Add links in part 0 of compiler series.

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

@@ -18,15 +18,15 @@ boolStringImpossible : BoolType = StringType -> Void
 boolStringImpossible Refl impossible
 
 decEq : (a : ExprType) -> (b : ExprType) -> Dec (a = b)
-decEq {a = IntType} {b = IntType} = Yes Refl
-decEq {a = BoolType} {b = BoolType} = Yes Refl
-decEq {a = StringType} {b = StringType} = Yes Refl
-decEq {a = IntType} {b = BoolType} = No intBoolImpossible
-decEq {a = BoolType} {b = IntType} = No $ intBoolImpossible . sym
-decEq {a = IntType} {b = StringType} = No intStringImpossible
-decEq {a = StringType} {b = IntType} = No $ intStringImpossible . sym
-decEq {a = BoolType} {b = StringType} = No boolStringImpossible 
-decEq {a = StringType} {b = BoolType} = No $ boolStringImpossible . sym
+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
@@ -46,7 +46,7 @@ data SafeExpr : ExprType -> Type where
   BoolLiteral : Bool -> SafeExpr BoolType
   StringLiteral : String -> SafeExpr StringType
   BinOperation : (repr a -> repr b -> repr c) -> SafeExpr a -> SafeExpr b -> SafeExpr c
-  IfThenElse : { t : ExprType } -> SafeExpr BoolType -> SafeExpr t -> SafeExpr t -> SafeExpr t
+  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 ** (+))

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

@@ -1,9 +1,8 @@
-baseURL = "https://danilafe.com"
-languageCode = "en-us"
+languageCode = "en"
 title = "Daniel's Blog"
 theme = "vanilla"
 pygmentsCodeFences = true
-pygmentsStyle = "github"
+pygmentsUseClasses = true
 summaryLength = 20
 
 [markup]
@@ -11,3 +10,9 @@ summaryLength = 20
     endLevel = 4
     ordered = false
     startLevel = 3
+
+[languages]
+  [languages.en]
+    baseURL = "https://danilafe.com"
+  [languages.ru]
+    baseURL = "https://ru.danilafe.com"

content/_index.ru.md

@@ -0,0 +1,8 @@
+---
+title: Daniel's Blog
+description: Персональный блог Данилы Федорина о функциональном программировании, дизайне компиляторов, и многом другом! 
+---
+## Привет!
+Добро пожаловать на мой сайт. Здесь, я пишу на многие темы, включая фунциональное программирование, дизайн компилляторов, теорию языков программирования, и иногда компьютерные игры. Я надеюсь, что здесь вы найдете что-нибуть интересное!
+
+Вы читаете русскою версию моего сайта. Я только недавно занялся его переводом, и до этого времени редко писал на русском. Я заранеее извиняюсь за присутствие орфографических или грамматических ошибок.

