#5012: haskell 98 program does not typecheck when compiled with -XTypeFamilies
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Reporter: jcpetruzza | Owner:
Type: bug | Status: closed
Priority: normal | Component: Compiler (Type
checker)
Version: 7.0.2 | Resolution: invalid
Keywords: | Testcase:
Blockedby: | Os: Unknown/Multiple
Blocking: | Architecture: Unknown/Multiple
Failure: GHC rejects valid program |
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Changes (by HolgerReinhardt):
* status: new => closed
* resolution: => invalid
Comment:
This is caused by a change in the type-checker in GHC 7:
http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7
This new behaviour gets triggered by activating Type families. The
solution is to give explicit local type signatures:
{{{
{-# OPTIONS -XTypeFamilies -XRankNTypes -XScopedTypeVariables #-}
f :: forall a m. Monad m => ([m a], [a]) -> m [a]
f x = do
a <- apply id fst
b <- apply return snd
return (a ++ b)
where
apply :: forall b c. (b -> m c) -> (([m a], [a]) -> [b]) -> m [c]
apply p r = mapM p (r x)
}}}
If you don't want to write the type signatures, you can just make "apply"
a global function and pass in "x" as a parameter:
{{{
f x = do
a <- apply x id fst
b <- apply x return snd
return (a ++ b)
apply x p r = mapM p (r x)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5012#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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