+In my research work, I was at one point inserting a data structure into
+a set; however, deep in the structure was a data type containing
+a 'future' value, and using the default `Eq` instance!
+Adding the data structure to a set ended up invoking `(==)` (or perhaps
+some function from the `Ord` typeclass),
+which, in turn, tried to compare the lazily evaluated values.
+My code therefore didn't terminate, much like `takeUntilMax`.
+
+Debugging time traveling code is, in general,
+a pain. This is especially true since future values don't look any different
+from regular values. You can see it in the type signatures
+of `repMax` and `takeUntilMax`: the maximum number is just an `Int`!
+And yet, trying to see what its value is will kill the entire program.
+As always, remember Brian W. Kernighan's wise words:
+
+> Debugging is twice as hard as writing the code in the first place.
+Therefore, if you write the code as cleverly as possible, you are,
+by definition, not smart enough to debug it.
+
+### Conclusion
+This is about it! In a way, time traveling can make code performing
+certain operations more expressive. Furthermore, even if it's not groundbreaking,
+thinking about time traveling is a good exercise to get familiar
+with lazy evaluation in general. I hope you found this useful!

content/blog/starbound.md

@@ -12,7 +12,7 @@ __py-starbound__, nicely enough, actually has a file named `FORMATS.md`. This fi
 > This section will contain information on how to retrieve a value from a BTreeDB5 database.
 
 Not very helpful. Before I go into what I managed to determine from the code, we may first take a look at one thing that we already know about the world format - it is a [B-Tree](https://en.wikipedia.org/wiki/B-tree).
-## Binary Search Trees
+### Binary Search Trees
 The B-Tree is a generalization of a Binary Search Tree, or BST for short. Binary Search trees (and B-Trees in general) operate on data that can be ordered consistently, the simplest example being numbers. For instance, as an example, I'll be using a BST that holds integers. A BST is made up of nodes, objects that actually hold the pieces of data that the tree itself organizes.
 
 In a BST, the nodes are organized in a simple way. Each node can have up to two _children_ (sub-nodes), and each of those can have up to two children, etc. The children are generally classified as _right_ and _left_. Conventionally, left children always have a value that is below (or comes before) the value of the node whose child they are (their _parent_), and right children have a bigger value.
@@ -45,7 +45,7 @@ __Although the average efficiency of a Binary Search Tree is \\(O(\log n)\\), me
 
 This isn't good enough, and many clever algorithms have been invented to speed up the lookup of the tree by making sure that it remains _balanced_ - that is, it _isn't_ arranged like a simple list. Some of these algorithms include [Red-Black Trees](https://en.wikipedia.org/wiki/Red%E2%80%93black_tree), [AVL Trees](https://en.wikipedia.org/wiki/AVL_tree), and, of course, B-Trees.
 
-## B-Trees
+### B-Trees
 B-Trees are a generalization of Binary Search Trees. That means that every Binary Search Tree is a B-Tree, but not all B-Trees are BSTs. The key difference lies in the fact that B-Trees' nodes aren't limited to having only two child nodes, and can also have more than one value.
 
 Each B-Tree node is a sorted array of values. That is, instead of a single number like the BST that we've looked at, it has multiple, and these numbers _must_ be sorted. Below are some examples of B-Tree nodes:
@@ -64,7 +64,7 @@ This is solved using another property of B-Trees - the number of children of a n
 
 If we were looking for the number 15, we'd look between the 10 and the 20, examining the 2nd node, and if we were looking for 45 we'd look past the 30, at the 4th node.
 
-## Starbound B-Trees and BTreeDB5
+### Starbound B-Trees and BTreeDB5
 The BTreeDB5 data structure uses something other than integers for its keys - it uses sequences of bytes. These bytes are compared in a very similar fashion to integers. The game first looks at the first number in the sequence of bytes (like the largest digit in an integer), and if that's the same, moves on to the next one. Also, Starbound B-Trees not only have the values, or _keys_, that they use to find data, but the data itself.
 
 The "nodes" in the BTreeDB are called "blocks" and are one of three types - "index", "leaf", and "free" nodes. "Index" nodes are like the `(10, 20, 30)` node in the above example - they point to other nodes, but actually store no data themselves. The "leaf" nodes actually contain the data, and, if that data is longer than the maximum block size, "leaf" nodes contain the index of the next leaf node where the user might continue to read the data. The "free" nodes are simply free data, empty and ready for Starbound to fill them with something useful.

