It seems reasonable to me, and GHC is happy.
Here's a working example:
Simon
class MyClass a b where
foo :: a -> b -> Int
data Special = forall b. (MyClass Int b)=> MkSpecial b
data General a = forall b. (MyClass a b)=> MkGeneral b
instance MyClass Int Bool where
foo x False = -x
foo x True = x
xs :: [General Int]
xs = [MkGeneral True, MkGeneral False]
main = print [foo (3::Int) x | MkGeneral x <- xs]
| -----Original Message-----
| From: Ashley Yakeley [mailto:[EMAIL PROTECTED]]
| Sent: 29 March 2001 10:47
| To: Haskell List
| Subject: Parameterising Class Constraints in Existential Types
|
|
| Is this valid (extended) Haskell?
|
| --
| class MyClass a b where
| foo :: a -> b -> Int
|
| data Special = forall b. (MyClass Int b)=> MkSpecial b
| data General a = forall b. (MyClass a b)=> MkGeneral b
| --
|
| Hugs complains about the 'General' line but has no problem with the
| 'Special' line...
|
| --
| Ashley Yakeley, Seattle WA
|
|
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