Playing with Future

future_example.js

var R = require('ramda');
var Future = require('ramda-fantasy').Future;
//Wrap ajax in a future
//:: String -> Future String
var fetch = function(url) {
    return new Future(function(rej, res){
        var oReq = new XMLHttpRequest();
        oReq.addEventListener("load", res, false);
        oReq.addEventListener("error", rej, false);
        oReq.addEventListener("abort", rej, false);
        oReq.open("get", url, true);
        oReq.send();
    });
};

//:: String -> Future Object
// Could use Either instead of Future but they work about the same.
var parseJSON = function(str){
    return new Future(function(rej, res){
        try {
            res(JSON.parse(str));
        } catch (err) {
            rej({error: "json parse error"});
        }
    });
};

//We have
// String -> Future String
// And
// String -> Future Object
// So we can .chain() them together
var fetchJSON = fetch.chain(parseJSON);

// Get the items out of it?
//:: Future Object -> Future []
var fetchItems = fetchJSON.map(R.prop('items'));

//BTW at this point in the code the request still hasn't been sent

// Filter the response?
// Have to map first to get at the contents of the future then filter
//:: Future [Object] -> Future [Object]
var fetchNewItems = fetchItems.map(R.filter(R.prop('new')))

// Just get the titles of the items
//:: Future [Object] -> Future [String]
var getNewTitles = fetchNewItems.map(R.map('title'));

//Finally do something
getNewTitles.fork(console.log, console.log);
//Now the AJAX req is sent and will log success or failure to console.