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Fantasy Do

Fantasy do is a generalization of mozilla's task.js from Promises/A+ to any monad, as defined by the Fantasy Land Spec. While usage is syntactically similar to Haskell's Do Notation, it's implemented completely at runtime with ES6 generators.

Fantasy Land Branding


Fantasy Do requires ES6 generators. In node, you can enable these with the --harmony option.

If you happen to have a browser with ES6 support, use the included fantasydo.min.js with your module loader of choice, or just with a script tag.

The library is available for node.js on npm under fantasydo

Normal mode

If your monad only calls the argument to the chain function once ('non-branching'), you can use Fantasy Do's normal, high-performance mode.

First, you'll need a monad. See the Fantasy Land Implementation List to find some monads that may work for you.

Here's a very simple Maybe monad:

var Maybe = {}
Maybe.chain = function(f) {
    if(this.val !== null)
        return f(this.val);
    return this;
Maybe.of = function(t) {
    return {"val" : t, "chain" : Maybe.chain};
Maybe.none = function() {
    return {"val" : null, "chain" : Maybe.chain};

Now, we can write a function that depends on multiple maybe values, and will only complete if all of them are not none:

    var a = yield Maybe.of(7);
    var b = yield Maybe.of(a + 9);
    return b;
 }, Maybe).val; // => 16

    var a = yield Maybe.of(7);
    var q = yield Maybe.none();
    var b = yield Maybe.of(a + 9);
    return b;
}, Maybe).val;  // => null


Fantasy Do also supports monads that may call their chain parameter multiple times, like the non-determinism context (aka the list monad). Without the ability to copy generator state, this works by re-runninng the generator from the beginning for each branch. This may behave strangely if your bind argument has significant side-effects.

Array.prototype.chain = function(f) {
  let next = [];
  let len = this.length;
      let v = f(it);
  return next;
Array.prototype.of = function(t) {return [t];}


    let c = yield [1, 2];
    let d = yield [c + 1, c + 2];
    return d;
}, Array.prototype); // => [2, 3, 3, 4]


To the extent possible under law, Russell McClellan has waived all copyright and related or neighboring rights to Fantasy Do. This work is published from: United States.


Do-notation for javascript fantasy land



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