Freer monad in javascript

monad.js

/* Copyright Freddy A Cubas "superstar64"
Boost Software License - Version 1.0 - August 17th, 2003

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// freer monads (copyible generators)
// based off http://okmij.org/ftp/Computation/free-monad.html

const pure = a => ({ id : 0, value : a });
const impure = f => m => ({ id : 1, value : {now : m, after : f}});
const match = (pure, impure) => m => [pure, ({now : m, after : f}) => impure(f)(m)][m.id](m.value);

const eta = impure(pure);
const rbind = k => match(k, k2 => impure(x => rbind(k)(k2(x))));
const bind = m => f => rbind(f)(m);
const apply = f => x => bind(f)(f => bind(x)(x => pure(f(x))));
const map = f => (...x) => x.reduce((f,x) => apply(f)(x), pure(f));

const run = ({pure, rbind}) => match(pure, f => rbind(x => run({pure,rbind}) (f(x))));

// dice example, trying out all possible cases

const list = {
    pure : x => [x],
    rbind : f => m => m.map(f).reduce((a,b) => a.concat(b), [])
};

// fair dice roll
const dice = eta([1,2,3,4,5,6]);

const possible = map(a => b => c => a + b + c)(
    dice,
    dice,
    dice
);

// sum of all possible triple dice rolls
console.log( run(list) (possible) );


// context example, implicitly passing context around

const reader = {
    pure : x => a => x,
    rbind : f => m => a => f(m(a))(a)
};

// get the context
const context = eta(a => a);

const square = map(x => x * x)(context);

const squareSum = map(a => b => a + b)(
    square,
    square
);

console.log ( run(reader) (squareSum) (10) );


// identity, the do nothing monad

const identity = {
  pure : x => ({wrapped : x}),
  rbind : f => ({wrapped : x}) => f(x)
}

const wrapped_five = eta ( { wrapped : 5 } );
const wrapped_six = map(x => x + 1)(wrapped_five);

console.log( run(identity) (wrapped_six).wrapped );