content/blog/00_compiler_intro.ru.md

@@ -0,0 +1,97 @@
+---
+title: Пишем Компилятор Для Функционального Языка на С++, Часть 0 - Вступление
+date: 2019-08-03T01:02:30-07:00
+tags: ["C and C++", "Functional Languages", "Compilers"]
+description: "todo"
+---
+Год назад, я был записан на курс по компиляторам. Я ждал этого момента почти два учебных года: еще со времени школы меня интересовало создание языков программирования. Однако я был разочарован - заданный нам финальный проект полностью состоял из склеивания вместе написанных профессором кусочков кода. Склеив себе такой грустный компилятор, я не почувствовал бы никакой гордости. А я хотел бы гордиться всеми своими проектами.
+
+Вместо стандартного задания, я решил -- с разрешением профессора -- написать компилятор для ленивого функционального языка, используя отличную книгу Саймона Пейтона Джоунса, _Implementing functional languages: a tutorial_. На курсе мы пользовались С++, и мой проект не был исключением. Получился прикольный маленький язык, и теперь я хочу рассказать вам, как вы тоже можете создать ваш собственный функциональный язык.  
+
+### Примечание к Русской Версии
+Вы читаете русскою версию этой статьи. Оригинал ее был написан год назад, и с тех пор объем всей серии немного изменился. Я планировал описать только те части компилятора, которые я успел закончить и сдать профессору: лексический анализ, синтаксический разбор, мономорфную проверку типов, и компиляцию простых выражений с помощью LLVM. Закончив и описав все эти части, я решил продолжать разрабатывать компилятор, и описал сборку мусора, полиморфную проверку типов, полиморфные структуры данных, а также компиляцию более сложных выражений. Вместо того чтобы писать наивный перевод английской версии -- притворяясь что я не знаю о перемене моих планов -- я буду вносить в эту версию изменения соответствующие сегодняшнему состоянию компилятора. Части статей не затронутые этими изменениями я тоже не буду переводить слово в слово, иначе они будут звучать ненатурально. Тем не менее техническое содержание каждой статьи будет аналогично содержанию ее английской версии, и код будет тот же самый. 
+
+### Мотивация
+Начать эту серию меня подтолкнули две причины. 
+
+Во-первых, почти все учебники и вступления к созданию компиляторов, с которыми я сталкивался, были написаны об императивных языках, часто похожих на C, C++, Python, или JavaScript. Я считаю, что в компиляции функциональных языков -- особенно ленивых -- есть много чего интересного, и все это относительно редко упоминается. 
+
+Во-вторых, меня вдохновили книги, как Software Foundations. Все содержание Software Foundations, например, написано в форме комментариев языка Coq. Таким образом, можно не только читать саму книгу, но и сразу же запускать находящийся рядом с комментариями код. Когда описываемый код под рукой, легче экспериментировать и интереснее читать. Принимая это во внимание, я выкладываю вместе с каждой статьей соответствующую версию компилятора; в самой статье описывается код именно из этой версии. Все части написанной мною программы полностью доступны. 
+
+### Обзор
+Прежде чем начинать наш проект, давайте обсудим, чего мы будем добиваться, и какими способами. 
+
+#### “Классические” Стадии Компилятора
+Части большинства компиляторов достаточно независимы друг от друга (по крайней мере в теории). Мы можем разделить их на следующие шаги:
+
+* Лексический анализ
+* Синтаксический разбор
+* Анализ и оптимизация
+* Генерация кода
+
+Не все вышеописанные шаги встречаются в каждом компиляторе. Например, компилятор в моих статьях совсем не оптимизирует код. Также, в некоторых компиляторах присутствуют шаги не упомянутые в этом списке. Язык Idris  -- как и многие другие функциональные языки -- переводится сначала в упрощённый язык “TT”, и только после этого проходит через анализ. Иногда, с целью ускорить компиляцию, несколько шагов производятся одновременно. В целом, все эти стадии помогут нам сориентироваться, но никаким образом нас не ограничат.  
+
+#### Темы, Которые Мы Рассмотрим
+Мы начнем с нуля, и пошагово построим компилятор состоящий из следующих частей:
+
+* Лексического анализа с помощью программы Flex.
+* Синтаксического разбора с помощью программы Bison.
+* Сначала мономорфной, а позже полиморфной проверки типов.
