[Haskell-cafe] Re: ANNOUNCE: fixpoint 0.1
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 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: fixpoint 0.1
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: fixpoint 0.1
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: fixpoint 0.1
apfelmus wrote: 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 Excellent point! When I originally developed the code, type functions didn't really work anyway. I'll try again now that they are more mature. Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: fixpoint 0.1
Roman Leshchinskiy wrote: apfelmus wrote: 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 Excellent point! When I originally developed the code, type functions didn't really work anyway. I'll try again now that they are more mature. Actually, I don't think that hylo :: Fixpoint f t = (f s - s) - (p - f p) - p - s hylo f g = cata f . ana g will typecheck, the t is still ambiguous. It's just that it's one of those cases where the type signature is ambiguous but the program isn't. Well, from a denotational point of view anyway, different t will generate different code for hylo . Regards, apfelmus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: fixpoint 0.1
[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. Bertram ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
