#5012: haskell 98 program does not typecheck when compiled with -XTypeFamilies
---------------------------------+------------------------------------------
Reporter: jcpetruzza | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.0.2 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
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Consider the following valid Haskell 98 function:
{{{
f x =
do let apply p r = mapM p (r x)
a <- apply id fst
b <- apply return snd
return (a ++ b)
}}}
It compiles fine with both ghc 6.12.x and ghc 7. However, if I add the
-XTypeFamilies flag to ghc 7 it is rejected with:
{{{
mono.hs:5:17:
Occurs check: cannot construct the infinite type: a0 = m0 a0
Expected type: m0 a0 -> m0 a0
Actual type: m0 a0 -> m0 (m0 a0)
In the first argument of `apply', namely `return'
In a stmt of a 'do' expression: b <- apply return snd
}}}
If one adds a type signature, the error message gets more intimidating.
A project of mine uses Type Families in some modules so TypeFamilies is
listed as an extension in cabal's configuration file; therefore, it is
being passed to every module. That is how I stumbled upon this problem.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5012>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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