+* Вычисления программ используя абстрактную машину G-machine.
+* Компиляции абстрактных инструкций G-machine используя LLVM.
+* Простого сбора мусора.
+
+Наша цель - создать ленивый, функциональный язык.
+
+#### Темы, Которые Мы Не Рассмотрим
+Для того, чтобы создать любую нетривиальную программу, нужно иметь значительный объем опыта и знаний; одному человеку было бы сложно научить всему этому. У меня буквально не хватило бы на это времени, да и исход такой попытки был бы неблагоприятным: опытным читателям было бы труднее извлечь из статей новую информацию, а неопытным читателям все равно было бы недостаточно подробно. Вместо того, чтобы портить таким образом свои статьи, я буду полагаться на то, что вы достаточно комфортно себя чувствуете с некоторыми темами. В число этих тем входят:
+
+* [Теория алгоритмов](https://ru.wikipedia.org/wiki/%D0%A2%D0%B5%D0%BE%D1%80%D0%B8%D1%8F_%D0%B0%D0%BB%D0%B3%D0%BE%D1%80%D0%B8%D1%82%D0%BC%D0%BE%D0%B2),
+более конкретно [теория автоматов](https://ru.wikipedia.org/wiki/%D0%A2%D0%B5%D0%BE%D1%80%D0%B8%D1%8F_%D0%B0%D0%B2%D1%82%D0%BE%D0%BC%D0%B0%D1%82%D0%BE%D0%B2).
+Детерминированные и недетерминированные автоматы кратко упоминаются в первой статье во время лексического анализа, a синтаксический разбор мы выполним используя контекстно-свободную грамматику.
+* [Функциональное программирование](https://ru.wikipedia.org/wiki/%D0%A4%D1%83%D0%BD%D0%BA%D1%86%D0%B8%D0%BE%D0%BD%D0%B0%D0%BB%D1%8C%D0%BD%D0%BE%D0%B5_%D0%BF%D1%80%D0%BE%D0%B3%D1%80%D0%B0%D0%BC%D0%BC%D0%B8%D1%80%D0%BE%D0%B2%D0%B0%D0%BD%D0%B8%D0%B5), с легкой примесью [лямбда-исчисления](https://ru.wikipedia.org/wiki/%D0%9B%D1%8F%D0%BC%D0%B1%D0%B4%D0%B0-%D0%B8%D1%81%D1%87%D0%B8%D1%81%D0%BB%D0%B5%D0%BD%D0%B8%D0%B5).
+Мы будем пользоваться лямбда-функциями, каррированием, и системой типов Хиндли-Мильнер, которая часто встречается в языках семейства ML.
+* С++. Я стараюсь писать код правильно и по последним стандартам, но я не эксперт. Я не буду объяснять синтаксис или правила С++, но разумеется буду описывать что именно делает мой код с точки зрения компиляторов.
+
+#### Синтаксис Нашего Языка
+Саймон Пейтон Джоунс, в одном из своих [~~двух~~ многочисленных](https://www.reddit.com/r/ProgrammingLanguages/comments/dsu115/compiling_a_functional_language_using_c/f6t52mh?utm_source=share&utm_medium=web2x&context=3) трудов на тему функциональных языков, отметил что большинство из этих языков по сути очень похожи друг на друга; часто, главная разница состоит именно в их синтаксисе. На данный момент, выбор синтаксиса - наша главная степень свободы. Нам точно нужно предоставить доступ к следующим вещам:
+
+* Декларациям функций
+* Вызову функций
+* Арифметике
+* Aлгебраическим типам данных
+* Сопоставлению с образцом
+
+Позже, мы добавим к этому списку выражения let/in и лямбда-функции. С арифметикой разобраться не сложно - числа будут писаться просто как `3`, значения выражений как `1+2*3` будут высчитываться по обычным математическим правилам. Вызов функций ненамного сложнее. Выражение `f x` будет значить “вызов функции `f` с параметром `x`”, а `f x + g y` - “сумма значений `f x` и `g y`”. Заметьте, что вызов функций имеет приоритет выше приоритета арифметических операций.
+
+Теперь давайте придумаем синтаксис для деклараций функций. Я предлогаю следующий вариант:
+
+```
+defn f x = { x + x }
+```
+
+А для типов данных:
+
+```
+data List = { Nil, Cons Int List }
+```
+
+Заметьте, что мы пока пользуемся мономорфными декларациями типов данных. Позже, в одиннадцатой части, мы добавим синтаксис для полиморфных деклараций.
+
+В последнюю очередь, давайте определимся с синтаксисом сопоставления с образцом:
+
+```
+case l of {
+    Nil -> { 0 }
+    Cons x xs -> { x }
+}
+```
+
+Представленная выше распечатка читается как: “если лист `l` сопоставим с `Nil`, то все выражение возвращает значение `0`; иначе, если лист сопоставим с `Cons x xs` (что, опираясь на декларацию `List`, означает, что лист состоит из значений `x`, с типом `Int`, и `xs`, с типом `List`), то выражение возвращает `x`”.   
+
+Вот и конец нашего обзора! В следующей статье, мы начнем с лексического анализа, что является первым шагом в процессе трансформации программного текста в исполняемые файлы.
+
+### Список Статей
+* Ой! Тут как-то пусто.
+* Вы, наверно, читаете черновик.
+* Если нет, то пожалуйста напишите мне об этом!

