functional.js

gistfile1.js

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

function reduce0(fn, l, init) {
  for(var i = 0; i < l.length; i++) {
    init = fn(init, l[i]);
  }
  return init;
}

function reduce(fn, l, init) {
  if (init === undefined) {
    init = l[0], l = l.slice(1);
  }
  return reduce0(fn, l, init);
}

function map(fn, l) {
  return reduce(function(acc, e) {
    acc.push(fn(e));
    return acc;
  }, l, []);
}

function filter(fn, l) {
  return reduce(function(acc, e) {
    if (fn(e)) {
      acc.push(e);
    }
    return acc;
  }, l, []);
}

function concat() {
  var args = argList(arguments);
  return reduce(function(acc, e) {
    return Array.prototype.concat.call(acc, e);
  }, args, []);
}

function mapcat(fn, l) {
  return reduce(function(acc, e) {
    return concat(acc, fn(e));
  }, l, []);
}

function partial(f) {
  var args =  argList(arguments).slice(1);
  return function() {
    return f.apply(null, concat(args, argList(arguments)));
  }
}

function compose0(f, g) {
  return function() {
    var args = argList(arguments);
    return f(g.apply(null, args));
  }
}

function compose() {
  return reduce(compose0, argList(arguments));
}

function juxt() {
  var fns = argList(arguments);
  return function() {
    var args = argList(arguments);
    return map(function(f) { return f.apply(null, args) }, fns);
  }
}


var inc = function(n) { return n + 1; }
var even = function(n) { return n % 2 == 0; }
var add = function(a, b) { return a + b; }
var mul = function(a, b) { return a * b; }
var double = partial(mul, 2);
var incDouble = compose(double, inc);
var both = juxt(inc, double);

console.log(map(inc, [1,2,3,4]));
console.log(filter(even, [1,2,3,4,5]));
console.log(reduce(add, [1]));
console.log(concat([1,2,3], [4,5,6]));
console.log(mapcat(both, [1,2,3]));
console.log(double(10));
console.log(incDouble(10));
console.log(both(10));