#5316: ghc rejects a program at first but will accept it if one insists
---------------------------------+------------------------------------------
Reporter: jcpetruzza | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.0.4 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
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Consider these two modules (boiled down example from the checkers
package):
{{{
{-# LANGUAGE ScopedTypeVariables, FlexibleContexts #-}
module T1
where
import Test.QuickCheck
import Text.Show.Functions ()
f :: forall m a b. ( Arbitrary (a->b) ) => m (a,b) -> Property
f = const $ property (undefined :: (a->b) -> Bool)
}}}
{{{
module T2 where
import Control.Concurrent
g = threadDelay maxBound
}}}
I see the following interaction:
{{{
$ rm *hi *.o
$ ghc --make -c -O T1 T2
[1 of 2] Compiling T2 ( T2.hs, T2.o )
[2 of 2] Compiling T1 ( T1.hs, T1.o )
T1.hs:12:13:
Overlapping instances for Show (a -> b)
arising from a use of `property'
Matching instances:
instance Show (a -> b) -- Defined in Text.Show.Functions
instance Show base:System.Event.Manager.IOCallback
-- Defined in base:System.Event.Manager
(The choice depends on the instantiation of `a, b'
To pick the first instance above, use -XIncoherentInstances
when compiling the other instance declarations)
In the second argument of `($)', namely
`property (undefined :: (a -> b) -> Bool)'
In the expression: const $ property (undefined :: (a -> b) -> Bool)
In an equation for `f':
f = const $ property (undefined :: (a -> b) -> Bool)
$ ghc --make -c -O T1 T2
[2 of 2] Compiling T1 ( T1.hs, T1.o )
$ ghc --make -c -O T1 T2
$ ls
T1.hi T1.hs T1.o T2.hi T2.hs T2.o
}}}
I see this consistent behaviour in versions 7.0.{1,2,3,4} but not with
6.12.1
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5316>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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