#1713: type synonym families are treated as being able to be instance of a class
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Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8
Severity: normal | Keywords:
Difficulty: Unknown | Os: Linux
Testcase: | Architecture: x86
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The following code doesn’t compile:
{{{
{-# LANGUAGE TypeFamilies #-}
module TypeFamilyBug where
type family TestFamily a :: *
type instance TestFamily () = [()]
testFunction :: value -> TestFamily value -> ()
testFunction = const (const ())
testApplication :: ()
testApplication = testFunction () (return ())
}}}
GHC 6.8.20070916 complains about {{{TestFamily}}} not being an instance
of {{{Monad}}}. Obviously, GHC recognizes that the second argument in the
application of {{{testApplication}}} has to be of type {{{TestFamily ()}}}
and tries to unify this type with {{{m ()}}} from the type of
{{{return}}}. However, in my opinion, GHC should reduce {{{TestFamily
()}}} to {{{[()]}}} and then see that the {{{[]}}} is the type which has
to be an instance of {{{Monad}}}.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1713>
GHC <http://www.haskell.org/ghc/>
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