It's like lisp, in JavaScript!

00_LICENSE.md

Functional JavaScript by Jesse Farmer is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

Contact Jesse at jesse@20bits.com if you have any questions!

01_tests.js

var isEven = function(n) { return n % 2 === 0; },
    isOdd = not(isEven);

var add = curry(function(a,b) { return a + b; }),
    inc = add(1),
    incAll = map(inc);

var times = curry(function(a,b) { return a * b; }),
    double = times(2),
    doubleAll = map(double);

console.log("" + cons(cons(1,2), cons(3,4)), "((1 2) (3 4))");
console.log("" + incAll(list(1,2,3)), "(2 (3 (4 null)))");

var evens = list(0,2,4,6,8),
    odds  = list(1,3,5,7,9);

var areAnyEven = any(isEven),
    areAnyOdd  = any(isOdd),
    areAllOdd  = all(isOdd);

console.log("" + areAnyOdd(odds), "true");
console.log("" + areAnyOdd(evens), "false");
console.log("" + areAllOdd(odds), "true");
console.log("" + areAllOdd(evens), "false");

console.log("" + areAnyOdd(doubleAll(odds)), "false");
console.log("" + areAnyEven(incAll(evens)), "false");
console.log("" + areAllOdd(incAll(evens)), "true");

var onlyEvens = filter(isEven),
    numbers = list(1,2,3,4);

console.log("" + onlyEvens(numbers), "(2 (4 null))");

functional.js

var argsToArray = Function.prototype.call.bind(Array.prototype.slice);

var curry = function(fn, filled) {
  var filled = filled || [];
  var arity = fn.length;

  return function() {
    var args = argsToArray(arguments);
    var newFilled = filled.concat(args);

    if (newFilled.length >= arity) {
      return fn.apply(this, newFilled);
    } else {
      return curry(fn, newFilled);
    }
  }
};

// A helper method to call 
var toString = function(o) {
  if (o !== null && typeof(o) !== 'object' && typeof(o) !== 'function') {
    return o.toString();
  } else {
    return "" + o;
  }
};

var pairify = function(fn) {
  fn.pair = true;

  fn.toString = function() {
    if (isEmpty(this)) { return "()"; }
    
    var head = first(this),
        tail = rest(this);

    // FIXME: This displays cons(1, cons(2, 3)) as (1 (2 3)) vs. (1 2 3)
    return "(" + toString(head) + " " + toString(tail) + ")";
  };

  return fn;
};

var cons = curry(function(x,y) {
  return pairify(function(fn) { return fn(x, y); });
});

var first = function(fn) {
  return fn(function(x, y) { return x });
};

var rest = function(fn) {
  return fn(function(x, y) { return y });
};

// e.g., list(1,2,3,4) == cons(1, cons(2, cons(3, cons(4, null))))
var list = function() {
  var args = argsToArray(arguments);
  return (args.length === 0) ? null : cons(args.shift(), list.apply(null, args));
};

var isAtom = function(x) {
  return (x !== null) && (x !== undefined) && !x.pair;
};
 
var isPair = function(x) {
  return x && x.pair;
};
 
var isNull = function(x) {
  return x === null;
}

var isZero = function(x) {
  return x === 0;
}

var isEmpty = function(lst) {
  return isNull(lst) || isNull(first(lst));
};

var foldl = curry(function(fn, acc, lst) {
    return isEmpty(lst) ? acc : foldl(fn, fn(acc, first(lst)), rest(lst));
});

var foldr = curry(function(fn, acc, lst) {
  return isEmpty(lst) ? acc : fn(first(lst), foldr(fn, acc, rest(lst)));
});

var map = curry(function(fn, lst) {
  return foldr(function(acc, lst) { return cons(fn(acc), lst) }, null, lst);
});

var filter = curry(function(fn, lst) {
  return foldr(function(acc, lst) { return fn(acc) ? cons(acc, lst) : lst }, null, lst);
});

var append = curry(function(to, lst) {
  return isEmpty(to) ? lst : cons(first(to), append(rest(to), lst));
});

var reverse = function(lst) {
  return isEmpty(lst) ? lst : append(reverse(rest(lst)), list(first(lst)));
};

var not = curry(function(pred, arg) {
  return !pred(arg)
});

var any = curry(function(pred, lst) {
  return isEmpty(lst) ? false : (pred(first(lst)) || any(pred, rest(lst)));
});

var all = curry(function(pred, lst) {
  return !any(not(pred), lst);
});

var flip = function(fn) {
  return curry(function(a,b) { return fn(b,a) });
};