{-# LANGUAGE TupleSections #-}
module Typecheck
(inferExpr,
 checkExpr,
 Environment)
  where
import Syntax
--import Control.Monad
import Control.Monad.Except
type Environment = [(Idnt,Typ)]

type Error = Except String

assertInt :: Typ -> Expr -> Error ()
assertInt TInt _ = return ()
assertInt t e = throwError $ "Expected type of expression "++ show e ++ " to be (TInt), instead got" ++ show t 
  
assertRec :: Typ -> Expr -> Error TypeDict
assertRec (TRecord ts) _ = return ts
assertRec t e = throwError $ "Expected type of expression "++ show e ++ " to be (TRecord _), instead got" ++ show t 

assertVar :: Typ -> Expr -> Error TypeDict
assertVar (TVariant ts) _ = return ts
assertVar t e = throwError $ "Expected type of expression "++ show e ++ " to be (TVariant _), instead got" ++ show t 

assertPtr :: Typ -> Expr -> Error Typ
assertPtr (TPtr t) _ = return t
assertPtr t e = throwError $ "Expected type of expression "++ show e ++ " to be (TPtr _), instead got" ++ show t 

envlookup :: Idnt -> TypeDict -> Expr -> Error Typ
envlookup name env expr = case lookup name env of
  Just t -> return t
  Nothing -> throwError $ "Variable "++ name ++ "not defined in expr" ++ show expr

inferExpr :: Expr -> Environment -> Error Typ
inferExpr e@(Var name) env = envlookup name env e
inferExpr (IntLit _) _ = return TInt
inferExpr (Record ts) env = do
  tts <- mapM (\(v,e) -> (v,) <$> inferExpr e env) ts
  return $ TRecord tts
inferExpr ee@(Variant t v e) env = do
  ts <- assertVar t e
  tt <- envlookup v ts ee
  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 e1
  return tt
inferExpr c@(Match e cases) env = do
  varType <- inferExpr e env
  variants <- assertVar varType e
  -- TODO ensure variants and cases cover the same names
  caseTypes <- mapM (\(v,e) ->(v,) <$> inferExpr e env) cases
  ret <- allSame caseTypes
  return $ snd ret
  where
    allSame :: [(Idnt,Typ)] -> Error (Idnt,Typ)
    allSame (x:[]) = return x
    allSame [] = throwError $ "Empty case "++ show c
    allSame ((xx,x):xs) = do
      (v,t) <- allSame xs
      if t==x then return (v,t) else
        throwError $ "Different types in branches " ++ show v ++ "," ++ show xx ++ "of expression" ++ show c
inferExpr (Proj name e) env = do
  t <- inferExpr e env
  tt <- assertRec t e
  case lookup name tt of
    Just typ -> return typ
    Nothing -> throwError $ "Record " ++show e ++ "doesn't have field" ++ name

checkExpr :: Expr -> Environment -> Typ -> Error ()
checkExpr e@(Add e1 e2) env t = do
  assertInt t e
  checkExpr e1 env TInt
  checkExpr e2 env TInt
checkExpr e@(Mult e1 e2) env t = do
  assertInt t e
  checkExpr e1 env TInt
  checkExpr e2 env TInt
checkExpr e@(Neg ee) env t = do
  assertInt t e
  checkExpr ee env TInt
checkExpr (Deref e1) env t = do
 checkExpr e1 env (TPtr t)
checkExpr e env t = do
  t' <- inferExpr e env
  if t == t' then return ()
    else throwError $ "Expected type of expression " ++
         show e ++ " to be"++ show t  ++" instead got" ++ show t'