what's the simplest monad implementation we could express in JS?

simplest-monad.js

function Identity(v) {
  return { val: v };
}

function chain(m,fn) {
  return fn(m.val);
}

// ^^^ that's it!



// extra helpers:
function IdentityMap(m,fn) {
  return chain( m, v => Identity(fn(v)) );
}

function inc(v) { return Identity(v + 1); }
function double(v) { return Identity(v * 2); }


// use Identity:
const fortyOne = Identity(41);   // { val: 41 }
// const fortyTwo = chain( fortyOne, inc );
const fortyTwo = IdentityMap( fortyOne, v => v + 1 );   // { val: 42 }


// monad law #1: Left Identity
chain( Identity(41), inc );    // { val: 42 }

// monad law #2: Right Identity
chain( Identity(42), Identity );   // { val: 42 }

// monad law #3: Associativity
chain( chain( Identity(20), inc ), double );           // { val: 42 }
chain( Identity(20), v => chain( inc(v), double ) );   // { val: 42 }