<div dir="ltr">Oh, I see your confusion - you're looking at `indexed` on Hackage, but I meant the unreleased library on Github. That uses the slice category presentation of higher-order monads/functors, which is what you're talking about:<div><br></div><div><a href="https://github.com/ekmett/indexed/blob/master/src/Indexed/Functor.hs#L53">https://github.com/ekmett/indexed/blob/master/src/Indexed/Functor.hs#L53</a><br></div><div><br></div><div>Though HFix isn't there, but I feel it would be a suitable addition to the library.</div><div><br></div><div>It is unfortunate that there is a name conflict.</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Mar 24, 2015 at 4:28 PM, Erik Hesselink <span dir="ltr"><<a href="mailto:hesselink@gmail.com" target="_blank">hesselink@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">That seems slightly different, more related to indexed monads with<br>
pre- and postconditions. They have many more type variables...<br>
<span class="HOEnZb"><font color="#888888"><br>
Erik<br>
</font></span><div class="HOEnZb"><div class="h5"><br>
On Tue, Mar 24, 2015 at 5:20 PM, Oliver Charles <<a href="mailto:ollie@ocharles.org.uk">ollie@ocharles.org.uk</a>> wrote:<br>
> Higher order stuff is very useful, but I don't think it needs to belong in<br>
> the same place as Data.Functor.Fix. Edward Kmett's 'indexed' library will<br>
> probably deal with this, when we have a GHC that's smart enough to<br>
> understand it ;)<br>
><br>
> - Ollie<br>
><br>
> On Tue, Mar 24, 2015 at 3:40 PM, Erik Hesselink <<a href="mailto:hesselink@gmail.com">hesselink@gmail.com</a>> wrote:<br>
>><br>
>> Ugh, you're right, I renamed the variables and made a mistake. The<br>
>> outer variable should stay the same of course, your signature is<br>
>> correct.<br>
>><br>
>> For a use site of HFix, see the multirec package [0], although that<br>
>> might not really clarify things :) We also have a use internally, but<br>
>> I just realized we can probably simplify that away.<br>
>><br>
>> Regards,<br>
>><br>
>> Erik<br>
>><br>
>> [0]<br>
>> <a href="http://hackage.haskell.org/package/multirec-0.7.5/docs/Generics-MultiRec-HFix.html" target="_blank">http://hackage.haskell.org/package/multirec-0.7.5/docs/Generics-MultiRec-HFix.html</a><br>
>><br>
>> On Tue, Mar 24, 2015 at 4:25 PM, David Feuer <<a href="mailto:david.feuer@gmail.com">david.feuer@gmail.com</a>><br>
>> wrote:<br>
>> > I have no comment on your higher-order version (because I don't<br>
>> > understand<br>
>> > it yet), but the type you give for hmap looks unlikely. Did you mean<br>
>> ><br>
>> > hmap :: (g :-> h) -> f g :-> f h ?<br>
>> ><br>
>> > On Mar 24, 2015 10:58 AM, "Erik Hesselink" <<a href="mailto:hesselink@gmail.com">hesselink@gmail.com</a>> wrote:<br>
>> >><br>
>> >> On Tue, Mar 24, 2015 at 7:23 AM, M Farkas-Dyck <<a href="mailto:strake888@gmail.com">strake888@gmail.com</a>><br>
>> >> wrote:<br>
>> >> > On 22/03/2015 at 23:01:47 -0400, David Feuer wrote:<br>
>> >> >> There are a good number of different packages that define either<br>
>> >> >><br>
>> >> >> newtype Fix f = Fix (f (Fix f))<br>
>> >> >><br>
>> >> >> or something equivalent except for naming. Most of the name<br>
>> >> >> variation<br>
>> >> >> is in the name of the data constructor and/or record selector. This<br>
>> >> >> does not look like an ideal situation to me. Most problematically,<br>
>> >> >> the<br>
>> >> >> only way to convert one to another is with unsafeCoerce.<br>
>> >> >><br>
>> >> >> I think it would be rather nice to choose one canonical place for<br>
>> >> >> this<br>
>> >> >> definition, and let everyone else use it.<br>
>> >> ><br>
>> >> > +1<br>
>> >> ><br>
>> >> > I propose<br>
>> >> ><br>
>> >> > module Data.Fix where<br>
>> >> ><br>
>> >> > newtype Fix f = Fix { unFix :: f (Fix f) }<br>
>> >><br>
>> >> I'm used to<br>
>> >><br>
>> >> newtype Fix f = In { out : f (Fix f) }<br>
>> >><br>
>> >> But all the other suggestions look fine to me, too.<br>
>> >><br>
>> >> I've also often used this variation:<br>
>> >><br>
>> >> newtype HFix f a = In { out :: f (HFix f) a }<br>
>> >><br>
>> >> This allows you to take the fixed point of e.g. monad stacks and<br>
>> >> indexed types. The function you propose below can then be a type class<br>
>> >> function of an indexed Functor class:<br>
>> >><br>
>> >> type f :-> g = forall a. f a -> g a<br>
>> >><br>
>> >> class HFunctor f where<br>
>> >> hmap :: (a :-> b) -> f a :-> g a<br>
>> >><br>
>> >> Does this deserve a place somewhere as well? Or is it too specialized?<br>
>> >><br>
>> >> > Perhaps too we ought to name and define _ :: (∀ a . f a -> g a) -><br>
>> >> > Fix f<br>
>> >> > -> Fix g there.<br>
>> >><br>
>> >> Some variation of 'map' seems sensible, like 'hmap' (but see above) or<br>
>> >> 'mapFix'.<br>
>> >><br>
>> >> Erik<br>
>> _______________________________________________<br>
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><br>
><br>
</div></div></blockquote></div><br></div>