At 2002-01-18 12:01, Hal Daume III wrote:

class MyClass f a | a -> f, f -> a where ...

I want to write a data type:

data (MyClass f a) => MyData f = MyData (f, a)

But this doesn't seem to be valid because "a" isn't in scope.

Class constraints in 'data' declarations only affect the type of constructor functions. In other words, data (Context) => MyData a b c = MkMyData whatever is equivalent to data MyData a b c = MkMyData whatever except for the type of MkMyData, which will be MkMyData :: (Context) => whatever -> MyData a b c instead of MkMyData :: whatever -> MyData a b c

But a is uniquely determined by f. What gives?

What you want is data MyData f = forall a. (MyClass f a) => MyData (f, a)

Furthermore, I can't even write:

data (MyClass f a) => MyData2 f = MyData2 f

Well now this is odd. I had no trouble simulating it with this: data MyData2 f = MyData2 f mkMyData2 :: (MyClass f a) => f -> MyData2 f mkMyData2 = MyData2 Looks like extended Haskell is being excessively restricted. Comments? -- Ashley Yakeley, Seattle WA