fantasy-land | Specification for interoperability of common algebraic structures in JavaScript | Functional Programming library

One way to think about it is to think about types versus functions.

Ramda offers a large collection of utility functions. They operate on arrays, on objects, on functions, on strings, on numbers, etc. But they also operate on user-defined types. So in your example, R.map operates on anything which matches the Functor specification. If the implementation of Either you use matches that specification, then Ramda's map will operate on it.

But Ramda does not supply types. It works with the built-in types such as Object, Array, Function, etc. But -- arguably outside Lens -- it does not supply any types of its own. Libraries such as Folktale provide large collections of types, such as Maybe, Result, Validation, Task and Future; more dedicated ones such as Fluture provide powerful versions of one specific type (Future). All of these types implement the Functor specification. A very incomplete list of such implementations is supplied by FantasyLand.

These two notions, functions on an abstract type and collections of types are complementary. A Ramda function which works on any functor will work on whatever version of Either you use (so long as it matches the specification.) More on this relationship is in this StackOverflow Q+A.

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