#6161: Program with missing instance accepted by compiler, program <<loops>>
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Reporter: wlguest | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.4.1 | Keywords: loop
Os: Linux | Architecture: x86
Failure: GHC accepts invalid program | Testcase:
Blockedby: | Blocking:
Related: |
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The following program is accepted by the compiler even though (Fam Float)
has no Super instance which is needed for Duper. testProg <<loops>> when
run.
Even with the Super (Fam Float) instance added, testProg continues to
<<loop>> unless the Duper (Fam a) constraint on Foo a is replaced by a
Super (Fam a) constraint.
The problem seems to be an instance of
[http://hackage.haskell.org/trac/ghc/ticket/5751 #5751], but I'm not
really sure.
{{{
{-# LANGUAGE TypeFamilies, FlexibleContexts, FlexibleInstances #-}
data family Fam a
data instance Fam Float = FamFloat Float
class Super a where
testSup :: a -> Float
class Super a => Duper a where
testDup :: a -> Float
--class ( Super (Fam a) ) => Foo a where
class Duper (Fam a) => Foo a where
testFoo :: Fam a -> Float
instance Foo a => Duper (Fam a) where
testDup x = testFoo x + testSup x
--instance Super (Fam Float) where
-- testSup (FamFloat x) = x
instance Foo Float where
testFoo (FamFloat _) = 5.0
testProg :: Float
testProg = testDup (FamFloat 3.0)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6161>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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