Another hint of intuition for C#'ers out there. Foldable is like IEnumerable, and Traversable is a bit like ICollection. Some are a bit unhappy with the names t

Another hint of intuition for C#'ers out there. Foldable is like IEnumerable, and Traversable is a bit like ICollection. Some are a bit unhappy with the names too, even though they really make sense from the category theory sense. A thing about Foldable is that you can fold over it (obviously) but you can't necessarily map over it because in Haskell the general map (fmap of the Functor typeclass) has the law that `map (f1.f2) enum == ((map f1) . (map f2)) enum` but for example for Set this law won't hold because the intermediate set might drop elements identical elements that would not have been identical would the f1.f2 been applied in direct succession. See [0] for an example. Why fmap has this law I don't know, but Functor is super fundamental in Haskell's types so you can bet it's there for a good reason. Note that this Foldable-Traversable-Functor introduction into Prelude is not complete yet. For example the map in prelude is: `map :: (a -> b) -> [a] -> [b]`. If this movement continues it will possibly be `map :: (Functor f) => (a -> b) -> f a -> f b`. People are super scared of the name Functor, so I think that's why it's not been adopted yet. It should possibly be renamed to Container or Mappable to be more in line with what general developers expect. Would prelude's map be made equal to fmap however, it would make prelude super powerful. It would suddenly work on any Functor type, not just List. [0] https://www.fpcomplete.com/user/chad/snippets/random-code-snippets/set-is-not-a-functor https://www.fpcomplete.com/user/chad/snippets/random-code-sn... p.s.: if you don't grok typeclass, call it classclass instead, i.e. class of classes. And then think about how C#'s Interface also defines a class of classes.