The Arrow class is redundant

[ekmett]

An Arrow isn't a Strong Category with extra stuff, it is precisely Strong + Category.

There isn't anything gained by having a separate class except now a risk that someone might have a constraint that requires it and which isn't satisfied by the logically equivalent pair of Strong and Category constraints.

If the purescript equivalent of UndecidableInstances works correctly here you could just make one instance for Arrow and call it good as a shorthand.

[paf31]

Yes, this library has been shrinking ever since purescript-profunctors came into existence. Now that we have Star, it probably makes sense to add Costar, deprecate (Co)Kleisli and move Static out.

Are there Profunctor versions of ArrowZero and ArrowPlus? (Not that we currently use them anywhere...)

[paf31]

Or, yes, the UndecidableInstances approach should work too.

[ekmett]

ArrowZero is a 'unit first' sort of definition, as such I tend to avoid defining things that way. I've found it tends to be less useful than I'd want. Mathematicians don't like writing down a unit then looking for operations that work with that unit. On the other hand an associative operation is useful even without a unit. With Arrow underneath it at least has the multiplicative structure to be cancellative for, though.

Modulo superclasses ArrowPlus (without ArrowZero) is basically a sum-based variant of the both based Wander type.

This gets pretty granular if you let it. ;)