Write You a Haskell Stephen Diehl

Lit (LInt _) -> return (nullSubst, typeInt)
Lit (LBool _) -> return (nullSubst, typeBool)
Let’s walk through each of the rule derivations and look how it translates into code:
T-Var
e T-Var rule, simply pull the type of the variable out of the typing context.
Var x -> lookupEnv env x
e function lookupVar looks up the local variable reference in typing environment and if found it
instantiates a fresh copy.
lookupEnv :: TypeEnv -> Var -> Infer (Subst, Type)
lookupEnv (TypeEnv env) x = do
case Map.lookup x env of
Nothing -> throwError $ UnboundVariable (show x)
Just s -> do t do
tv