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In normal Javascript, we do undefine check or null check:
var person = {age: 14, name: "Suvi"}; var name = person.name ? person.name: null;
Sometime backend data return may contain or not contain ‘name‘ prop.
So let‘s see how to define a Maybe() functor:
var _Maybe.prototype.map = function(f) { return this.val ? Maybe(f(this.val)) : Maybe(null); } map(capitalize, Maybe("flamethrower")) //=> Maybe(“Flamethrower”)
The idea of Maybe is check whetehr the Container has value or not, if not return Maybe(null), if has then return Maybe(val).
// Exercise 3 // ========== // Use safeGet and _.head to find the first initial of the user
var _ = R;
var P = PointFree;
var map = P.fmap;
var compose = P.compose;
var Maybe = P.Maybe;
var Identity = P.Id;
var safeGet = _.curry(function(x,o){ return Maybe(o[x]) }) var user = {id: 2, name: "Albert"} console.log("--------Start exercise 3--------") var ex3 = compose(map(_.head), safeGet(‘name‘)); assertDeepEqual(Maybe(‘A‘), ex3(user)) console.log("exercise 3...ok!")
So after the "safeGet(‘name‘)" run, it return "Maybe(‘Albert‘)";
Then we use map(_.head): map--> get into the Maybe to check val;
_.head --> Get teh ‘A‘ or ‘Albert‘.
The good thing about this is if use ‘safeGet(‘address‘)‘ which inside ‘user‘ object doesn‘t provide, then we will get ‘Maybe(null)‘:
"Uncaught expected Maybe(A) to equal Maybe(null) (line 68)"
// Exercise 4 // ========== // Use Maybe to rewrite ex4 without an if statement console.log("--------Start exercise 4--------")
var _ = R;
var P = PointFree;
var map = P.fmap;
var compose = P.compose;
var Maybe = P.Maybe;
var Identity = P.Id;
var ex4 = function(n) { if(n){ return parseInt(n); } } var ex4 = compose(map(parseInt), Maybe) assertDeepEqual(Maybe(4), ex4("4")) console.log("exercise 4...ok!")
So when the value comes into the ex4 which we wrote, first we will get "Maybe(4)";
Because Maybe(4) is inside a Functor, then we need to call map() on it to get value;
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原文地址：http://www.cnblogs.com/Answer1215/p/5847818.html
