---
title: Meaningfully Typechecking a Language in Idris, Revisited
date: 2020-07-22T14:37:35-07:00
tags: ["Idris", "Programming Languages"]
series: "Meaningfully Typechecking a Language in Idris"
favorite: true
---

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.
