Hands-On Functional Programming with TypeScript

Applicative

An Applicative is a Functor that implements a method named of. However, an Applicative is not just a Functor type; it is also an Apply type. For a type to be an implementation of Apply, it must implement a method named ap that takes a Functor that wraps a function as an argument.

The following code snippet implements an Applicative and, as a result, it has an of, a map, and an ap method:

class Container<T> {    public static of<TVal>(val: TVal) {        return new Container(val);    }    private _value!: T;    public constructor(val: T) {        this._value = val;    }    public map<TMap>(fn: (val: T) => TMap) {        return new Container<TMap>(fn(this._value));    }    public ap<TMap>(c: Container<(val: T) => TMap>) {        return c.map(fn => this.map(fn));    }}

We can use the ...