/Writing a simple evaluator and type-checker in Haskell

## Writing a simple evaluator and type-checker in Haskell

This tutorial serves as a very short and quick summary of the first few chapters of TAPL. Huge thanks to the ##dependent@freenode community (pgiarrusso, lyxia, mietek, …) for clearing all my questions, which provided a good sense of the design of such systems.

## 1.1. Syntax

Our syntax, per BNF is defined as follows:

```<term>  ::= <bool> | <num> | If <bool> Then <expr> Else <expr> | <arith>
<bool>  ::= T | F | IsZero <num>
<num>   ::= O
<arith> ::= Succ <num> | Pred <num>
```

For simplicity, we merge all them together in a single `Term`.

```data Term =
T
| F
| O
| IfThenElse Term Term Term
| Succ Term
| Pred Term
| IsZero Term
deriving (Show, Eq)
```

## 1.2. Inference rules (evaluator)

The semantics we use here are based on so called small-step style, which state how a term is rewritten to a specific value, written $t to v$. In contrast, big-step style states how a specific term evaluates to a final value, written $t Downarrow v$.

Evaluation of a term is just pattern matching the inference rules. Given a term, it should produce a term:

```eval :: Term -> Term
```

The list of inference rules:

As an example, the rule E-IfTrue written using big-step semantics would be $fract_1 Downarrow textT, t2 Downarrow vtextIf T text Then t_2 text Else t_3 Downarrow t_2$.

Given the rules, by pattern matching them we will reduce terms. Implementation in Haskell is mostly “copy-paste” according to the rules:

```eval (IfThenElse T t2 t3) = t2
eval (IfThenElse F t2 t3) = t3
eval (IfThenElse t1 t2 t3) = let t' = eval t1 in IfThenElse t' t2 t3
eval (Succ t1) = let t' = eval t1 in Succ t'
eval (Pred O) = O
eval (Pred (Succ k)) = k
eval (Pred t1) = let t' = eval t1 in Pred t'
eval (IsZero O) = T
eval (IsZero (Succ t)) = F
eval (IsZero t1) = let t' = eval t1 in IsZero t'
eval _ = error "No rule applies"
```

## 1.3. Conclusion

As an example, evaluating the following:

```Main> eval \$ Pred \$ Succ \$ Pred O
Pred O
```

Corresponds to the following inference rules:

```             ----------- E-PredZero
pred O -> O
----------------------- E-Succ
succ (pred O) -> succ O
------------------------------------- E-Pred
pred (succ (pred O)) -> pred (succ O)
```

However, if we do:

```Main> eval \$ IfThenElse O O O
*** Exception: No rule applies
```

It’s type-checking time!

## 2.1. Type syntax

In addition to the previous syntax, we create a new one for types, so per BNF it’s defined as follows:

```<type> ::= Bool | Nat
```

```data Type =
TBool
| TNat
```

## 2.2. Inference rules (type)

Getting a type of a term expects a term, and either returns an error or the type derived:

```typeOf :: Term -> Either String Type
```

Next step is to specify the typing rules.

```typeOf T = Right TBool
typeOf F = Right TBool
typeOf O = Right TNat
typeOf (IfThenElse t1 t2 t3) =
case typeOf t1 of
Right TBool ->
let t2' = typeOf t2
t3' = typeOf t3 in
if t2' == t3'
then t2'
else Left "Types mismatch"
_ -> Left "Unsupported type for IfThenElse"
typeOf (Succ k) =
case typeOf k of
Right TNat -> Right TNat
_ -> Left "Unsupported type for Succ"
typeOf (Pred k) =
case typeOf k of
Right TNat -> Right TNat
_ -> Left "Unsupported type for Pred"
typeOf (IsZero k) =
case typeOf k of
Right TNat -> Right TBool
_ -> Left "Unsupported type for IsZero"
```

