#1061: default class methods can be given too general a type as GHC tries to
share
them between instances
----------------------+-----------------------------------------------------
Reporter: igloo | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.8
Component: Compiler | Version: 6.6
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: tc199 | Architecture: Unknown
Os: Unknown |
----------------------+-----------------------------------------------------
Old description:
> GHC does not accept this program:
>
> {{{
> class Baz v x where
> foo :: x -> x
> foo y = y
>
> instance Baz Int Int
> }}}
>
> even though it does accept this one:
>
> {{{
> class Baz v x where
> foo :: x -> x
> foo y = y
>
> instance Baz Int Int where
> foo y = y
> }}}
>
> I can't find an explicit answer in the report, but I think both should be
> accepted.
>
> The bug says, in a comment,
>
> {{{
> This code defines a default method with a highly dubious type,
> because 'v' is not mentioned, and there are no fundeps
>
> However, arguably the instance declaration should be accepted,
> beause it's equivalent to
> instance Baz Int Int where { foo x = x }
> which *does* typecheck
>
> GHC does not actually macro-expand the instance decl. Instead, it
> defines a default method function, thus
>
> $dmfoo :: Baz v x => x -> x
> $dmfoo y = y
>
> Notice that this is an ambiguous type: you can't call $dmfoo
> without triggering an error. And when you write an instance decl,
> it calls the default method:
>
> instance Baz Int Int where foo = $dmfoo
>
> I'd never thought of that. You might think that we should just
> *infer* the type of the default method (here forall a. a->a), but
> in the presence of higher rank types etc we can't necessarily do
> that.
> }}}
New description:
GHC does not accept this program:
{{{
class Baz v x where
foo :: x -> x
foo y = y
instance Baz Int Int
}}}
even though it does accept this one:
{{{
class Baz v x where
foo :: x -> x
foo y = y
instance Baz Int Int where
foo y = y
}}}
I can't find an explicit answer in the report, but I think both should be
accepted.
The tc199 test says, in a comment,
{{{
This code defines a default method with a highly dubious type,
because 'v' is not mentioned, and there are no fundeps
However, arguably the instance declaration should be accepted,
beause it's equivalent to
instance Baz Int Int where { foo x = x }
which *does* typecheck
GHC does not actually macro-expand the instance decl. Instead, it
defines a default method function, thus
$dmfoo :: Baz v x => x -> x
$dmfoo y = y
Notice that this is an ambiguous type: you can't call $dmfoo
without triggering an error. And when you write an instance decl,
it calls the default method:
instance Baz Int Int where foo = $dmfoo
I'd never thought of that. You might think that we should just
*infer* the type of the default method (here forall a. a->a), but
in the presence of higher rank types etc we can't necessarily do
that.
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1061>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
