[Javascript]Functor Baic Intro

Well,this stuff will be a little bit strange if you deal with it first time.
 
Continer Object:

Just a wrapper/contianer for values

No Method

      No Nouns

var _Container = function(val){
  this.val = val;
}

var Container = function(x){
  return new _Container(x);
}

console.log(Container(3)) // _Container( {val: 3})

Every time we use Continer,it will just add the value to the Continer oject and assign value to prop val.
 
map function on Continer:
This map is not the map what you think it is… 
The map you think is the method on the Aray.
Here the map is the one get into the Continer,grep the value,then appy function on this value.The return value is still Continer.

_Container.prototype.map = function(f){
  return Container(f(this.val))
}

var res = Container("flamethrower").map(function(s){ return _.capitalize(s) })
console.log(res) //_Container( {val: "Flamethrower"} )

So in the example,map function goes into the Continer,get the value「flame ethrower」,and applythe function'u.capitialize'to the value and return the new value to the Conttainer.
Or you can write like this:

var capitalize = _.capitalize;
var res = Container("flamethrower").map(capitalize);

 
More examples:

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


Container("flamethrower").map(length).map(add(1))
//=> Container(13)

 
So「Continer」…what you feel about it?It is nothing...you don't need to care about it.Just evry time you cal map on it,the return value will still inside the Continer,so that you can chain map on it.
 
Curry map:
Define a global map function、which use Ramda curry method:

var map = R.curry(function(f, obj) {
  return obj.map(f) 
})

Later we Will pass Continer as obj、and on our Continer、we already defined map function、so we can use here as'obj.map'.
So,again,here'obj.map'->Means,goes into the obj,grep the value and apply function f.
 
So now what we can do:

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

map(add(1), Container(3)) // Container(4), or map(add(1))(Container(3)), since map is curry method

More exmaples:

map(R.compose(R.head, R.reverse), Container("dog"))
//=> Container(“g”)

 
So all what we have seen so far,we give a name call“Funtor”.
 
Functor
「An object or data struct you map over」
 function:map
 

// Exercise 1
// ==========
// Use _.add(x,y) and map(f,x) to make a function that increments a value inside a functor
console.log("--------Start exercise 1--------")
//map(): Go inside the object and run the function on the value of the object
//map(): Here map is curry function, so take the function as first arguement and object as second arguement.
//map(_.add(1), Identity(2)) --> Identity(3)
var ex1 = map(_.add(1));


assertDeepEqual(Identity(3), ex1(Identity(2)))
console.log("exercise 1...ok!")





// Exercise 2
// ==========
// Use _.head to get the first element of the list
var xs = Identity(['do', 'ray', 'me', 'fa', 'so', 'la', 'ti', 'do'])
console.log("--------Start exercise 2--------")


var ex2 = map(_.head)


assertDeepEqual(Identity('do'), ex2(xs))
console.log("exercise 2...ok!")