apfelmus wrote:
Bertram Felgenhauer wrote:
[redirecting from [EMAIL PROTECTED]
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
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