module Typecheck
(inferExpr,
 checkExpr)
  where
import Syntax
import Control.Monad
type Environment = [(Idnt,Typ)]

assertInt TInt = Just ()
assertInt _ = Nothing
assertRec (TRecord ts) = Just ts
assertRec _ = Nothing
assertVar (TVariant ts) = Just ts
assertVar _ = Nothing
assertPtr (TPtr t) = Just t
assertPtr _ = Nothing

inferExpr :: Expr -> Environment -> Maybe Typ
inferExpr (Var name) env = lookup name env
inferExpr (IntLit _) _ = Just TInt
inferExpr (Record _) _ = undefined
inferExpr (Variant t v e) env = do
  ts <- assertVar t
  tt <- lookup v ts
  checkExpr e env tt
  return t
inferExpr (Add e1 e2) env = do
  checkExpr e1 env TInt
  checkExpr e2 env TInt
  return TInt
inferExpr (Mult e1 e2) env = do
  checkExpr e1 env TInt
  checkExpr e2 env TInt
  return TInt
inferExpr (Neg e) env = do
  checkExpr e env TInt
  return TInt
inferExpr (Deref e1) env = do
  t <- inferExpr e1 env
  tt <- assertPtr t
  return tt

checkExpr :: Expr -> Environment -> Typ -> Maybe ()
checkExpr e env t = do
  t' <- inferExpr e env
  guard (t==t')
  return ()