#5736: Possible regression with functional dependencies
------------------------------+---------------------------------------------
Reporter: magnus | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.2.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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The following program typechecks in GHC-7.0.4, but not in 6.12.3, 7.2.2 or
7.5.20111229:
{{{
{-# LANGUAGE FunctionalDependencies, MultiParamTypeClasses,
FlexibleInstances, UndecidableInstances, TypeFamilies #-}
class C a b | a -> b where
c :: a -> b
instance C b b where
c = undefined
instance C Bool b where
c = undefined
m :: Int
m = c True
}}}
7.5.20111229 says
{{{
Couldn't match type `Bool' with `Int'
When using functional dependencies to combine
C b b,
arising from the dependency `a -> b'
in the instance declaration at Test.hs:7:10
C Bool Int, arising from a use of `c' at Test.hs:14:5
In the expression: c True
In an equation for `m': m = c True
}}}
Was I just lucky that this worked in 7.0.4, or is it a true regression?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5736>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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