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 class Fixpoint f t where type F t a ... instance Fixpoint f (Mu f) where .. type F (Mu f) a = f a Otherwise, we would have to deal with some kind of newtype data F (Mu f) a = MuF f a Hm, but does F (Mu f) qualify as a type constructor of kind * -> * for type inference/checking? Or is the situation the same as with normal type synonyms? Regards, apfelmus