#5596: "f c = a $ b c", "f = a . b" does not.
-------------------------------+--------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.2.1 | Keywords:
Testcase: | Blockedby:
Os: Linux | Blocking:
Architecture: x86_64 (amd64) | Failure: GHC rejects valid program
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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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5596>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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