#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 | Keywords:
Difficulty: Unknown | Testcase: tc199
Architecture: Unknown | Os: Unknown
-------------------------+--------------------------------------------------
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.
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1061>
GHC <http://www.haskell.org/ghc/>
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