JavaScript Functional JavaScript Identity Monad
Example
This is an example of an implementation of the identity monad in JavaScript, and could serve as a starting point to create other monads.
Based on the conference by Douglas Crockford on monads and gonads
Using this approach reusing your functions will be easier because of the flexibility this monad provides, and composition nightmares:
f(g(h(i(j(k(value), j1), i2), h1, h2), g1, g2), f1, f2)
readable, nice and clean:
identityMonad(value)
.bind(k)
.bind(j, j1, j2)
.bind(i, i2)
.bind(h, h1, h2)
.bind(g, g1, g2)
.bind(f, f1, f2);
function identityMonad(value) {
var monad = Object.create(null);
// func should return a monad
monad.bind = function (func, ...args) {
return func(value, ...args);
};
// whatever func does, we get our monad back
monad.call = function (func, ...args) {
func(value, ...args);
return identityMonad(value);
};
// func doesn't have to know anything about monads
monad.apply = function (func, ...args) {
return identityMonad(func(value, ...args));
};
// Get the value wrapped in this monad
monad.value = function () {
return value;
};
return monad;
};
It works with primitive values
var value = 'foo',
f = x => x + ' changed',
g = x => x + ' again';
identityMonad(value)
.apply(f)
.apply(g)
.bind(alert); // Alerts 'foo changed again'
And also with objects
var value = { foo: 'foo' },
f = x => identityMonad(Object.assign(x, { foo: 'bar' })),
g = x => Object.assign(x, { bar: 'foo' }),
h = x => console.log('foo: ' + x.foo + ', bar: ' + x.bar);
identityMonad(value)
.bind(f)
.apply(g)
.bind(h); // Logs 'foo: bar, bar: foo'
Let's try everything:
var add = (x, ...args) => x + args.reduce((r, n) => r + n, 0),
multiply = (x, ...args) => x * args.reduce((r, n) => r * n, 1),
divideMonad = (x, ...args) => identityMonad(x / multiply(...args)),
log = x => console.log(x),
substract = (x, ...args) => x - add(...args);
identityMonad(100)
.apply(add, 10, 29, 13)
.apply(multiply, 2)
.bind(divideMonad, 2)
.apply(substract, 67, 34)
.apply(multiply, 1239)
.bind(divideMonad, 20, 54, 2)
.apply(Math.round)
.call(log); // Logs 29