content/blog/backend_math_rendering.md

@@ -1,8 +1,7 @@
 ---
 title: Rendering Mathematics On The Back End
-date: 2020-07-15T15:27:19-07:00
-draft: true
-tags: ["Website", "Nix", "Ruby", "KaTeX", "Hugo"]
+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
@@ -26,7 +25,7 @@ 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 is rendered on the back end.
+* 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__
@@ -90,7 +89,7 @@ to `node2nix`:
 ]
 ```
 
-The Ruby script I wrote for this (more on that soon) required the `nokigiri` gem, which
+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.
 
@@ -105,10 +104,13 @@ page advertises server-side rendering. Their documentation [(link)](https://kate
 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 remain that provide the software that actually takes your HTML page and substitutes
-LaTeX for math.
+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,
@@ -119,8 +121,14 @@ 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. I decided
-to write my own Ruby script to get the job done. From this script, I
+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.
 
@@ -170,13 +178,16 @@ 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! Thus, we can't just
-search-and-replace over the entire HTML document; we need to be context
-aware. This is where `nokigiri` comes in. We parse the HTML, and iterate
+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 is pretty easy to specify thanks to something called XPath.
+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
@@ -211,7 +222,7 @@ All in all:
 //*[not(self::code)]/text()
 ```
 
-Finally, we use this XPath from `nokigiri`:
+Finally, we use this XPath from `nokogiri`:
 
 ```Ruby {linenos=table}
 files = ARGV[0..-1]
@@ -236,7 +247,8 @@ I used Nix for this, but the below script will largely be compatible with a non-
 I came up with the following, commenting on Nix-specific commands:
 
 ```Bash {linenos=table}
-source $stdenv/setup # Nix-specific; set up paths.
+# 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.
@@ -266,7 +278,7 @@ 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 `nokigiri` here, though, and I hope that an equivalently
+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
@@ -275,6 +287,15 @@ 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,

content/blog/dell_is_horrible/index.md

@@ -1,7 +1,6 @@
 ---
 title: DELL Is A Horrible Company And You Should Avoid Them At All Costs
-date: 2020-07-17T16:23:34-07:00
-draft: true
+date: 2020-07-23T13:40:05-07:00
 tags: ["Electronics"]
 ---
 
@@ -230,13 +229,10 @@ 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 from my almost-abandoned account. DELL Support's Twitter
-account quickly responded, eager as always to destroy any semblance of
-transparency by switching to private messages:
-
-{{< tweet 1268064691416334344 >}}
-
-I let them know my thoughts on the matter. I wanted a new machine.
+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." >}}
 
@@ -261,7 +257,8 @@ paper which included my name and email address. I obliged, and quickly got a res
 
 {{< 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. Try to remember that name; you will never hear it again, not in this post.
+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?" >}}

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/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 languages with finitely many types. 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 +1,216 @@
+---
+title: Meaningfully Typechecking a Language in Idris, With Tuples
+date: 2020-08-12T15:48:04-07:00
+tags: ["Idris"]
+---
+
+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/style.scss

@@ -31,31 +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;
-    border: $code-border;
-}
-
-div.highlight table {
-    border: $code-border !important;
-    border-radius: 0px;
-
-    pre {
-        margin: 0;
-    }
-
-    code {
-        border: none;
-    }
-}
-
 .container {
   position: relative;
   margin: auto;
@@ -241,6 +216,18 @@ figure {
     figcaption {
         text-align: center;
     }
+
+    &.tiny img {
+        max-height: 15rem;
+    }
+
+    &.small img {
+        max-height: 20rem;
+    }
+
+    &.medium img {
+        max-height: 30rem;
+    }
 }
 
 .twitter-tweet {

themes/vanilla/i18n/en.toml

@@ -0,0 +1,35 @@
+[Home]
+other = "Home"
+
+[About]
+other = "About"
+
+[Resume]
+other = "Resume"
+
+[Tags]
+other = "Tags"
+
+[RecentPosts]
+other = "Recent posts"
+
+[AllPosts]
+other = "All Posts"
+
+[PostedOn]
+other = "Posted on {{ .Date.Format \"January 2, 2006\" }}."
+
+[TableOfContents]
+other = "Table of Contents"
+
+[ReadingTime]
+other = "{{ .WordCount }} words, about {{ .ReadingTime }} minutes to read."
+
+[Note]
+other = "note"
+
+[Tagged]
+other = "Tagged \"{{ .Title }}\""
+
+[AllTags]
+other = "Below is a list of all the tags ever used on this site."

