[aaaaaaaaaaab]

  Constant Functor

  Object whose map doesn't transform the contents. See Functor

  Constant(1).map(n => n + 1)

Ummm… how is this constant?

[debugnik]

It ignores the mapping completely:

    constant.map(...) == constant

Constant functors are only ever useful if you need to thread a simple value through code that asks for an arbitrary functor. I'd say that's rare even for abstract, type-level heavy code.