Jim
You've found a strange corner case. Here's a cut-down example, which
fails in exactly the same way.
class Baz v x where
foo :: x - x-- Notice that v is not mentioned
foo y = y
instance Baz Int Int
But it succeeds if you put the instance explicitly:
instance Baz Int Int where foo y = y
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.
So I'm not sure what to do. I've added it as an expected-failure test
(tc199). I have not added a SourceForge bug because the original
program seems so implausible.
Meanwhile, the workaround is to define your own default method, thus:
dmfoo :: x - x
dmfoo y = y
instance Baz Int Int where foo = dmfoo
Simom
| -Original Message-
| From: [EMAIL PROTECTED]
[mailto:glasgow-haskell-users-
| [EMAIL PROTECTED] On Behalf Of Jim Apple
| Sent: 04 December 2005 17:24
| To: glasgow-haskell-users@haskell.org
| Subject: Data.Typeable and default instances
|
| {-# OPTIONS -fglasgow-exts #-}
|
| import Maybe
| import Data.Typeable
|
| data Nil = Nil deriving (Eq,Typeable,Show)
|
| class (Typeable t) = List a t where
| init :: (t - b) - (forall y . (List a y) = y - b)
| init f z = fromJust $ do x - cast z
|return $ f x
|
| instance List a Nil where
|
| Could not deduce (List a1 y)
|from the context (List a Nil, Typeable Nil, List a y)
|arising from use of `Main.$dminit' at Main.hs:21:0
| Probable fix: add (List a1 y) to the class or instance method
| `Main.init'
| In the definition of `init': init = Main.$dminit
| In the definition for method `Main.init'
| In the instance declaration for `List a Nil'
|
| but copying and pasting the code from init to the instance declaration
| works fine.
|
| Jim
|
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