Write You a Haskell Stephen Diehl

type Env = [(Name, Type)]
extend :: (Name, Type) -> Env -> Env
extend xt env = xt : env
inEnv :: (Name, Type) -> Check a -> Check a
inEnv (x,t) = local (extend (x,t))
lookupVar :: Name -> Check Type
lookupVar x = do
env return e
Nothing -> throwError $ NotInScope x
e typechecker will be a ExceptT + Reader monad transformer stack, with the reader holding the
typing environment. ere are three possible failure modes for our simply typed lambda calculus typechecker:
• e case when we try to unify two unlike types.
• e case when we try to apply a non-function to an argument.
• e case when a variable is referred to that is not in scope.
data TypeError
= Mismatch Type Type
| NotFunction Type
| NotInScope Name
type Check = ExceptT TypeError (Reader Env)
ere is a direct equivalence between syntax patterns here and the equivalent typing judgement for it.
is will not always be the case in general though. e implementation of the type checker is as follows:
check :: Expr -> Check Type
check expr = case expr of
Lit (LInt{}) -> return TInt
Lit (LBool{}) -> return TBool
Lam x t e -> do
rhs do
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