On Tue, Mar 24, 2015 at 7:23 AM, M Farkas-Dyck
On 22/03/2015 at 23:01:47 -0400, David Feuer wrote:
There are a good number of different packages that define either
newtype Fix f = Fix (f (Fix f))
or something equivalent except for naming. Most of the name variation is in the name of the data constructor and/or record selector. This does not look like an ideal situation to me. Most problematically, the only way to convert one to another is with unsafeCoerce.
I think it would be rather nice to choose one canonical place for this definition, and let everyone else use it.
+1
I propose
module Data.Fix where
newtype Fix f = Fix { unFix :: f (Fix f) }
I'm used to newtype Fix f = In { out : f (Fix f) } But all the other suggestions look fine to me, too. I've also often used this variation: newtype HFix f a = In { out :: f (HFix f) a } This allows you to take the fixed point of e.g. monad stacks and indexed types. The function you propose below can then be a type class function of an indexed Functor class: type f :-> g = forall a. f a -> g a class HFunctor f where hmap :: (a :-> b) -> f a :-> g a Does this deserve a place somewhere as well? Or is it too specialized?
Perhaps too we ought to name and define _ :: (∀ a . f a -> g a) -> Fix f -> Fix g there.
Some variation of 'map' seems sensible, like 'hmap' (but see above) or 'mapFix'. Erik