Favoring curry

Mar 16, 2017

Currying is the process of turning a function that expects multiple parameters into one that, when supplied fewer parameters, returns a new function that awaits the remaining ones.

The basics look like this. Here’s a plain function:

// uncurried version
var formatName1 = function(first, middle, last) {
    return first + ' ' + middle + ' ' + last;
};
formatName1('John', 'Paul', 'Jones');
//=> 'John Paul Jones' // (Ah, but the musician or the admiral?)
formatName1('John', 'Paul');
//=> 'John Paul undefined');

But a curried version behaves more usefully:

// curried version
var formatNames2 = R.curry(function(first, middle, last) {
    return first + ' ' + middle + ' ' + last;
});
formatNames2('John', 'Paul', 'Jones');
//=> 'John Paul Jones' // (definitely the musician!)
var jp = formatNames2('John', 'Paul'); //=> returns a function
jp('Jones'); //=> 'John Paul Jones' (maybe this one's the admiral)
jp('Stevens'); //=> 'John Paul Stevens' (the Supreme Court Justice)
jp('Pontiff'); //=> 'John Paul Pontiff' (ok, so I cheated.)
jp('Ziller'); //=> 'John Paul Ziller' (magician, a wee bit fictional)
jp('Georgeandringo'); //=> 'John Paul Georgeandringo' (rockers)

['Jones', 'Stevens', 'Ziller'].map(jp);
//=> ['John Paul Jones', 'John Paul Stevens', 'John Paul Ziller']

And you can do this in multiple passes, as well:

var james = formatNames2('James'); //=> returns a function
james('Byron', 'Dean'); //=> 'James Byron Dean' (rebel)
var je = james('Earl'); also returns a function
je('Carter'); //=> 'James Earl Carter' (president)
je('Jones'); //=> 'James Earl Jones' (actor, Vader)

example

var data = {
    result: "SUCCESS",
    interfaceVersion: "1.0.3",
    requested: "10/17/2013 15:31:20",
    lastUpdated: "10/16/2013 10:52:39",
    tasks: [
        {id: 104, complete: false,            priority: "high",
                  dueDate: "2013-11-29",      username: "Scott",
                  title: "Do something",      created: "9/22/2013"},
        {id: 105, complete: false,            priority: "medium",
                  dueDate: "2013-11-22",      username: "Lena",
                  title: "Do something else", created: "9/22/2013"},
        {id: 107, complete: true,             priority: "high",
                  dueDate: "2013-11-22",      username: "Mike",
                  title: "Fix the foo",       created: "9/22/2013"},
        {id: 108, complete: false,            priority: "low",
                  dueDate: "2013-11-15",      username: "Punam",
                  title: "Adjust the bar",    created: "9/25/2013"},
        {id: 110, complete: false,            priority: "medium",
                  dueDate: "2013-11-15",      username: "Scott",
                  title: "Rename everything", created: "10/2/2013"},
        {id: 112, complete: true,             priority: "high",
                  dueDate: "2013-11-27",      username: "Lena",
                  title: "Alter all quuxes",  created: "10/5/2013"}
        // , ...
    ]
};

getIncompleteTaskSummaries = function(membername) {
    return fetchData()
        .then(function(data) {
            return data.tasks;
        })
        .then(function(tasks) {
            var results = [];
            for (var i = 0, len = tasks.length; i < len; i++) {
                if (tasks[i].username == membername) {
                    results.push(tasks[i]);
                }
            }
            return results;
        })
        .then(function(tasks) {
            var results = [];
            for (var i = 0, len = tasks.length; i < len; i++) {
                if (!tasks[i].complete) {
                    results.push(tasks[i]);
                }
            }
            return results;
        })
        .then(function(tasks) {
            var results = [], task;
            for (var i = 0, len = tasks.length; i < len; i++) {
                task = tasks[i];
                results.push({
                    id: task.id,
                    dueDate: task.dueDate,
                    title: task.title,
                    priority: task.priority
                })
            }
            return results;
        })
        .then(function(tasks) {
            tasks.sort(function(first, second) {
                var a = first.dueDate, b = second.dueDate;
                return a < b ? -1 : a > b ? 1 : 0;
            });
            return tasks;
        });
};

var getIncompleteTaskSummaries = function(membername) {
    return fetchData()
        .then(R.get('tasks'))
        .then(R.filter(R.propEq('username', membername)))
        .then(R.reject(R.propEq('complete', true)))
        .then(R.map(R.pick(['id', 'dueDate', 'title', 'priority'])))
        .then(R.sortBy(R.get('dueDate')));
};

Without currying:

var getIncompleteTaskSummaries = function(membername) {
    return fetchData()
        .then(function(data) {
            return R.get('tasks', data)
        })
        .then(function(tasks) {
            return R.filter(function(task) {
                return R.propEq('username', membername, task)
            }, tasks)
         })
        .then(function(tasks) {
            return R.reject(function(task) {
                return R.propEq('complete', true, task);
            }, tasks)
        })
        .then(function(tasks) {
            return R.map(function(task) {
                return R.pick(['id', 'dueDate', 'title', 'priority'], task);
            }, tasks);
        })
        .then(function(abbreviatedTasks) {
            return R.sortBy(function(abbrTask) {
                return R.get('dueDate', abbrTask);
            }, abbreviatedTasks);
        });
};

That, I think, is the equivalent. It’s still better than the original code. Ramda’s utility functions have some – er, utility – even in the absence of currying. But I don’t think it’s even close to as readable as the curried version.