[Haskell-cafe] Re: ANNOUNCE: fixpoint 0.1

[apfelmus]

Roman Leshchinskiy wrote:
> apfelmus wrote:
>>
>> 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
> 
> 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.

Ah, right. But unlike  size , this is unambiguous since  t  can (and 
probably should) be fused away:

   hylo :: Functor f => (f s -> s) -> (p -> f p) -> p -> s
   hylo f g = f . fmap (hylo f g) . g


Regards,
apfelmus