文章目錄

1. 1. Evalulation Argunment first
   1. 1.1. Call-By-Value
   2. 1.2. Call-By-Reference
2. 2. Evaluation Argunment when needed
   1. 2.1. Call-By-Name
      1. 2.1.1. Clousre in Javascript
   2. 2.2. Call-By-Need
   3. 2.3. Call-By-Text

主要分為兩種Evalulation first, or Evaluation when needed

Evalulation Argunment first

Call-By-Value

最common 的做法，先計算parameter 的value，然後把value copy 進function內
不會影響出面的數值

#include <iostream>

using namespace std;

void addOne(int n){
    n = n + 1;
}

int main(){
    int x = 5;
    cout << "before function is called: " << x << endl;
    addOne(x);
    cout << "after function is called: " << x << endl;

    return 0;
}


Output:
before function is called: 5
after function is called: 5

Call-By-Reference

把呼叫者(caller)的variable 直接在function內使用, 但叫第二個名字（Reference)
可以有side effect 出面的數值

#include <iostream>

using namespace std;

void addOne(int &n){
    n = n + 1;
}

int main(){
    int x = 5;
    cout << "before function is called: " << x << endl;
    addOne(x);
    cout << "after function is called: " << x << endl;

    return 0;
}


Output:
before function is called: 5
after function is called: 6

Evaluation Argunment when needed

Call-By-Name

用的時侯才把parameter evaluate
在passing as parameter是產生closure (enclosing environement) 裝著evaluation所需的variable

以下add function 中的n 是call-by-name的，
n 是array(index), 當中的index 是記憶著上面的index

object CallByName {
  var array = Array(0, 1, 2, 3)
  var index = 0 // 這是closure 內的variable
  
  def add(n: => Int) : Int = {
    return n;
  }
  
  def callAdd() = {
    add(array(index)) // 我連著上面的 index !!
  }
  
}

defined object CallByName

scala> CallByName.callAdd
res2: Int = 0

所以如果closure 內的variable數值被外內改變
會影響evaluation的結果

object CallByName {
  var array = Array(0, 1, 2, 3)
  var index = 0 
  
  def add(n: => Int) : Int = {
    index = 1; // 改變了closure 內的variable 數值
    return n;
  }
  
  def callAdd() = {
    add(array(index))
  }
  
}



defined object CallByName

scala> CallByName.callAdd
res3: Int = 1

改變local variable 是沒有作用的

object CallByName {
  var array = Array(0, 1, 2, 3)
  var index = 0
  
  def add(n: => Int) : Int = {
    var index = 1; // 這不是closure內的variable
    return n;
  }
  
  def callAdd() = {
    add(array(index))
  }
  
}

defined object CallByName

scala> CallByName.callAdd
res2: Int = 0

Clousre in Javascript

利用javascript 達成類似call-by-name的效果

function CallByName(){
    var array = [0, 1, 2, 3];
    var index = 0;

    this.add = function(f){
        index = 1;
        return f();
    }

    this.callAdd = function(){
        return this.add(function(){
            return array[index];
        });
    }
}

var callByName = new CallByName();
console.log(callByName.callAdd()); // 1

Call-By-Need

Call-By-Name + memorized the evaluation result, evalute argunment 後把結果儲起來
就算之後改變argunment也不會影響evaluation的結果

Call-By-Text

類似Call-By-Name ，但passing as parameter時沒有closure 裝著所需的variable
到evaluation 時才找所需的variable

個人認為javascript內的eval 是類似效果？

function CallByText(){
    var array = [0, 1, 2, 3];
    var index = 0;

    this.add = function(n){
        var index = 1;
        return eval(n);
    }

    this.callAdd = function(){
        return this.add('array[index]');
    }
}

var callByText = new CallByText();
console.log(callByText.callAdd());