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the following code produces a type error, and i think it shouldn't:

{-# LANGUAGE ScopedTypeVariables, MultiParamTypeClasses #-}
module Main
where

import Control.Monad.ST
import Data.STRef
import Text.Parsec


type P v a = ParsecT TokStream () (ST v) a

data TokStream = TokStream [Char]

instance Stream TokStream (ST v) Char where
    uncons (TokStream []) = return Nothing
    uncons (TokStream (t:ts)) = return (Just (t, TokStream ts))

c :: P v ()
c = return ()

works :: [Char] -> Either ParseError ()
works toks = runST $ f $ TokStream toks
    where
      f :: forall v . TokStream -> ST v (Either ParseError ())
      f = runPT c () "<sourcefile>"

doesnt :: [Char] -> Either ParseError ()
doesnt = runST . f . TokStream
    where
      f :: forall v . TokStream -> ST v (Either ParseError ())
      f = runPT c () "<sourcefile>"

doesnt should be equivalent to works, but works works and doesnt doesn't. the type error:

    Couldn't match expected type `forall s.
                                  ST s (Either ParseError ())'
                with actual type `ST v0 (Either ParseError ())'
    Expected type: TokStream -> forall s. ST s (Either ParseError ())
      Actual type: TokStream -> ST v0 (Either ParseError ())
    In the first argument of `(.)', namely `f'
    In the second argument of `(.)', namely `f . TokStream'

I tried this on 7.2.1 and 7.0.3. may be related to tickets 4347 or 4310, but I don't know enough about the ghc type engine to tell.

Trac metadata

Trac field	Value
Version	7.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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