content/blog/typesafe_interpreter_revisited.md

@@ -0,0 +1,375 @@
+---
+title: Meaningfully Typechecking a Language in Idris, Revisited
+date: 2020-07-22T14:37:35-07:00
+tags: ["Idris"]
+---
+
+Some time ago, I wrote a post titled [Meaningfully Typechecking a Language in Idris]({{< relref "typesafe_interpreter.md" >}}). The gist of the post was as follows:
+
+* _Programming Language Fundamentals_ students were surprised that, despite
+having run their expression through (object language) typechecking, they still had to
+have a `Maybe` type in their evaluation functions. This was due to
+the fact that the (meta language) type system was not certain that
+(object language) typechecking worked.
+* A potential solution was to write separate expression types such
+as `ArithExpr` and `BoolExpr`, which are known to produce booleans
+or integers. However, this required the re-implementation of most
+of the logic for `IfElse`, for which the branches could have integers,
+booleans, or strings.
+* An alternative solution was to use dependent types, and index
+the `Expr` type with the type it evaluates to. We defined a data type
+`data ExprType = IntType | StringType | BoolType`, and then were able
+to write types like `SafeExpr IntType` that we _knew_ would evaluate
+to an integer, or `SafeExpr BoolType`, which we also _knew_ would
+evaluate to a boolean. We then made our `typecheck` function
+return a pair of `(type, SafeExpr of that type)`.
+
+Unfortunately, I think that post is rather incomplete. I noted
+at the end of it that I was not certain on how to implement
+if-expressions, which were my primary motivation for not just
+sticking with `ArithExpr` and `BoolExpr`. It didn't seem too severe
+then, but now I just feel like a charlatan. Today, I decided to try
+again, and managed to figure it out with the excellent help from
+people in the `#idris` channel on Freenode. It required a more
+advanced use of dependent types: in particular, I ended up using
+Idris' theorem proving facilities to get my code to pass typechecking.
+In this post, I will continue from where we left off in the previous
+post, adding support for if-expressions.
+
+Let's start with the new `Expr` and `SafeExpr` types. Here they are:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV2.idr" 37 49 >}}
+
+For `Expr`, the `IfElse` constructor is very straightforward. It takes
+three expressions: the condition, the 'then' branch, and the 'else' branch.
+With `SafeExpr` and `IfThenElse`, things are more rigid. The condition
+of the expression has to be of a boolean type, so we make the first argument
+`SafeExpr BoolType`. Also, the two branches of the if-expression have to
+be of the same type. We encode this by making both of the input expressions
+be of type `SafeExpr t`. Since the result of the if-expression will be
+the output of one of the branches, the whole if-expression is also
+of type `SafeExpr t`.
+
+### What Stumped Me: Equality
+Typechecking if-expressions is where things get interesting. First,
+we want to require that the condition of the expression evaluates
+to a boolean. For this, we can write a function `requireBool`,
+that takes a dependent pair produced by `typecheck`. This
+function does one of two things:
+
+* If the dependent pair contains a `BoolType`, and therefore also an expression
+of type `SafeExpr BoolType`, `requireBool` succeeds, and returns the expression.
+* If the dependent pair contains any type other than `BoolType`, `requireBool`
+fails with an error message. Since we're using `Either` for error handling,
+this amounts to using the `Left` constructor.
+
+Such a function is quite easy to write:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV2.idr" 58 60 >}}
+
+We can then write all of the recursive calls to `typecheck` as follows:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV2.idr" 71 75 >}}
+
+Alright, so we have the types of the `t` and `e` branches. All we have to
+do now is use `(==)`. We could implement `(==)` as follows:
+
+```Idris
+implementation Eq ExprType where
+  IntType == IntType = True
+  BoolType == BoolType = True
+  StringType == StringType = True
+  _ == _ = False
+```
+
+Now we're golden, right? We can just write the following:
+
+```Idris {linenos=table, linenostart=76}
+    if tt == et
+       then pure (_ ** IfThenElse ce te ee)
+       else Left "Incompatible branch types."
+```
+
+No, this is not quire right. Idris complains:
+
+```
+Type mismatch between et and tt
+```
+
+Huh? But we just saw that `et == tt`! What's the problem?
+The problem is, in fact, that `(==)` is meaningless as far
+as the Idris typechecker is concerned. We could have just
+as well written,
+
+```Idris
+implementation Eq ExprType where
+  _ == _ = True
+```
+
+This would tell us that `IntType == BoolType`. But of course,
+`SafeExpr IntType` is not the same as `SafeExpr BoolType`; I
+would be very worried if the typechecker allowed me to assert
+otherwise. There is, however, a kind of equality that we can
+use to convince the Idris typechecker that two types are the
+same. This equality, too, is a type.
+
+### Curry-Howard Correspondence
+Spend enough time learning about Programming Language Theory, and
+you will hear the term _Curry Howard Correspondence_. If you're
+the paper kind of person, I suggest reading Philip Wadler's
+_Propositions as Types_ paper. Alternatively, you can take a look
+at _Logical Foundations_' [Proof Objects](https://softwarefoundations.cis.upenn.edu/lf-current/ProofObjects.html)
