purescript/purescript-profunctor

Add Loop

paf31 opened this issue · 3 comments

paf31 commented

Suggested:

class (Category a, Strong a) <= Loop a where
  loop :: forall b c d. a (Tuple b d) (Tuple c d) -> a b c

with laws (cribbed from https://hackage.haskell.org/package/base-4.7.0.2/docs/Control-Arrow.html#t:ArrowLoop):

loop (f <<< first h) = loop f <<< h
loop (f >>> first h) = loop f >>> h

.. and the rest which I haven't figured out yet (TODO).

DavidHarrison commented

Does ArrowLoop depend on Haskell's lazy evaluation? It seems that Halogen's loop relies on an initial state.

  • 👍1
garyb commented

Does Costrong cover this? unfirst is loop anyway, but there's also unsecond.

  • 👍1
paf31 commented

I think it possibly does. I don't fully understand the intent of Costrong yet, so I'm not sure.