Function

Functions x → b can be treated as functors (→ x) b (or Functor f ⇒ f b). The functor f or (→ x) represents all functions accepting x as argument.

For an introduction into that concept, explanations and examples see the article from Tom Harding.

Reader

Treating functions as functors for instance makes it possible to compose pipelines of functions which are applied sequentially to their respective return values – as usual – but which are only executed when called with an additional, possibly mutable, argument – usually called "environment". Invoked with just their "primary" pipeline-entry argument, the pipeline is not executed and is thus referentially transparent. The responsibility to permit reading from the environment is left for the final caller of this so-called Reader. This is much like a binary curried function a => b => someOperation(a, b) which happens to be the of factory for Reader of = x => env => x.

map

map :: Functor f ⇒ (b → c) → f b → f c
map :: (b → c) → (→ a) b → (→ a) c
map :: (b → c) → (a → b) → (a → c)

simply compose, really.

not implemented – use Ramda.map

Using Ramda

The Ramda version of the ADT functions ap, map and chain support not only StaticLand (i.e. native JavaScript) List functors (that is: Array) but also function functors.

Some function combinators – which are useful for programming in point-free style – can be implemented with help of the ADT functions: