Add some bifunctors-style newtypes by garyb · Pull Request #22 · purescript/purescript-profunctor

garyb · 2017-06-09T11:15:06Z

The name Cowrap is probably a bit odd, but I haven't come up with anything better yet.

garyb · 2017-06-09T11:15:55Z

paf31 · 2017-06-09T16:42:37Z

src/Data/Profunctor/Join.purs

+
+-- | Turns a `Profunctor` into a `Invariant` functor by equating the two type
+-- | arguments.
+newtype Join p a = Join (p a a)


I wonder if it's worth providing the adjoint, which constructs a profunctor from an invariant.

Personally, I'd rather just remove Invariant, but that's a separate, larger conversation 😄

Wait! I just realised something, this is purescript-deprecated/purescript-monoid#34 if we provide

instance semigroupJoin :: Semigroupoid p => Semigroup (Join p a) where append (Join a) (Join b) = Join (a <<< b) instance monoidJoin :: Category p => Monoid (Join p a) where mempty = Join id

Any idea what the adjoint would look like? I just took a stab at it, but not really sure what I'm doing there and couldn't figure out how to implement dimap.

I found the previous discussion of this here.

Ah yeah! I remember now. Will add that too then.

Feel free to give it a better name 😄

Cojoin was all I had, I'm not sure it's any better 😆

Split possibly? Since we're trying to split the type argument into two.

paf31 · 2017-06-09T16:43:44Z

👍 LGTM. Curious though - is there a use case for these?

garyb · 2017-06-09T17:17:53Z

Curious though - is there a use case for these?

Uhh... most of them are pretty speculative 😉

It came from a discussion with @safareli, he was asking if we could impose Functor and Contravariant superclasses, or provide instances for Functor and Contravariant for every Profunctor - since we have endofunctors, no, but I figured providing newtypes for them would "prove" the relationship is there, and perhaps be occasionally useful.

Then when I started on that, I figured that profunctors are still a bifunctors, just not in the purescript sense, so I went back to BF to see which of the various newtypes may translate to PF. So Clown and Joker are definitely a bit 🤷‍♂️, and Join was right up until two minutes ago when I realised it's our abstracted Endo 😆

paf31 · 2017-06-09T17:21:20Z

Do you think the names Clown and Joker might be confusing though?

safareli · 2017-06-09T17:23:39Z

there is already Joker and Clown defined in https://pursuit.purescript.org/packages/purescript-bifunctors

garyb · 2017-06-09T17:24:07Z

I'm happy to change them, just figured since they work the same way for bifunctors, reusing the names here was reasonable.

garyb · 2017-06-09T18:27:06Z

There we go, Split sounded good to me.

paf31 · 2017-06-09T18:38:11Z

src/Data/Profunctor/Split.purs

+
+newtype Split f a b = Split (Exists (SplitF f a b))
+
+data SplitF f a b x = SplitF (a -> x) (x -> b) (f x)


Can we hide the constructors here?

Oops, good call.

paf31 · 2017-06-09T18:38:22Z

👍 LGTM, thanks!

paf31 · 2017-06-09T19:45:38Z

I just thought - should this have reused the existing newtypes? Bit late now, but probably worth considering.

garyb · 2017-06-09T21:55:53Z

I just explored that a bit...

We could reuse Clown and Joker, defining the instances in Data.Profunctor
Wrap doesn't work as it has an overlapping Functor instance (one with Bifunctor and one with Profunctor constraints on p)
Join could work, but we'd have to define those "endo" instances with the newtype, which would mean some dependency reordering - it probably works out, but bifunctors and monoid are both near the top of the hierarchy, so I'm a little hesitant to mess with them right now.

I think next time we do breaking releases for all the core libraries we can take a look at it again though 👍

Add some bifunctors-style newtypes

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paf31 reviewed Jun 9, 2017

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garyb added 2 commits June 9, 2017 18:25

Add Endo style instances for Join

9f1fc8b

Add Split

8353b58

paf31 reviewed Jun 9, 2017

View reviewed changes

garyb added 2 commits June 9, 2017 20:25

Hide implementation details of Split

82244de

Add Functor instance for Split

0f7bb5b

garyb merged commit 3939c48 into master Jun 9, 2017

garyb deleted the newtypes branch June 9, 2017 19:33

garyb mentioned this pull request Jun 9, 2017

Re-use Bifunctor newtypes where appropriate #23

Closed

paf31 mentioned this pull request Jun 11, 2017

Generalize Endo purescript-deprecated/purescript-monoid#34

Closed


		newtype Split f a b = Split (Exists (SplitF f a b))

		data SplitF f a b x = SplitF (a -> x) (x -> b) (f x)

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