+chapter. I will give a very brief
+explanation here, too, for the sake of completeness. The general
+gist is as follows: __propositions (the logical kind) correspond
+to program types__, and proofs of the propositions correspond
+to values of the types.
+
+To get settled into this idea, let's look at a few 'well-known' examples:
+
+* `(A,B)`, the tuple of two types `A` and `B` is equivalent to the
+proposition \\(A \land B\\), which means \\(A\\) and \\(B\\). Intuitively,
+to provide a proof of \\(A \land B\\), we have to provide the proofs of
+\\(A\\) and \\(B\\).
+* `Either A B`, which contains one of `A` or `B`, is equivalent
+to the proposition \\(A \lor B\\), which means \\(A\\) or \\(B\\).
+Intuitively, to provide a proof that either \\(A\\) or \\(B\\)
+is true, we need to provide one of them.
+* `A -> B`, the type of a function from `A` to `B`, is equivalent
+to the proposition \\(A \rightarrow B\\), which reads \\(A\\)
+implies \\(B\\). We can think of a function `A -> B` as creating
+a proof of `B` given a proof of `A`. 
+
+Now, consider Idris' unit type `()`:
+
+```Idris
+data () = ()
+```
+
+This type takes no arguments, and there's only one way to construct
+it. We can create a value of type `()` at any time, by just writing `()`.
+This type is equivalent to \\(\\text{true}\\): only one proof of it exists,
+and it requires no premises. It just is.
+
+Consider also the type `Void`, which too is present in Idris:
+
+```Idris
+-- Note: this is probably not valid code.
+data Void = -- Nothing
+```
+
+The type `Void` has no constructors: it's impossible
+to create a value of this type, and therefore, it's 
+impossible to create a proof of `Void`. Thus, as you may have guessed, `Void`
+is equivalent to \\(\\text{false}\\).
+
+Finally, we get to a more complicated example:
+
+```Idris
+data (=) : a -> b -> Type where
+   Refl : x = x
+```
+
+This defines `a = b` as a type, equivalent to the proposition
+that `a` is equal to `b`. The only way to construct such a type
+is to give it a single value `x`, creating the proof that `x = x`.
+This makes sense: equality is reflexive. 
+
+This definition isn't some loosey-goosey boolean-based equality! If we can construct a value of 
+type `a = b`, we can prove to Idris' typechecker that `a` and `b` are equivalent. In
+fact, Idris' standard library gives us the following function:
+
+```Idris
+replace : {a:_} -> {x:_} -> {y:_} -> {P : a -> Type} -> x = y -> P x -> P y
+```
+
+This reads, given a type `a`, and values `x` and `y` of type `a`, if we know
+that `x = y`, then we can rewrite any proposition in terms of `x` into
+another, also valid proposition in terms of `y`. Let's make this concrete.
+Suppose `a` is `Int`, and `P` (the type of which is now `Int -> Type`),
+is `Even`, a proposition that claims that its argument is even.
+{{< sidenote "right" "specialize-note" "Then, we have:" >}}
+I'm only writing type signatures for <code>replace'</code>
+to avoid overloading. There's no need to define a new function;
+<code>replace'</code> is just a specialization of <code>replace</code>,
+so we can use the former anywhere we can use the latter.
+{{< /sidenote >}}
+
+```Idris
+replace' : {x : Int} -> {y : Int} -> x = y -> Even x -> Even y
+```
+
+That is, if we know that `x` is equal to `y`, and we know that `x` is even,
+it follows that `y` is even too. After all, they're one and the same!
+We can take this further. Recall:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV2.idr" 44 44 >}}
+
+We can therefore write:
+
+```Idris
+replace'' : {x : ExprType} -> {y : ExprType} -> x = y -> SafeExpr x -> SafeExpr y
+```
+
+This is exactly what we want! Given a proof that one `ExprType`, `x`, is equal to
+another `ExprType`, `y`, we can safely convert `SafeExpr x` to `SafeExpr y`.
+We will use this to convince the Idris typechecker to accept our program.
+
+### First Attempt: `Eq` implies Equality
+It's pretty trivial to see that we _did_ define `(==)` correctly (`IntType` is equal
+to `IntType`, `StringType` is equal to `StringType`, and so on). Thus,
+if we know that `x == y` is `True`, it should follow that `x = y`. We can thus
+define the following proposition:
+
+```Idris
+eqCorrect : {a : ExprType} -> {b : ExprType} -> (a == b = True) -> a = b
+```
+
+We will see shortly why this is _not_ the best solution, and thus, I won't bother
+creating a proof / implementation for this proposition / function.
+It reads:
+
+> If we have a proof that `(==)` returned true for some `ExprType`s `a` and `b`,
+it must be that `a` is the same as `b`.
+
+We can then define a function to cast
+a `SafeExpr a` to `SafeExpr b`, given that `(==)` returned `True` for some `a` and `b`:
+
+```Idris
+safeCast : {a : ExprType} -> {b : ExprType} -> (a == b = True) -> SafeExpr a -> SafeExpr b
+safeCast h e = replace (eqCorrect h) e
+```
+
+Awesome! All that's left now is to call `safeCast` from our `typecheck` function:
+
+```Idris {linenos=table, linenostart=76}
+    if tt == et
+       then pure (_ ** IfThenElse ce te (safeCast ?uhOh ee))
+       else Left "Incompatible branch types."
+```
+
+No, this doesn't work after all. What do we put for `?uhOh`? We need to have
+a value of type `tt == et = True`, but we don't have one. Idris' own if-then-else
+expressions do not provide us with such proofs about their conditions. The awesome
+people at `#idris` pointed out that the `with` clause can provide such a proof.
+We could therefore write:
