In actual programming, you can always define `fold...

# arrow

[CLOVIS]

In actual programming, you can always define fold on any Functor, but not all operations can be called on it (there needs to be a default value). So it sounds like fold is defined neither on a category nor on a morphism but on the pair of both of them?

[raulraja]

Fold shows up in Algebras that are higher kinded frequently in List etc, but you can also see it in types like String -> Array<Char>

[CLOVIS]

Wouldn't that be some kind of flatMap or something of the sort?

It looks like (Functor f, Functor g) => f a -> g a (sorry if the notation is wrong)

[raulraja]

that is a natural transformation

[raulraja]

F<A> -> G<A>

[raulraja]

in Arrow FunctionK

[raulraja]

In Kotlin we have to make the distinctions between higher order (more than one type arg)

[raulraja]

but that usually is implemented with a fold

[raulraja]

because you go from one ADT or Algebra to another.

[raulraja]

When your encoding is based on ADT that is frequently just folding the ADT and contemplating all cases