## 2.3. Conclusion

Going back to the previous example, we can now “safely” evaluate (by type-checking first), depending on type-check results:

```Main> typeOf \$ IfThenElse O O O
Left "Unsupported type for IfThenElse"
Main> typeOf \$ IfThenElse T O (Succ O)
Right TNat
Main> typeOf \$ IfThenElse F O (Succ O)
Right TNat
Main> eval \$ IfThenElse T O (Succ O)
O
Main> eval \$ IfThenElse F O (Succ O)
Succ O
```

Our neat language supports evaluation and type checking, but does not allow for defining constants. To do that, we will need kind of an environment which will hold information about constants.

```type TyEnv = [(String, Type)] -- Type env
type TeEnv = [(String, Term)] -- Term env
```

We also extend our data type to contain `TVar` for defining variables, and meanwhile also introduce the `Let ... in ...` syntax:

```data Term =
...
| TVar String
| Let String Term Term
```

Here are the rules for variables:

 Name Rule Add binding $fracGamma, a text : TGamma vdash a text : T$ Retrieve binding $fraca text : T in GammaGamma vdash a text : T$

```addType :: String -> Type -> TyEnv -> TyEnv
addType varname b env = (varname, b) : env

getTypeFromEnv :: TyEnv -> String -> Maybe Type
getTypeFromEnv [] _ = Nothing
getTypeFromEnv ((varname', b) : env) varname = if varname' == varname then Just b else getTypeFromEnv env varname
```

Analogously for terms:

```addTerm :: String -> Term -> TeEnv -> TeEnv
addTerm varname b env = (varname, b) : env

getTermFromEnv :: TeEnv -> String -> Maybe Term
getTermFromEnv [] _ = Nothing
getTermFromEnv ((varname', b) : env) varname = if varname' == varname then Just b else getTermFromEnv env varname
```

## 3.1. Inference rules (evaluator)

At this point we stop giving mathematical inference rules, but it’s a good homework if you want to do it 🙂

`eval'` is exactly the same as `eval`, with the following additions:
1. New parameter (the environment) to support retrieval of values for constants
2. Pattern matching for the new `Let ... in ...` syntax

```eval' :: TeEnv -> Term -> Term
eval' env (TVar v) = case getTermFromEnv env v of
Just ty -> ty
_       -> error "No var found in env"
eval' env (Let v t t') = eval' (addTerm v (eval' env t) env) t'
```

We will modify `IfThenElse` slightly to allow for evaluating variables:

```> eval' env (IfThenElse T t2 t3) = eval' env t2
> eval' env (IfThenElse F t2 t3) = eval' env t3
> eval' env (IfThenElse t1 t2 t3) = let t' = eval' env t1 in IfThenElse t' t2 t3
```

Copy-pasting the others:

```eval' env (Succ t1) = let t' = eval' env t1 in Succ t'
eval' _ (Pred O) = O
eval' _ (Pred (Succ k)) = k
eval' env (Pred t1) = let t' = eval' env t1 in Pred t'
eval' _ (IsZero O) = T
eval' _ (IsZero (Succ t)) = F
eval' env (IsZero t1) = let t' = eval' env t1 in IsZero t'
```

Also since we modified `IfThenElse` to recursively evaluate, we also need to consider base types:

```eval' _ T = T
eval' _ F = F
eval' _ O = O
```

## 3.2. Inference rules (type)

`typeOf'` is exactly the same as `typeOf`, with the only addition to support env (for retrieval of types for constants in an env) and the new let syntax.

```typeOf' :: TyEnv -> Term -> Either String Type
typeOf' env (TVar v) = case getTypeFromEnv env v of
Just ty -> Right ty
_       -> Left "No type found in env"
typeOf' _ T = Right TBool
typeOf' _ F = Right TBool
typeOf' _ O = Right TNat
typeOf' env (IfThenElse t1 t2 t3) =
case typeOf' env t1 of
Right TBool ->
let t2' = typeOf' env t2
t3' = typeOf' env t3 in
if t2' == t3'
then t2'
else Left \$ "Type mismatch between " ++ show t2' ++ " and " ++ show t3'
_ -> Left "Unsupported type for IfThenElse"
typeOf' env (Succ k) =
case typeOf' env k of
Right TNat -> Right TNat
_ -> Left "Unsupported type for Succ"
typeOf' env (Pred k) =
case typeOf' env k of
Right TNat -> Right TNat
_ -> Left "Unsupported type for Pred"
typeOf' env (IsZero k) =
case typeOf' env k of
Right TNat -> Right TBool
_ -> Left "Unsupported type for IsZero"
typeOf' env (Let v t t') = case typeOf' env t of
Right ty -> typeOf' (addType v ty env) t'
_        -> Left "Unsupported type for Let"
```