+
+```Idris
+createIfThenElse ce (tt ** et) (et ** ee) with (et == tt) proof p
+  | True = pure (tt ** IfThenElse ce te (safeCast p ee))
+  | False = Left "Incompatible branch types."
+```
+
+Here, the `with` clause effectively adds another argument equal to `(et == tt)` to `createIfThenElse`,
+and tries to pattern match on its value. When we combine this with the `proof` keyword,
+Idris will give us a handle to a proof, named `p`, that asserts the new argument
+evaluates to the value in the pattern match. In our case, this is exactly
+the proof we need to give to `safeCast`.
+
+However, this is ugly. Idris' `with` clause only works at the top level of a function,
+so we have to define a function just to use it. It also shows that we're losing
+information when we call `(==)`, and we have to reconstruct or recapture it using
+some other means.
+
+
+### Second Attempt: Decidable Propositions
+More awesome folks over at `#idris` pointed out that the whole deal with `(==)`
+is inelegant; they suggested I use __decidable propositions__, using the `Dec` type.
+The type is defined as follows:
+
+```Idris
+data Dec : Type -> Type where
+  Yes : (prf : prop) -> Dec prop
+  No : (contra : prop -> Void) -> Dec prop
+```
+
+There are two ways to construct a value of type `Dec prop`:
+
+* We use the `Yes` constructor, which means that the proposition `prop`
+is true. To use this constructor, we have to give it a proof of `prop`,
+called `prf` in the constructor.
+* We use the `No` constructor, which means that the proposition `prop`
+is false. We need a proof of type `prop -> Void` to represent this:
+if we have a proof of `prop`, we arrive at a contradiction.
+
+This combines the nice `True` and `False` of `Bool`, with the
+'real' proofs of the truthfulness or falsity. At the moment
+that we would have been creating a boolean, we also create
+a proof of that boolean's value. Thus, we don't lose information.
+Here's how we can go about this:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV2.idr" 20 29 >}}
+
+We pattern match on the input expression types. If the types are the same, we return
+`Yes`, and couple it with `Refl` (since we've pattern matched on the types
+in the left-hand side of the function definition, the typechecker has enough
+information to create that `Refl`). On the other hand, if the expression types
+do not match, we have to provide a proof that their equality would be absurd.
+For this we use helper functions / theorems like `intBoolImpossible`:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV2.idr" 11 12 >}}
+
+I'm not sure if there's a better way of doing this than using `impossible`.
+This does the job, though: Idris understands that there's no way we can get
+an input of type `IntType = BoolType`, and allows us to skip writing a right-hand side.
+
+We can finally use this new `decEq` function in our type checker:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV2.idr" 76 78 >}}
+
+Idris is happy with this! We should also add `IfThenElse` to our `eval` function.
+This part is very easy:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV2.idr" 80 85 >}}
+
+Since the `c` part of the `IfThenElse` is indexed with `BoolType`, we know
+that evaluating it will give us a boolean. Thus, we can use that
+directly in the Idris if-then-else expression. Let's try this with a few
+expressions:
+
+```Idris
+BinOp Add (IfElse (BoolLit True) (IntLit 6) (IntLit 7)) (BinOp Multiply (IntLit 160) (IntLit 2))
+```
+
+This evaluates `326`, as it should. What if we make the condition non-boolean?
+
+```Idris
+BinOp Add (IfElse (IntLit 1) (IntLit 6) (IntLit 7)) (BinOp Multiply (IntLit 160) (IntLit 2))
+```
+
+Our typechecker catches this, and we end up with the following output:
+
+```
+Type error: Not a boolean.
+```
+
+Alright, let's make one of the branches of the if-expression be a boolean, while the
+other remains an integer.
+
+```Idris
+BinOp Add (IfElse (BoolLit True) (BoolLit True) (IntLit 7)) (BinOp Multiply (IntLit 160) (IntLit 2))
+```
+
+Our typechecker catches this, too:
+
+```
+Type error: Incompatible branch types.
+```
+
+### Conclusion
+I think this is a good approach. Should we want to add more types to our language, such as tuples,
+lists, and so on, we will be able to extend our `decEq` approach to construct more complex equality
+proofs, and keep the `typecheck` method the same. Had we not used this approach,
+and instead decided to pattern match on types inside of `typecheck`, we would've quickly
+found that this only works for types with finitely many values. When we add polymorphic tuples
+and lists, we start being able to construct an arbitrary number of types: `[a]`. `[[a]]`, and
+so on. Then, we cease to be able to enumerate all possible pairs of types, and require a recursive
+solution. I think that this leads us back to `decEq`.
+
+I also hope that I've now redeemed myself as far as logical arguments go. We used dependent types
+and made our typechecking function save us from error-checking during evaluation. We did this
+without having to manually create different types of expressions like `ArithExpr` and `BoolExpr`,
+and without having to duplicate any code.
+
+That's all I have for today, thank you for reading! As always, you can check out the
+[full source code for the typechecker and interpreter](https://dev.danilafe.com/Web-Projects/blog-static/src/branch/master/code/typesafe-interpreter/TypesafeIntrV2.idr) on my Git server.

