[Haskell-cafe] mapM and Control.Arrow
olf at aatal-apotheke.de
Mon Jul 8 20:37:05 UTC 2019
consider the declaration
mapM :: (SomeArrow a, Traversable t) => a b c -> a (t b) (t c)
For the Arrow a = (->), the above mapM is just an over-constrained fmap, since Functor is a superclass of Traversable. For the arrow a = Kleisi m, the above mapM is the Control.Monad.mapM we know. Currently I fail to see how it could be defined with any of the existing sub-classes in Control.Arrow.
One might say that fmap and mapM are on the same spectrum: If you specialize SomeArrow to (->) you can generalize Traversable to Functor, if you specialize Functor to Traversable you can generalize (->) to SomeArrow.
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