themes/vanilla/i18n/ru.toml

@@ -0,0 +1,35 @@
+[Home]
+other = "Главная"
+
+[About]
+other = "О Сайте"
+
+[Resume]
+other = "Резюме"
+
+[Tags]
+other = "Метки"
+
+[RecentPosts]
+other = "Недавние статьи"
+
+[AllPosts]
+other = "Все Статьи"
+
+[PostedOn]
+other = "Статья опубликована {{ .Date.Format \"January 2, 2006\" }}."
+
+[TableOfContents]
+other = "Оглавление"
+
+[ReadingTime]
+other = "{{ .WordCount }} слов, чтение займет примерно {{ .ReadingTime }} минут."
+
+[Note]
+other = "примечание"
+
+[Tagged]
+other = "Статьи c меткой \"{{ .Title }}\""
+
+[AllTags]
+other = "Ниже приводится список всех меток на сайте."

themes/vanilla/layouts/blog/single.html

@@ -6,14 +6,14 @@
         <a class="button" href="{{ $.Site.BaseURL }}/tags/{{ . | urlize }}">{{ . }}</a>
         {{ end }}
     </p>
-    <p>Posted on {{ .Date.Format "January 2, 2006" }}.</p>
+    <p>{{ i18n "PostedOn" . }}</p>
 </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>
+            <em>{{ i18n "TableOfContents" }}</em>
             {{ .TableOfContents }}
         </div>
     </div>

themes/vanilla/layouts/index.html

@@ -1,7 +1,7 @@
 {{ define "main" }}
 {{ .Content }}
 
-Recent posts:
+{{ i18n "RecentPosts" }}:
 <ul class="post-list">
     {{ range first 10 (where (where .Site.Pages.ByDate.Reverse "Section" "blog") ".Kind" "!=" "section") }}
     {{ partial "post.html" . }}

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

@@ -3,11 +3,11 @@
 </div>
 <nav>
     <div class="container">
-        <a href="/">Home</a>
-        <a href="/about">About</a>
+        <a href="/">{{ i18n "Home" }}</a>
+        <a href="/about">{{ i18n "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>
+        <a href="/Resume-Danila-Fedorin.pdf">{{ i18n "Resume" }}</a>
+        <a href="/tags">{{ i18n "Tags" }}</a>
+        <a href="/blog">{{ i18n "AllPosts" }}</a>
     </div>
 </nav>

themes/vanilla/layouts/partials/post.html

@@ -1,5 +1,5 @@
 <li>
     <a href="{{ .Permalink }}" class="post-title">{{ .Title }}</a>
-    <p class="post-wordcount">{{ .WordCount }} words, about {{ .ReadingTime }} minutes to read.</p>
+    <p class="post-wordcount">{{ i18n "ReadingTime" . }}</p>
     <p class="post-preview">{{ .Summary }} . . .</p>
 </li>

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")) }}

themes/vanilla/layouts/shortcodes/numberedsidenote.html

@@ -4,7 +4,7 @@
 <label class="sidenote-label" for="numbernote-{{ $id }}">({{ $id }})</label>
 <input class="sidenote-checkbox" type="checkbox" id="numbernote-{{ $id }}"></input>
 <span class="sidenote-content sidenote-{{ .Get 0 }}">
-<span class="sidenote-delimiter">[note:</span>
+<span class="sidenote-delimiter">[{{ i18n "note" }}:</span>
 {{ .Inner }}
 <span class="sidenote-delimiter">]</span>
 </span>

themes/vanilla/layouts/shortcodes/sidenote.html

@@ -2,7 +2,7 @@
 <label class="sidenote-label" for="{{ .Get 1 }}">{{ .Get 2 }}</label>
 <input class="sidenote-checkbox" type="checkbox" id="{{ .Get 1 }}"></input>
 <span class="sidenote-content sidenote-{{ .Get 0 }}">
-<span class="sidenote-delimiter">[note:</span>
+<span class="sidenote-delimiter">[{{ i18n "Note" }}:</span>
 {{ .Inner }}
 <span class="sidenote-delimiter">]</span>
 </span>

themes/vanilla/layouts/tags/list.html

@@ -1,5 +1,5 @@
 {{ define "main" }}
-<h2>Tagged "{{ .Title }}"</h2>
+<h2>{{ i18n "Tagged" . }}</h2>
 
 <ul class="post-list">
     {{ range .Pages.ByDate.Reverse }}

themes/vanilla/layouts/tags/terms.html

@@ -1,6 +1,6 @@
 {{ define "main" }}
-<h2>{{ .Title }}</h2>
-Below is a list of all the tags ever used on this site.
+<h2>{{ i18n "Tags" }}</h2>
+{{ i18n "AllTags" }}
 
 <ul>
     {{ range sort .Pages "Title" }}