#1624: internal error caused by adding an instance to a type class with a
functional dependency and a default method
----------------------------------------+-----------------------------------
Reporter: int-e | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 6.10
Component: Compiler (Type checker) | Version: 6.7
Severity: minor | Resolution:
Keywords: | Difficulty: Unknown
Os: Linux | Testcase:
Architecture: x86 |
----------------------------------------+-----------------------------------
Changes (by simonpj):
* milestone: => 6.10
* severity: normal => minor
* owner: => simonpj
Comment:
Indeed! This is a well-known problem with fundeps. The best example is
{{{
class Foo a b | a -> b
instance Foo a (Maybe a)
foo :: Foo a b => a -> b
foo = undefined
}}}
The instance decl says that for ''any'' first argument `a`, the second
argument must be `Maybe a`. But the type sig for `foo` says `foo` has
type `for all `a,b. a->b`. But it doesn't hold for all `b`, because `b`
must be a `Maybe`.
GHC rejects this because there is no translation into System F. All this
will become much clearer when we complete implementing indexed type
families.
Meanwhile, I agree that the error message from your first program is
horribly obscure, so I'll leave this bug open at low priority to improve
the error message. It arises because GHC is trying to verify that the
rhs of the default method for `Foo` has the right type; that is:
{{{
undefined :: Foo a b => a -> b
}}}
which is an example of the same problem again.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1624>
GHC <http://www.haskell.org/ghc/>
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