By
Jason Liu
更新日期:
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
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| #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
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| #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
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| object CallByName {
var array = Array(0, 1, 2, 3)
var index = 0
def add(n: => Int) : Int = {
return n;
}
def callAdd() = {
add(array(index))
}
}
defined object CallByName
scala> CallByName.callAdd
res2: Int = 0
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所以如果closure 內的variable數值被外內改變
會影響evaluation的結果
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| object CallByName {
var array = Array(0, 1, 2, 3)
var index = 0
def add(n: => Int) : Int = {
index = 1;
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
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| object CallByName {
var array = Array(0, 1, 2, 3)
var index = 0
def add(n: => Int) : Int = {
var index = 1;
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
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| 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());
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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？
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| 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());
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