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ArrowChoices are Traversing. #40

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ArrowChoices are Traversing. #40

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Zemyla
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@Zemyla Zemyla commented Jun 25, 2016

Add functions wanderA and traverseA that work like wander and traverse' with ArrowChoices, and make WrappedArrow Traversing as well.

Zemyla added 3 commits June 24, 2016 18:42
@Zemyla
ArrowChoices are Traversing.
16d112c 
Add functions to implement wander and traverse' for ArrowChoices; also
make WrappedArrow a Traversing.
@Zemyla
Fix errors on pre-7.10 GHC versions.
343de4f 
There was another Prelude import that brought id and (.) into scope.
That was moved.
@Zemyla
Applicative/Monad instances for WrappedArrow
2b62479 
WrappedArrow p is now a Functor, Applicative, Monad, Alternative,
MonadPlus, and MonadFix for appropriate values of p, as well as an
ArrowPlus (which it wasn't before for some reason).
@sjoerdvisscher
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This works for just (Choice p, ProductProfunctor p) too:

wander :: forall p a b s t. (Choice p, ProductProfunctor p)
  => (forall f. (Applicative f) => (a -> f b) -> s -> f t)
  -> p a b -> p s t
wander tr p = lmap (runTrav . tr tLift) go where
  go :: forall u. p (Coyo (TList a b) u) u
  go = dimap unTList (either id id) . right' $ lmap fst go **** lmap snd p

Note: this makes Costar w Traversing for Traversable w, and therefore also Strong! This should not be possible, and when I tried to ask for something impossible, I got a stack overflow. So something is fishy, it might be this whole idea, or my translation to Choice/ProductProfunctor, or just the Choice instance for Costar.

@ekmett
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ekmett commented Mar 26, 2017

The Choice Costar instance is known to be wrong.

Base automatically changed from master to main March 25, 2021 09:48
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3 participants
@Zemyla @sjoerdvisscher @ekmett