[Haskell-cafe] Re: ANNOUNCE: fixpoint 0.1
Roman Leshchinskiy
rl at cse.unsw.edu.au
Wed Nov 21 03:55:27 EST 2007
apfelmus wrote:
> 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
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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