content/blog/typesafe_interpreter_tuples.md

@@ -0,0 +1,217 @@
+---
+title: Meaningfully Typechecking a Language in Idris, With Tuples
+date: 2020-08-11T19:57:26-07:00
+tags: ["Idris"]
+draft: true
+---
+
+Some time ago, I wrote a post titled
+[Meaningfully Typechecking a Language in Idris]({{< relref "typesafe_interpreter.md" >}}).
+I then followed it up with
+[Meaningfully Typechecking a Language in Idris, Revisited]({{< relref "typesafe_interpreter_revisited.md" >}}).
+In these posts, I described a hypothetical
+way of 'typechecking' an expression data type `Expr` into a typesafe form `SafeExpr`.
+A `SafeExpr` can be evaluated without any code to handle type errors,
+since it's by definition impossible to construct ill-typed expressions using
+it. In the first post, we implemented the method only for simple arithmetic
+expressions; in my latter post, we extended this to support `if`-expressions.
+Near the end of the post, I made the following comment:
+
+> When we add polymorphic tuples and lists, we start being able to construct an
+arbitrary number of types: `[a]`. `[[a]]`, and so on. Then, we cease to be able t
+enumerate all possible pairs of types, and require a recursive solution. I think
+that this leads us back to [our method].
+
+Recently, I thought about this some more, and decided that it's rather simple
+to add tuples into our little language. The addition of tuples mean that our
+language will have an infinite number of possible types. We would have
+`Int`, `(Int, Int)`, `((Int, Int), Int)`, and so on. This would make it
+impossible to manually test every possible case in our typechecker,
+but our approach of returning `Dec (a = b)` will work just fine.
+
+### Extending The Syntax
+First, let's extend our existing language with expressions for
+tuples. For simplicity, let's use pairs `(a,b)` instead of general
+`n`-element tuples. This would make typechecking less cumbersome while still
+having the interesting effect of making the number of types in our language
+infinite. We can always represent the 3-element tuple `(a,b,c)` as `((a,b), c)`,
+after all. To be able to extract values from our pairs, we'll add the `fst` and
+`snd` functions into our language, which accept a tuple and return its
+first or second element, respectively.
+
+Our `Expr` data type, which allows ill-typed expressions, ends up as follows:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 31 39 "hl_lines=7 8 9" >}}
+
+I've highlighted the new lines. The additions consist of the `Pair` constructor, which
+represents the tuple expression `(a, b)`, and the `Fst` and `Snd` constructors,
+which represent the `fst e` and `snd e` expressions, respectively. In
+a similar manner, we extend our `SafeExpr` GADT:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 41 49 "hl_lines=7 8 9" >}}
+
+Finally, to provide the `PairType` constructor, we extend the `ExprType` and `repr` functions:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 1 11 "hl_lines=5 11" >}}
+
+### Implementing Equality
+An important part of this change is the extension of the `decEq` function,
+which compares two types for equality. The kind folks over at `#idris` previously
+recommended the use of the `Dec` data type for this purpose. A value of
+type `Dec P`
+{{< sidenote "right" "decideable-note" "is either a proof that \(P\) is true, or a proof that \(P\) is false." >}}
+It's possible that a proposition \(P\) is not provable, and neither is \(\lnot P\).
+It is therefore not possible to construct a value of type <code>Dec P</code> for
+any proposition <code>P</code>. Having a value of type <code>Dec P</code>, then,
+provides us nontrivial information.
+{{< /sidenote >}} Our `decEq` function, given two types `a` and `b`, returns
+`Dec (a = b)`. Thus, it will return either a proof that `a = b` (which we can
+then use to convince the Idris type system that two `SafeExpr` values are,
+in fact, of the same type), or a proof of `a = b -> Void` (which tells
+us that `a` and `b` are definitely not equal). If you're not sure what the deal with `(=)`
+and `Void` is, check out
+[this section]({{< relref "typesafe_interpreter_revisited.md" >}}#curry-howard-correspondence)
+of the previous article.
+
+A lot of the work in implementing `decEq` went into constructing proofs of falsity.
+That is, we needed to explicitly list every case like `decEq IntType BoolType`, and create
+a proof that `IntType` cannot equal `BoolType`. However, here's how we use `decEq` in
+the typechecking function:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV2.idr" 76 78 >}}
+
+We always throw away the proof inequality! So, rather than spending the time
+constructing useless proofs like this, we can just switch `decEq` to return
+a `Maybe (a = b)`. The `Just` case will tell us that the two types are equal
+(and, as before, provide a proof); the `Nothing` case will tell us that
+the two types are _not_ equal, and provide no further information. Let's
+see the implementation of `decEq` now:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 13 23 >}}
+
+Lines 14 through 16 are pretty simple; in this case, we can tell at a glance
+that the two types are equal, and Idris can infer an equality proof in
+the form of `Refl`. We return this proof by writing it in a `Just`.
+Line 23 is the catch-all case for any combination of types we didn't handle.
+Any combination of types we don't handle is invalid, and thus, this case
+returns `Nothing`.
+
+What about lines 17 through 22? This is the case for handling the equality
+of two pair types, `(lt1, lt2)` and `(rt1, rt2)`. The equality of the two
+types depends on the equality of their constituents. That is, if we
+know that `lt1 = rt1` and `lt2 = rt2`, we know that the two pair types
+are also equal. If one of the two equalities doesn't hold, the two
+pairs obviously aren't equal, and thus, we should return `Nothing`.
+This should remind us of `Maybe`'s monadic nature: we can first compute
+`decEq lt1 rt1`, and then, if it succeeds, compute `decEq lt2 rt2`.
+If both succeed, we will have in hand the two proofs, `lt1 = rt1`
+and `lt2 = rt2`. We achieve this effect using `do`-notation,
+storing the sub-proofs into `subEq1` and `subEq2`.
+
+What now? Once again, we have to use `replace`. Recall its type:
+
+```Idris
+replace : {a:_} -> {x:_} -> {y:_} -> {P : a -> Type} -> x = y -> P x -> P y
+```
+
+Given some proposition in terms of `a`, and knowing that `a = b`, `replace`
+returns the original proposition, but now in terms of `b`. We know for sure
+that:
+
+```Idris
+PairType lt1 lt2 = PairType lt1 lt2
+```
+
+We can start from there. Let's handle one thing at a time, and try
+to replace the second `lt1` with `rt1`. Then, we can replace the second
+`lt2` with `rt2`, and we'll have our equality!
+
+Easier said than done, though. How do we tell Idris which `lt1`
+we want to substitute? After all, of the following are possible:
+
+```Idris
+PairType rt1 lt2 = PairType lt1 lt2 -- First lt1 replaced
+PairType lt1 lt2 = PairType rt1 lt2 -- Second lt1 replaced
+PairType rt1 lt2 = PairType rt1 lt2 -- Both replaced
+```
+
+The key is in the signature, specifically the expressions `P x` and `P y`.
+We can think of `P` as a function, and of `replace` as creating a value
+of `P` applied to another argument. Thus, the substitution will occur
+exactly where the argument of `P` is used. Then, to achieve each
+of the above substitution, we can write `P` as follows:
+
+```Idris {linenos=table, hl_lines=[2]}
+t1 => PairType t1 lt2 = PairType lt1 lt2
+t1 => PairType lt1 lt2 = PairType t1 lt2
+t1 => PairType t1 lt2 = PairType t1 lt2
+```
+
+The second function (highlighted) is the one we'll need to use to attain
+the desired result. Since `P` is an implicit argument to `replace`,
+we can explicitly provide it with `{P=...}`, leading to the following
+line:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 20 20>}}
+
+We now have a proof of the following proposition:
+
+```Idris
+PairType lt1 lt2 = PairType rt1 lt2
+```
+
+We want to replace the second `lt2` with `rt2`, which means that we
+write our `P` as follows:
+
+```Idris
+t2 => PairType lt1 lt2 = PairType rt1 t2
+```
+
+Finally, we perform the second replacement, and return the result:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 21 22 >}}
+
+Great! We have finished implement `decEq`.
+
+### Adjusting The Typechecker
+It's time to make our typechecker work with tuples.
+First, we need to fix the `IfElse` case to accept `Maybe (a=b)` instead
+of `Dec (a=b)`:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 71 78 "hl_lines=7 8" >}}
+
+Note that the only change is from `Dec` to `Maybe`; we didn't need to add new cases
+or even to know what sort of types are available in the language.
+
+Next, we can write the cases for the new expressions in our language. We can
+start with `Pair`, which, given expressions of types `a` and `b`, creates
+an expression of type `(a,b)`. As long as the arguments to `Pair` are well-typed,
+so is the `Pair` expression itself; thus, there are no errors to handle.
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 79 83 >}}
+
+The case for `Fst` is more complicated. If the argument to `Fst` is a tuple
+of type `(a, b)`, then `Fst` constructs from it an expression
+of type `a`. Otherwise, the expression is ill-typed, and we return an error.
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 84 89 >}}
+
+The case for `Snd` is very similar:
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 90 96 >}}
+
+### Evaluation Function and Conclusion
+We conclude with our final `eval` and `resultStr` functions,
+which now look as follows.
+
+{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 97 111 "hl_lines=7-9 13-15" >}}
+
+As you can see, we require no error handling in `eval`; the expressions returned by
+`typecheck` are guaranteed to evaluate to valid Idris values. We have achieved our goal,
+with very little changes to `typecheck` other than the addition of new language
+constructs. In my opinion, this is a win!
+
+As always, you can see the code on my Git server. Here's
+[the latest Idris file,](https://dev.danilafe.com/Web-Projects/blog-static/src/branch/master/code/typesafe-interpreter/TypesafeIntrV3.idr)
+if you want to check it out (and maybe verify that it compiles). I hope you found
+this interesting!

