#1241: Functional dependencies not checked.
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Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.6
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
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Hello,
GHC 6.6 (with flags {{{-fglasgow-exts -fallow-undecidable-instances}}}) is
too liberal when accepting instances. Here is an example:
{{{
class F a b | a -> b where f :: (a,b)
instance F Int b
}}}
The instance violates the functional dependency constraint of {{{F}}}
because (in theory) it can be used to solve both {{{F Int Bool}}} and {{{F
Int Char}}}. In practice, it seems that some free variable gets bound
upon the first use of {{{f}}} with a concrete type, leading to some rather
confusing behavior. Example:
{{{
x :: (Int,a)
x = f
y :: (Int,Bool)
y = f
z :: (Int,Char)
z = x -- works, but 'f' does not
}}}
-Iavor
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1241>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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