maybe.md

Maybe {#maybe}

A common problem in programming is checking for null or undefined (hereafter called "nothing," while all other values including 0, [] and false will be called "something"). Languages like JavaScript do not strongly enforce the notion that a particular variable or particular property be something, so programs are often written to account for values that may be nothing.

This recipe concerns a pattern that is very common: A function fn takes a value as a parameter, and its behaviour by design is to do nothing if the parameter is nothing:

function isSomething (value) {
  return value !== null && value !== void 0;
}

function checksForSomething (value) {
  if (isSomething(value)) {
    // function's true logic
  }
}

Alternately, the function may be intended to work with any value, but the code calling the function wishes to emulate the behaviour of doing nothing by design when given nothing:

var something = isSomething(value) ? 
  doesntCheckForSomething(value) : value;

Naturally, there's a recipe for that, borrowed from Haskell's maybe monad, Ruby's andand, and CoffeeScript's existential method invocation:

function maybe (fn) {
  return function () {
    var i;
    
    if (arguments.length === 0) {
      return
    }
    else {
      for (i = 0; i < arguments.length; ++i) {
        if (arguments[i] == null) return
      }
      return fn.apply(this, arguments)
    }
  }
}

maybe reduces the logic of checking for nothing to a function call, either:

var checksForSomething = maybe(function (value) {
  // function's true logic
});

Or:

var something = maybe(doesntCheckForSomething)(value);

As a bonus, maybe plays very nicely with instance methods, we'll discuss those later:

function Model () {};

Model.prototype.setSomething = maybe(function (value) {
  this.something = value;
});

If some code ever tries to call model.setSomething with nothing, the operation will be skipped.