themes/vanilla/assets/scss/code.scss

@@ -0,0 +1,78 @@
+@import "variables.scss";
+
+$code-color-lineno: grey;
+$code-color-keyword: black;
+$code-color-type: black;
+$code-color-comment: grey;
+
+code {
+  font-family: $font-code;
+  background-color: $code-color;
+  border: $code-border;
+  padding: 0 0.25rem 0 0.25rem;
+}
+
+pre code {
+    display: block;
+    box-sizing: border-box;
+    padding: 0.5rem;
+    overflow: auto;
+}
+
+.chroma {
+    .lntable {
+        border-spacing: 0;
+        padding: 0.5rem 0 0.5rem 0;
+        background-color: $code-color;
+        border-radius: 0;
+        border: $code-border;
+        display: block;
+        overflow: auto;
+
+        td {
+            padding: 0;
+        }
+
+        code {
+            border: none;
+            padding: 0;
+        }
+
+        pre {
+            margin: 0;
+        }
+
+        .lntd:last-child {
+            width: 100%;
+        }
+    }
+
+    .lntr {
+        display: table-row;
+    }
+
+    .lnt {
+        display: block;
+        padding: 0 1rem 0 1rem;
+        color: $code-color-lineno;
+    }
+
+    .hl {
+        display: block;
+        background-color: #fffd99; 
+    }
+}
+
+.kr, .k {
+    font-weight: bold;
+    color: $code-color-keyword;
+}
+
+.kt {
+    font-weight: bold;
+    color: $code-color-type;
+}
+
+.c, .c1 {
+    color: $code-color-comment;   
+}

themes/vanilla/assets/scss/margin.scss

@@ -2,20 +2,21 @@
 @import "mixins.scss";
 
