BAM Weblog

Roy - Monad Syntax

Brian McKenna2011-06-19

I added monad syntax to Roy. It is very close to do-notation, with the exception that the monad implementation is explicit:

data Option a =
  Some a | None

let optionMonad = {
  return: fn x =
    Some x
  bind: fn x f = match x
    case (Some a) = f a
    case None = None

console.log (do optionMonad
  bind x = Some 1
  let y = 2
  bind z = Some 3
  return x + y + z

Note the do optionMonad. Explicitness is only necessary because Roy doesn't have any form of interfaces or type classes. An upside is that a single datatype could have multiple monad implementations, something not allowed in most other languages.

I also worked on prettying up the JS output. It now looks like this:

"use strict";
var Some = function(a){this._0 = a};
var None = function(){};
var optionMonad = {
    "return": function(x) {
        return new Some(x);
    "bind": function(x, f) {
        return (function() {
            if(x instanceof Some) {
                var a = x._0;
                return f(a)
            } else if(x instanceof None) {
                return new None()
    var __monad__ = optionMonad;
    return __monad__["bind"](new Some(1), function(x) {
        var y = 2;
        return __monad__["bind"](new Some(3), function(z) {
            return __monad__["return"](x + y + z);
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