{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}

{-# OPTIONS_GHC
  -fplugin     BinderAnn.Pure
  -fplugin-opt BinderAnn.Pure:infix=@@
#-}

module Pure where

import Control.Monad.Writer
import Control.Monad.Identity

import BinderAnn.Pure

data Exp =
    Val Int
  | Add Exp Exp
  | Ann Exp SrcInfo
  deriving Show

type Eval = WriterT [SrcInfo] Identity

instance Annotated Exp where
  annotate = Ann

runEval :: Eval Exp -> (Int, [SrcInfo])
runEval e = runIdentity (runWriterT (eval =<< e))

runEval' :: Eval Exp -> Exp
runEval' = fst . runIdentity . runWriterT

eval :: Exp -> Eval Int
eval (Val n) = return n
eval (Add x y) = liftM2 (+) (eval x) (eval y)
eval (Ann x a) = tell [a] >> eval x

val :: Int -> Eval Exp
val n = return (Val n)

(|+|) :: Exp -> Exp -> Eval Exp
x |+| y = return (Add x y)


test1 = runEval @@ do
  (x, y) <- (,) <$> val 10 <*> val 5
  z  <- return False
  w  <- (if z then val 1 else val 2)
  s1 <- x |+| y
  s1 |+| w

tests :: IO ()
tests = do
  putStrLn "test1:"
  print test1