[Haskell-cafe] Re: ANNOUNCE: fixpoint 0.1
apfelmus at quantentunnel.de
Wed Nov 21 03:46:25 EST 2007
Bertram Felgenhauer wrote:
> [redirecting from haskell at ...]
> 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
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