#2247: GHC accepts FD violations, unless the conflicing instances are used
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Reporter: claus | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.9
Severity: normal | Resolution:
Keywords: TF vs FD | Testcase:
Architecture: Unknown | Os: Unknown
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Comment (by claus):
here is a slight variation that does not rely on a GHCi session to raise
the issue
{{{
module Improve where
class FD a b | a -> b
instance CFD a b => FD a b
class {- FD a b => -} CFD a b
instance CFD Bool Char
instance CFD Bool Bool
f' :: FD Bool Bool => Bool
f' = True
g' :: FD Bool Char => Bool
g' = False
x = f'
}}}
{{{
module Main where
import Improve
y = g'
main = print (x,y)
}}}
GHC (6.9.20080217) accepts this, and the executable outputs
`(True,False)`, even though the first component depends on `instance FD
Bool Bool` while the second depends on `instance FD Bool Char`.
Inlining `x` as `f'` is sufficient to raise the expected error message.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2247#comment:1>
GHC <http://www.haskell.org/ghc/>
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