On Jul 18, 2013, at 2:35 PM, harry
Sjoerd Visscher-2 wrote
equal pair@Pair{} = foo pair == bar pair
Interesting solution, I didn't know you could do that. (Do all those who suggested GADTs - you can add a type context to the constructor of a regular data type as well, they don't bring you anything here.)
What it brings in my opinion is clarity, but that's subjective of course.
I've also been experiencing this a lot in class instances, such as:
class Foo f where foo :: a -> f a
data Bar f a = Foo f => Bar {bar :: f a}
instance Foo (Bar f) where foo a = Bar (foo a)
Is there any way to avoid repeating the Foo f constraint in the Bar f instance?
No, you can only omit it where you provide Foo f in another way. With equal you provide it with the Pair value. But here you're constructing a value, so you'll have to provide the Foo f. If all the methods in the class receive a value, then you can omit it, f.e.: class Baz f where baz :: f a -> a data Bar f a = Baz f => Bar {bar :: f a} instance Baz (Bar f) where baz (Bar fa) = baz fa -- Sjoerd