pure functions

12 April 2015

pure function

Functions that don’t change anything are called pure

example:

impure

this function become hard to test because you can’t easily mock window.location object

function getQueryVariable(variable) {
	var query = window.location.search.substring(1);
	var vars = query.split('&');
    for(var i = 0; i < vars.length; i++) {
		var pair = vars[i].split('=');
		if(decodeURIComponent(pair[0]) === variable) {
			return decodeURIComponent(pair[1]);
		}
	}
}

map

we all know [array].map(doSth) will iterator the array and execute doSth on it, actually map can also apply on container, which means, open the container, execute the callback on the value in the container then put it back to the container

Container([1,2,3]).map(reverse).map(first)  // return Container(3)


var map = _.curry(function(f, object) {
	return object.map(f);
});

Container(3).map(add(1))  //Container(4)
map(add(1), Container(3)) //Container(4)

map(match(/cat/g), Container('catsup'));
map(compose(first, reverse), Container('dog'));

Functor

An object or data structure you can map over

Maybe

var _Maybe.prototype.map = function(f) {
	return this.val? Maybe(f(this.val)):Maybe(null);
}

map(capitalize, Maybe(null));  //=> Maybe(null)

var firstMatch = compose(first, match(/cat/g));

firstMatch("dogsup");


var firstMatch = compose(map(first), Maybe, match(/cat/g));

Functor Laws

_Identity.prototype.map = function(f) {
    return Identity(f(this.val));
}

map = _.curry(function(fn, obj) {
	return obj.map(fn);
})

map(id) == id

compose(map(f), map(g)) == map(compose(f,g))

compose(map(compose(toUpper,reverse)), toArray)("bingo")
==
compose(map(toUpper), map(reverse), toArray)("bingo")

javascript 21

function 3