 $margin-width: 30rem;
-$margin-offset: 1.5rem;
+$margin-inner-offset: 0.5rem;
+$margin-outer-offset: 1rem;
 
 @mixin below-two-margins {
     @media screen and
-        (max-width: $container-width +
-            2 * ($margin-width + 2 * $margin-offset)) {
+        (max-width: $container-width-threshold +
+            2 * ($margin-width + $margin-inner-offset + $margin-outer-offset)) {
         @content;
     }
 }
 
 @mixin below-one-margin {
     @media screen and
-        (max-width: $container-width +
-            ($margin-width + 3 * $margin-offset)) {
+        (max-width: $container-width-threshold +
+            ($margin-width + $margin-inner-offset + $margin-outer-offset)) {
         @content;
     }
 }
@@ -29,10 +30,18 @@ $margin-offset: 1.5rem;
 
 @mixin margin-content-left {
     left: 0;
-    margin-left: -($margin-width + $margin-offset);
+    margin-left: -($margin-width + $container-min-padding + $margin-inner-offset);
+
+    @include below-two-margins {
+        display: none;
+    }
 }
 
 @mixin margin-content-right {
     right: 0;
-    margin-right: -($margin-width + $margin-offset);
+    margin-right: -($margin-width + $container-min-padding + $margin-inner-offset);
+
+    @include below-one-margin {
+        display: none;
+    }
 }

themes/vanilla/assets/scss/mixins.scss

@@ -6,7 +6,7 @@
 }
 
 @mixin below-container-width {
-    @media screen and (max-width: $container-width){
+    @media screen and (max-width: $container-width-threshold){
         @content;
     }
 }

themes/vanilla/assets/scss/sidenotes.scss

@@ -2,8 +2,6 @@
 @import "mixins.scss";
 @import "margin.scss";
 
-$sidenote-width: 30rem;
-$sidenote-offset: 1.5rem;
 $sidenote-padding: 1rem;
 $sidenote-highlight-border-width: .2rem;
 
@@ -56,7 +54,6 @@ $sidenote-highlight-border-width: .2rem;
     margin-top: 1rem;
     margin-bottom: 1rem;
     width: 100%;
-    display: none;
 
     .sidenote-checkbox:checked ~ & {
         display: block;
@@ -71,10 +68,6 @@ $sidenote-highlight-border-width: .2rem;
 }
 
 @include below-one-margin {
-    .post-content {
-        max-width: 100%;
-    }
-
     .sidenote-content.sidenote-right {
         @include hidden-sidenote;
         margin-right: 0rem;

themes/vanilla/assets/scss/style.scss

@@ -1,6 +1,7 @@
 @import "variables.scss";
 @import "mixins.scss";
 @import "margin.scss";
+@import "toc.scss";
 
 body {
   font-family: $font-body;
@@ -30,22 +31,6 @@ h1, h2, h3, h4, h5, h6 {
   }
 }
 
-code {
-  font-family: $font-code;
-  background-color: $code-color;
-}
-
-pre code {
-    display: block;
-    padding: 0.5rem;
-    overflow-x: auto;
-    background-color: $code-color;
-}
-
-div.highlight table pre {
-    margin: 0;
-}
-
 .container {
   position: relative;
   margin: auto;
@@ -54,15 +39,17 @@ div.highlight table pre {
   box-sizing: border-box;
 
   @include below-container-width {
-      padding: 0rem 1rem 0rem 1rem;
+      padding: 0 $container-min-padding 0 $container-min-padding;
+      margin: 0;
+      max-width: $container-width + 2 * $container-min-padding;
   }
 
   @include below-two-margins {
-      left: -($margin-width + $margin-offset)/2;
+      left: -($margin-width + $margin-inner-offset + $margin-outer-offset)/2;
   }
 
   @include below-one-margin {
-      position: initial;
+      left: 0;
   }
 }
 
@@ -71,8 +58,7 @@ div.highlight table pre {
   background-color: $primary-color;
   border: none;
   color: white;
-  transition: color 0.25s;
-  transition: background-color 0.25s;
+  transition: color 0.25s, background-color 0.25s;
   text-align: left;
 
