#1614: Type checker does not use fundep to avoid ambiguity
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Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.8
Component: Compiler (Type checker) | Version: 6.7
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Os: MacOS X | Testcase:
Architecture: x86 |
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Comment (by guest):
What I'm really trying to use this for is to solve a different problem,
which might have a more direct solution. I have an instance 'Eq (E RValue
a)' and I want the type checker to pick that one when it runs into the
constraint 'Eq (E v a)', because there's never going to be any other
instance of 'Eq (E ...)'.
Of course, the type checker needs to be convinced of this. So I tried
'IsRValue v => Eq (E v a)', where IsRValue has a fundep that forces it to
have (at most) one instance.
But then I ran into the bug I reported.
I think using this idiom is a reasonable way to tell the type checker that
there will be at most one instance of a particular class.
The code:
{{{
{-# OPTIONS_GHC -fglasgow-exts #-}
module E where
data E v a = E a
data RValue
{-
-- This fails with the message below.
instance (Eq a) => Eq (E RValue a) where
E x == E y = x == y
-- No instance for (Eq (E v Int))
-- arising from a use of `==' at E.hs:13:6-11
-}
-- This succeeds, but doesn't force v to be RValue
instance (Eq a) => Eq (E v a) where
E x == E y = x == y
{-
-- This fails with the message below.
instance (IsRValue v, Eq a) => Eq (E v a) where
E x == E y = x == y
class IsRValue a | -> a
instance IsRValue RValue
-- Ambiguous type variable `v' in the constraint:
-- `IsRValue v' arising from a use of `==' at E.hs:30:6-11
-}
a :: E v Int
a = undefined
foo = a == a
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1614>
GHC <http://www.haskell.org/ghc/>
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