crockford-exercises - functional programming

crockford.js

/*
 * This is a JavaScript Scratchpad.
 *
 * Enter some JavaScript, then Right Click or choose from the Execute Menu:
 * 1. Run to evaluate the selected text (Ctrl+R),
 * 2. Inspect to bring up an Object Inspector on the result (Ctrl+I), or,
 * 3. Display to insert the result in a comment after the selection. (Ctrl+L)
 */

function identityf(x) {
  return function() {
    return x;
  }
}


function add(x, y) {
  return x + y;
}

function mul(x, y) {
  return x * y;
}

function addf(x) {
  return function(y) {
    return x + y;
  }
}

// addf(3)(4)


function applyf(f) {
  return function (x) {
    return function (y) {
      return f(x, y)
    }
  }
}

// applyf(add)(3)(5)
// var mulf = applyf(mul)
// mulf(2)(8)

function curry(f, x) {
  return function (y) {
    return f(x, y)
  }
}

// curry(mul, 5)(6) 

function methodize(f) {
  return function (x) {
    return f(this, x)
  } 
}

// Number.prototype.add = methodize(add);

// (3).add(4)

function demethodize(f) {
  return function (x, y) {
    return f.call(x, y)
  }
}
// Number.prototype.add = methodize(add);
// demethodize(Number.prototype.add)(5, 6)

function twice(f) {
  return function(x) {
    return f(x, x)
  }
}
var double = twice(add);
var square = twice(mul);


// double(11)
// 22

function composeu(f, g) {
  return function(a) {
    return g(f(x))
  }
}

// composeu(double, square)(3)
/*
36
*/

function composeb(f, g) {
  return function(x, y, z) {
    return g(f(x, y), z)
  }
}

// composeb(add, mul)(2,3,5)
/*
25
*/

// function once(f) {
//   var called = false;
//   return function (x, y) {
//     if (called) {
//       throw Error('throw!')
//     }
//     called = true;
//     return f(x,y);
//   }
// }

// better approach
function once(func) {
  return function() {
    var f = func;
    func = null;
    return f.apply(this, arguments)
  }
}

// add_once = once(add);

// add_once(3,4)
// add_once(3,4)


function counterf(x) {
  return {
    inc: function() {
      return x++;
    },
    dec: function() {
      return x--;
    }
  }
}

// var counter = counterf(10);

// counter.inc();
// counter.dec();