Fixing Fix

[Oliver Charles]

Higher order stuff is very useful, but I don't think it needs to belong in
the same place as Data.Functor.Fix. Edward Kmett's 'indexed' library will
probably deal with this, when we have a GHC that's smart enough to
understand it ;)

- Ollie

On Tue, Mar 24, 2015 at 3:40 PM, Erik Hesselink <hesselink at gmail.com> wrote:

> Ugh, you're right, I renamed the variables and made a mistake. The
> outer variable should stay the same of course, your signature is
> correct.
>
> For a use site of HFix, see the multirec package [0], although that
> might not really clarify things :) We also have a use internally, but
> I just realized we can probably simplify that away.
>
> Regards,
>
> Erik
>
> [0]
> http://hackage.haskell.org/package/multirec-0.7.5/docs/Generics-MultiRec-HFix.html
>
> On Tue, Mar 24, 2015 at 4:25 PM, David Feuer <david.feuer at gmail.com>
> wrote:
> > I have no comment on your higher-order version (because I don't
> understand
> > it yet), but the type you give for hmap looks unlikely. Did you mean
> >
> > hmap :: (g :-> h) -> f g :-> f h ?
> >
> > On Mar 24, 2015 10:58 AM, "Erik Hesselink" <hesselink at gmail.com> wrote:
> >>
> >> On Tue, Mar 24, 2015 at 7:23 AM, M Farkas-Dyck <strake888 at gmail.com>
> >> wrote:
> >> > 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
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