chapter05.js

function dispatch(/* funs */) {
  var funs = _.toArray(arguments);
  var size = funs.length;

  return function(target /*, args */) {
    var ret = undefined;
    var args = _.rest(arguments);

    for (var funIndex = 0; funIndex < size; funIndex++) {
      var fun = funs[funIndex];
      ret = fun.apply(fun, construct(target, args));

      if (existy(ret)) return ret;
    }

    return ret;
  };
}

var str = dispatch(invoker('toString', Array.prototype.toString),
invoker('toString', String.prototype.toString));

str("a");
//=> "a"

str(_.range(10));
//=> "0,1,2,3,4,5,6,7,8,9"

function stringReverse(s) {
  if (!_.isString(s)) return undefined;
  return s.split('').reverse().join("");
}

stringReverse("abc");
//=> "cba"

stringReverse(1);
//=> undefined

var rev = dispatch(invoker('reverse', Array.prototype.reverse),
stringReverse);

rev([1,2,3]);
//=> [3, 2, 1]

rev("abc");
//=> "cba"

function performCommandHardcoded(command) {
  var result;

  switch (command.type) 
  {
  case 'notify':
    result = notify(command.message);
    break;
  case 'join':
    result = changeView(command.target);
    break;
  default:
    alert(command.type);
  }

  return result;
}

function isa(type, action) {
  return function(obj) {
    if (type === obj.type)
      return action(obj);
  }
}

var performCommand = dispatch(
  isa('notify', function(obj) { return notify(obj.message) }),
  isa('join',   function(obj) { return changeView(obj.target) }),
  function(obj) { alert(obj.type) });

var performAdminCommand = dispatch(
  isa('kill', function(obj) { return shutdown(obj.hostname) }),
  performCommand);

var performTrialUserCommand = dispatch(
  isa('join', function(obj) { alert("Cannot join until approved") }),
  performCommand);

function rightAwayInvoker() {
  var args = _.toArray(arguments);
  var method = args.shift();
  var target = args.shift();

  return method.apply(target, args);
}

rightAwayInvoker(Array.prototype.reverse, [1,2,3])
//=> [3, 2, 1]

function leftCurryDiv(n) {
  return function(d) {
    return n/d;
  };
}

function rightCurryDiv(d) {
  return function(n) {
    return n/d;
  };
}

function curry(fun) {
  return function(arg) {
    return fun(arg);
  };
}

function curry2(fun) {
  return function(secondArg) {
    return function(firstArg) {
        return fun(firstArg, secondArg);
    };
  };
}

function div(n, d) { return n / d }

var div10 = curry2(div)(10);

div10(50);
//=> 5

var parseBinaryString = curry2(parseInt)(2);

parseBinaryString("111");
//=> 7

parseBinaryString("10");
//=> 2

var plays = [{artist: "Burial", track: "Archangel"},
             {artist: "Ben Frost", track: "Stomp"},
             {artist: "Ben Frost", track: "Stomp"},
             {artist: "Burial", track: "Archangel"},
             {artist: "Emeralds", track: "Snores"},
             {artist: "Burial", track: "Archangel"}];

_.countBy(plays, function(song) {
  return [song.artist, song.track].join(" - ");
});

//=> {"Ben Frost - Stomp": 2,
//    "Burial - Archangel": 3,
//    "Emeralds - Snores": 1}

function songToString(song) {
  return [song.artist, song.track].join(" - ");
}

var songCount = curry2(_.countBy)(songToString);

songCount(plays);
//=> {"Ben Frost - Stomp": 2,
//    "Burial - Archangel": 3,
//    "Emeralds - Snores": 1}

function curry3(fun) {
  return function(last) {
    return function(middle) {
      return function(first) {
        return fun(first, middle, last);
      };
    };
  };
};

var songsPlayed = curry3(_.uniq)(false)(songToString);

songsPlayed(plays);

//=> [{artist: "Burial", track: "Archangel"},
//    {artist: "Ben Frost", track: "Stomp"},
//    {artist: "Emeralds", track: "Snores"}]

function toHex(n) {
  var hex = n.toString(16);
  return (hex.length < 2) ? [0, hex].join(''): hex;
}

function rgbToHexString(r, g, b) {
  return ["#", toHex(r), toHex(g), toHex(b)].join('');
}

rgbToHexString(255, 255, 255);
//=> "#ffffff"

var blueGreenish = curry3(rgbToHexString)(255)(200);

blueGreenish(0);
//=> "#00c8ff"

var greaterThan = curry2(function (lhs, rhs) { return lhs > rhs });
var lessThan    = curry2(function (lhs, rhs) { return lhs < rhs });

var withinRange = checker(
  validator("arg must be greater than 10", greaterThan(10)),
  validator("arg must be less than 20", lessThan(20)));

function divPart(n) {
  return function(d) {
    return n / d;
  };
}

var over10Part = divPart(10);
over10Part(2);
//=> 5

function partial1(fun, arg1) {
  return function(/* args */) {
    var args = construct(arg1, arguments);
    return fun.apply(fun, args);
  };
}

function partial2(fun, arg1, arg2) {
  return function(/* args */) {
    var args = cat([arg1, arg2], arguments);
    return fun.apply(fun, args);
  };
}

var div10By2 = partial2(div, 10, 2)

div10By2()
//=> 5

function partial(fun /*, pargs */) {
  var pargs = _.rest(arguments);

  return function(/* arguments */) {
    var args = cat(pargs, _.toArray(arguments));
    return fun.apply(fun, args);
  };
}

var zero = validator("cannot be zero", function(n) { return 0 === n
});
var number = validator("arg must be a number", _.isNumber);

function sqr(n) {
  if (!number(n)) throw new Error(number.message);
  if (zero(n))    throw new Error(zero.message);

  return n * n;
}

function condition1(/* validators */) {
  var validators = _.toArray(arguments);

  return function(fun, arg) {
    var errors = mapcat(function(isValid) {
      return isValid(arg) ? [] : [isValid.message];
    }, validators);

    if (!_.isEmpty(errors))
      throw new Error(errors.join(", "));

    return fun(arg);
  };
}

var sqrPre = condition1(
  validator("arg must not be zero", complement(zero)),
  validator("arg must be a number", _.isNumber));

function uncheckedSqr(n) { return n * n };

uncheckedSqr('');
//=> 0

var checkedSqr = partial1(sqrPre, uncheckedSqr);

var sillySquare = partial1(
  condition1(validator("should be even", isEven)),
  checkedSqr);

var validateCommand = condition1(
  validator("arg must be a map", _.isObject),
  validator("arg must have the correct keys", hasKeys('msg',
'type')));

var createCommand = partial(validateCommand, _.identity);

var createLaunchCommand = partial1(
  condition1(
    validator("arg must have the count down", hasKeys('countDown'))),
  createCommand);

var isntString = _.compose(function(x) { return !x }, _.isString);

isntString([]);
//=> true

function not(x) { return !x }

var composedMapcat = _.compose(splat(cat), _.map);

composedMapcat([[1,2],[3,4],[5]], _.identity);
//=> [1, 2, 3, 4, 5]

var sqrPost = condition1(
  validator("result should be a number", _.isNumber),
  validator("result should not be zero", complement(zero)),
  validator("result should be positive", greaterThan(0)));

var megaCheckedSqr = _.compose(partial(sqrPost, _.identity),
checkedSqr);