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kotlinlang

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?

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

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

It looks like `(Functor f, Functor g) =&gt; f a -&gt; g a` (sorry if the notation is wrong)

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

but that usually is implemented with a fold

because you go from one ADT or Algebra to another.

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