   &:focus {
@@ -230,4 +216,20 @@ figure {
     figcaption {
         text-align: center;
     }
+
+    &.tiny img {
+        max-height: 15rem;
+    }
+
+    &.small img {
+        max-height: 20rem;
+    }
+
+    &.medium img {
+        max-height: 30rem;
+    }
+}
+
+.twitter-tweet {
+    margin: auto;
 }

themes/vanilla/assets/scss/toc.scss

@@ -0,0 +1,49 @@
+@import "variables.scss";
+@import "mixins.scss";
+
+$toc-color: $code-color;
+$toc-border-color: $code-border-color;
+
+.table-of-contents {
+    @include margin-content;
+    @include margin-content-left;
+    display: flex;
+    flex-direction: column;
+    align-items: end;
+    margin-bottom: 1rem;
+
+    em {
+        font-style: normal;
+        font-weight: bold;
+        font-size: 1.2em;
+        display: block;
+        margin-bottom: 0.5rem;
+    }
+
+    #TableOfContents > ul {
+        padding-left: 0;
+    }
+
+    nav {
+        margin: 0px;
+    }
+
+    ul {
+        list-style: none;
+        padding-left: 2rem;
+        margin: 0px;
+    }
+
+    a {
+        padding: 0;
+    }
+
+    div.wrapper {
+        @include bordered-block;
+        padding: 1rem;
+        background-color: $toc-color;
+        border-color: $toc-border-color;
+        box-sizing: border-box;
+        max-width: 100%;
+    }
+}

themes/vanilla/assets/scss/variables.scss

@@ -1,14 +1,16 @@
 $container-width: 45rem;
+$container-min-padding: 1rem;
+$container-width-threshold: $container-width + 2 * $container-min-padding;
 $standard-border-width: .075rem;
 
 $primary-color: #36e281;
-$primary-color-dark: darken($primary-color, 10%);
-$code-color: #f0f0f0;
-$code-color-dark: darken($code-color, 10%);
 $border-color: #bfbfbf;
+$code-color: #f0f0f0;
+$code-border-color: darken($code-color, 10%);
 
 $font-heading: "Lora", serif;
 $font-body: "Raleway", serif;
 $font-code: "Inconsolata", monospace;
 
 $standard-border: $standard-border-width solid $border-color;
+$code-border: $standard-border-width solid $code-border-color;

themes/vanilla/layouts/blog/single.html

@@ -10,6 +10,14 @@
 </div>
 
 <div class="post-content">
+    {{ if not (eq .TableOfContents "<nav id=\"TableOfContents\"></nav>") }}
+    <div class="table-of-contents">
+        <div class="wrapper">
+            <em>Table of Contents</em>
+            {{ .TableOfContents }}
+        </div>
+    </div>
+    {{ end }}
     {{ .Content }}
 </div>
 {{ end }}

themes/vanilla/layouts/partials/head.html

@@ -6,14 +6,16 @@
     <meta name="description" content="{{ .Description }}">
     {{ end }}
 
-    <link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Inconsolata&family=Raleway&family=Lora&display=block" media="screen"> 
+    <link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Inconsolata:wght@400;700&family=Raleway&family=Lora&display=block" media="screen"> 
     <link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/normalize/5.0.0/normalize.min.css" media="screen">
     {{ $style := resources.Get "scss/style.scss" | resources.ToCSS | resources.Minify }}
     {{ $sidenotes := resources.Get "scss/sidenotes.scss" | resources.ToCSS | resources.Minify }}
+    {{ $code := resources.Get "scss/code.scss" | resources.ToCSS | resources.Minify }}
     {{ $icon := resources.Get "img/favicon.png" }}
     {{- partial "sidenotes.html" . -}}
     <link rel="stylesheet" href="{{ $style.Permalink }}" media="screen">
     <link rel="stylesheet" href="{{ $sidenotes.Permalink }}" media="screen">
+    <link rel="stylesheet" href="{{ $code.Permalink }}" media="screen">
     <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.11.1/dist/katex.min.css" integrity="sha384-zB1R0rpPzHqg7Kpt0Aljp8JPLqbXI3bhnPWROx27a9N0Ll6ZP/+DiW/UqRcLbRjq" crossorigin="anonymous" media="screen">
     <link rel="icon" type="image/png" href="{{ $icon.Permalink }}">
 

themes/vanilla/layouts/partials/header.html

@@ -4,8 +4,9 @@
 <nav>
     <div class="container">
         <a href="/">Home</a>
-        <a href="https://github.com/DanilaFe">GitHub</a>
         <a href="/about">About</a>
+        <a href="https://github.com/DanilaFe">GitHub</a>
+        <a href="/Resume-Danila-Fedorin.pdf">Resume</a>
         <a href="/tags">Tags</a>
         <a href="/blog">All Posts</a>
     </div>

themes/vanilla/layouts/shortcodes/codelines.html

@@ -6,4 +6,9 @@
 {{ .Scratch.Set "u" $t }}
 {{ end }}
 {{ $v := first (add (sub (int (.Get 3)) (int (.Get 2))) 1) (.Scratch.Get "u") }}
-{{ highlight (delimit $v "\n") (.Get 0) (printf "linenos=table,linenostart=%d" (.Get 2)) }}
+{{ if (.Get 4) }}
+{{ .Scratch.Set "opts" (printf ",%s" (.Get 4)) }}
+{{ else }}
+{{ .Scratch.Set "opts" "" }}
+{{ end }}
+{{ highlight (delimit $v "\n") (.Get 0) (printf "linenos=table,linenostart=%d%s" (.Get 2) (.Scratch.Get "opts")) }}