A functor is a value-containing structure with a map operation. Map applies a function to the contained value and returns a structure of the expected type. JavaScript arrays offer a familiar example: mapping transforms their elements and produces another array. The same interface can make custom objects participate in function-composition pipelines. section 5.4.1 section 5.4
A custom map should obey two rules:
- Identity: mapping the identity function leaves the contained value equivalent, inside a new container of the same type. section 5.4.1
- Composition: mapping one function and then another is equivalent to mapping their composition. section 5.4.1
A reusable map
An Id container can provide `of(value)` to wrap a value and `get()` to retrieve it. A Functor mixin then defines `map(f = identity) { return this.constructor.of(f(this.get())); }`. Thus `Id.of('aabbcc').map(unique).map(join).map(toUpper).get()` yields `'ABC'`. Each map creates another Id, so transformations remain chainable without changing the original container. section 5.4.1 section 1.3
Map can also express conditional processing. With a Validation type, Success maps a transformation over its value, while Failure skips it; both branches retain a composable interface. If a mapped function itself returns a container, ordinary map can produce nested containers. A monad adds flatMap for that case. section 5.5.4 section 5.4.2
A shared map contract lets standalone functions from Ramda delegate to custom objects. For implementations where the resulting constructor matters, `Symbol.species` can document and select the type produced by map. section 5.5.8 section 7.5.3
