This type class is exposed in addition to Applicative because there are data types for which we can't implement pure, but that could still benefit from an ap definition. For example in case of a Map<K, ?> we couldn't define pure for it because we don't have a K key.

MUST obey the laws defined in ApplyLaws.

Note that having an Apply instance implies that a Functor implementation is also available, which is why Apply is a subtype of Functor.

Implementation notes

Even though in TypeScript the Funfix library is using abstract class to express type classes, when implementing this type class it is recommended that you implement it as a mixin using "implements", instead of extending it directly with "extends". See TypeScript: Mixins for details and note that we already have applyMixins defined.

Implementation example:

import {
  HK, Apply,
  registerTypeClassInstance,
  applyMixins
} from "funfix"

// Type alias defined for readability.
// HK is our encoding for higher-kinded types.
type BoxK<T> = HK<Box<any>, T>

class Box<T> implements HK<Box<any>, T> {
  constructor(public value: T) {}

  // Implements HK<Box<any>, A>, not really needed, but useful in order
  // to avoid type casts. Note these can and should be undefined:
  readonly _funKindF: Box<any>
  readonly _funKindA: T
}

class BoxApply implements Apply<Box<any>> {
  map<A, B>(fa: BoxK<A>, f: (a: A) => B): Box<B> {
    const a = (fa as Box<A>).value
    return new Box(f(a))
  }

  ap<A, B>(fa: BoxK<A>, ff: BoxK<(a: A) => B>): Box<B> {
    const a = (fa as Box<A>).value
    const f = (ff as Box<(a: A) => B>).value
    return new Box(f(a))
  }

  // Mixed-in, as these have default implementations
  map2: <A, B, Z>(fa: BoxK<A>, fb: BoxK<B>, f: (a: A, b: B) => Z) => Box<Z>
  product: <A, B> (fa: BoxK<A>, fb: BoxK<B>) => Box<[A, B]>
}

// Call needed in order to implement `map2` and `product` using
// the default implementations defined by `Apply`, because
// we are using `implements` instead of `extends` above and
// because in this sample we want the default implementations,
// but note that you can always provide your own definitions
applyMixins(BoxApply, [Apply])

// Registering global Apply instance for Box, needed in order
// for the `applyOf(Box)` calls to work
registerTypeClassInstance(Apply)(Box, new BoxApply())

We are using implements in order to support multiple inheritance and to avoid inheriting any static members. In the Flow definitions (e.g. .js.flow files) for Funfix these type classes are defined with "interface", as they are meant to be interfaces that sometimes have default implementations and not classes.

Credits

This type class is inspired by the equivalent in Haskell's standard library and the implementation is inspired by the Typelevel Cats project.