See blog post http://tech.pro/blog/6742/callback-to-future-functor-applicative-monad

Future.js

/*
The MIT License (MIT)

Copyright (c) 2014 

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
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The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

function toArray(args) {
    return Array.prototype.slice.call(args);
}

function Future() {
    this.slots = [];
    this.failslots = [];
    this.actions = [];
    this.failactions = [];
}

Future.prototype.ready = function(slot) {    
    if(this.completed)
        slot(this.value);
    else
        this.slots.push(slot);
}

Future.prototype.failed = function(slot) {        
    if(this.hasFailed)
        slot(this.error);
    else
        this.failslots.push(slot);
}

Future.prototype.complete = function(val) {    
    var me = this;
    if(this.completed || this.hasFailed)
        throw "Can't complete an already settled future!";
    
    this.value = val;
    this.completed = true;
    
    for(var i=0, len=this.slots.length; i<len; i++) {
        this.slots[i](val);
    }
    this.slots = null;
    
    setTimeout(function() {
        for(var i=0, len=me.actions.length; i<len; i++) {
            me.actions[i](val);
        }
        me.actions = null;
    });
}

Future.prototype.fail = function(err) {
    var me = this;    
    if(this.completed || this.hasFailed)
        throw "Can't complete an already settled future!"
    
    this.hasFailed = true;
    this.error = err;
    
    for(var i=0, len=this.failslots.length; i<len; i++) {
        this.failslots[i](err);
    }
    this.failslots = null; 
    
    
    setTimeout(function() {
        for(var i=0, len=me.failactions.length; i<len; i++) {
            me.failactions[i](err);
        }
        me.failactions = null;
    });
}

Future.prototype.fmap = function(fn) {
    var fut = new Future();
    this.ready(function(val) {
        try {
            fut.complete( fn(val) );
        } catch(err) {
            fut.fail(err);
        }
        
    });
    this.failed(function(err) {
        fut.fail( err );
    });
    return fut;
}

Future.prototype.fmapError = function(fn) {
    var fut = new Future();
    this.ready(function(val) {
        fut.complete( val );
    });
    this.failed(function(err) {
        try {
            fut.complete( fn(err) );
        } catch(err1) {
            fut.fail( err1 );
        }
        
    });
    return fut;
}

Future.prototype.flatten = function() {
    var fut = new Future();
    this.failed(function(err) {
        fut.fail(err);
    });
    this.ready(function(fut2) {
        fut2.failed( function(err){
            fut.fail(err);
        } );
    }); 
    this.ready(function(fut2) {
        fut2.ready( function(val){
            fut.complete(val);
        } );
    });      
    return fut;
}

Future.prototype.flatMap = function( fn ) {
    return this.fmap(fn).flatten();
}

Future.prototype.flatMapError = function( fn ) {
    return this.fmapError(fn).flatten();
}

Future.lift1 = function(fn) {
    return function(fut) {
        return fut.fmap(f);
    }
}

Future.lift2 = function(fn) {
    return function(fut1, fut2) {
        return fut1.flatMap(function(value1) {
            return fut2.flatMap(function(value2) {
                return Future.unit( fn(value1, value2) );
            });
        });
    }
}

Future.lift3 = function(fn) {
    return function(fut1, fut2, fut3) {
        return fut1.flatMap(function(value1) {
            return fut2.flatMap(function(value2) {
                return fut3.flatMap(function(value3) {
                    return Future.unit( fn(value1, value2, value3) );
                });                
            });
        });
    }
}

Future.lift = function(fn) {
    return function() {
        var args = toArray(arguments),
            ctx = this;
        
        return bindArg(0, []);
        
        function bindArg(index, actArgs) {                
            return args[index].flatMap(function(val) {
                actArgs = actArgs.concat(val);
                return (index <  args.length - 1) ?
                            bindArg(index+1, actArgs) :
                            Future.unit( fn.apply(ctx, actArgs) );
            });           
        }
    }
}

Future.never = function() {
    return new Future();   
}

Future.unit = function(val) {
    var fut = new Future();
    fut.complete(val);
    return fut;
}

Future.delay = function(v, ms) {
    var f = new Future();
    setTimeout(function(){
        f.complete(v);
    }, ms);
    return f;
}

function logF(f) {
    f.do(function(v) {
        console.log('Future completed with', v);
    });
}

function logErrF(f) {
    f.doError(function(err) {
        console.log('Future failed with', err);
    });
}


var fs = require('fs');

function readFileF(file, options) {
    var f = new Future();
    fs.readFile(file, options, function (err, data) {
        if (err) f.fail(err);
        else
            f.complete(data);
    });
    return f;
}

function readDirF(path) {
    var f = new Future();
    fs.readdir(path, function (err, files) {
        if (err) f.fail(err);
        else 
            f.complete(files);
    });
    return f;
}

var concat = Future.lift(function() {
    return toArray(arguments).join(' ');
});

result = readDirF('testdir').flatMap(function(files) {
    var filesF = files.map( function(file) {        
        return readFileF('testdir/' + file, {encoding: 'utf8'});
    });
    return concat.apply(null, filesF);
});

logF( result );

I found that do/doErr/ready/failed are each only checking one of the settled conditions (this.completed or this.hasFailed) before they add the given function to one of the arrays. They should all check both before they do that since the arrays are all null in either case.

The changes I needed to get the behaviour correct are here: davidmason/composable-future@8f3b8e4

Great post, appreciated!

(fut2 should prob be fut3)

Really helpful, thanks.

I think it should be this.failactions.push ?

Should this become a package on npm?