Some examples:

```Main> let termEnv = addTerm "a" O \$ addTerm "b" (Succ O) \$ addTerm "c" F []
Main> let e = IfThenElse T (TVar "a") (TVar "b") in (eval' termEnv e, typeOf' typeEnv e)
(O,Right TNat)
Main> let e = IfThenElse T (TVar "a") (TVar "c") in (eval' termEnv e, typeOf' typeEnv e)
(O,Left "Type mismatch between Right TNat and Right TBool")
Main> let e = IfThenElse T F (TVar "c") in (eval' termEnv e, typeOf' typeEnv e)
(F,Right TBool)
Main> let e = (Let "y" (TVar "a") (Succ (TVar "y"))) in eval' e termEnv
Succ O
Main> let e = (Let "y" (TVar "a") (Succ (TVar "y"))) in typeOf' e typeEnv
Right TNat
```

Products and unions are awesome, so we will implement them! We extend the value constructors as follows:

```data Term =
...
| Pair Term Term
| EitherL Term
| EitherR Term
```

…and for the type checker:

```data Type =
...
| TyVar String       -- constants, use later
| TyPair Type Type   -- pairs, use later
| TyEither Type Type -- sum, use later
```

We add handling to the evaluator:

```eval' _ (Pair a b) = Pair a b
eval' env (EitherL a) = eval' env a
eval' env (EitherR a) = eval' env a
```

…and the type checker:

```> typeOf' env (Pair a b) =
>     let a' = typeOf' env a
>         b' = typeOf' env b in
>         case a' of
>             Right ta -> case b' of
>                 Right tb -> Right \$ TyPair ta tb
>                 Left _ -> Left "Unsupported type for Pair"
>             Left _  -> Left "Unsupported type for Pair"
> typeOf' env (EitherL a) = case (typeOf' env a) of
>     Right t -> Right \$ TyEither t (TyVar "x")
>     _       -> Left "Unsupported type for EitherL"
> typeOf' env (EitherR a) = case (typeOf' env a) of
>     Right t -> Right \$ TyEither t (TyVar "x")
>     _       -> Left "Unsupported type for EitherR"
```

Where `TyVar "x"` represents a polymorphic variable, and is unhandled since our system has no real support for polymorphism.

```Main> let e = IfThenElse T (Pair (TVar "a") (TVar "b")) O in (eval' termEnv e, typeOf' typeEnv e)
(Pair (TVar "a") (TVar "b"),Left "Type mismatch between Right (TyPair TNat TNat) and Right TNat")
Main> let e = IfThenElse T (Pair (TVar "a") (TVar "b")) (Pair O O) in (eval' termEnv e, typeOf' typeEnv e)
(Pair (TVar "a") (TVar "b"),Right (TyPair TNat TNat))
Main> eval' termEnv (EitherL (TVar "a"))
O
Main> typeOf' typeEnv (EitherL (TVar "a"))
Right (TyEither TNat (TyVar "x"))
```

The evaluator and the type checker almost live in two separate worlds — they do two separate tasks. If we want to ensure the evaluator will produce the correct results, the first thing is to assure that the type-checker returns no error. It was really interesting to understand the mechanism of type-checking in-depth 🙂

TAPL starts with untyped lambda calculus and proceeds to typed lambda calculus, but is focusing only on type environment, while leaving the variables environment. While this is a good approach, I feel producing a system like in this tutorial is a good first step before jumping to lambda calculus.

My next steps are to finish reading TAPL, and continue playing with toy languages. The code used in this tutorial is available as a literate Haskell file. I also had some fun with De Bruijn index.