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 _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
