apfelmus wrote:
Bertram Felgenhauer wrote:
[redirecting from haskell@...] apfelmus wrote: [...]
I wonder whether a multi parameter type class without fundeps/associated types would be better.
class Fixpoint f t where inject :: f t -> t project :: t -> f t
[...]
Interestingly, this even gives slightly shorter type signatures
cata :: Fixpoint f t => (f s -> s) -> t -> s size :: (Fixpoint f t, Foldable f) => t -> Int
size can't be used now though, because there is no way to infer f.
Ah, of course, stupid me.
Making f an associacted type synonym / fundep instead of a associated data type is still worth it, since we can use it for Mu f
I originally considered the following: class Functor (Pre t) => Fixpoint t where type Pre t :: * -> * instance Fixpoint (Mu f) where type Pre (Mu f) = f But alas, this breaks hylomorphisms: hylo :: Fixpoint t => (Pre t s -> s) -> (p -> Pre t p) -> p -> s If Pre is a type function, there is no way to infer t. Roman