Dear Tom,
I really appreciate your help, but If I could ask the perfect question
I probably would already know the answer... My example should not
prove anything, instead they collectively show, that I am missing
something. And it is not the fact, that "pure f does not depend on f."
If, however, this makes all the difference, I have to ask, why was
plausability and looking at the actual definition (not just the types)
not important for the other examples.
But I think my problem lies somewhere else. Maybe all would become
evident, if I knew the rigorous definition of "A is more general than
B" in this context. Especially when A is a class of type, that takes
two arguments (i.e. Unit and Arrow) and B for ones, that takes only
one (like Monad, Pure,..)
Thanks again!
Johannes
On Tue, May 28, 2013 at 11:11 PM, Tom Ellis
On Tue, May 28, 2013 at 09:09:48PM +0200, Johannes Gerer wrote:
What about these two very simple type classes. Are they equivalent? [...] class Pointed f where pure :: a -> f a
class Unit f where unit :: f a a
newtype UnitPointed f a = UnitPointed f a a instance Unit f => Pointed (UnitPointed f) where pure f = UnitPointed unit
newtype Kleisli f a b = Kleisli (a -> f b) instance Pointed f => Unit (Kleisli f) where unit = Kleisli pure
This is implausible, since "pure f" does not depend on "f".
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