[Udacity CS 262] Programming Languages (Week #5)

previously：
第四个unit  http://www.1point3acres.com/bbs/thread-37362-1-1.html
第三个unit  http://www.1point3acres.com/bbs/thread-37239-1-1.html
第二个unit  http://www.1point3acres.com/bbs/thread-37174-1-1.html
第一个unit  http://www.1point3acres.com/bbs/thread-37089-1-1.html


经过前几个unit讲述的tokenizing, parsing之后，本章接着讲interpreting的部分。把这几个unit的代码合在一起，就解释运行一个简单的javascript代码了，很有趣的。

本章讲了一下几个知识点

1. 解释器
An interpreter finds the meaning of a program by traversing its parse tree.
讲义里给了一个例子：
We're going to write a recursive procedure called interpret that's going to walk over abstract syntax trees and figure out what they mean.
For example, you could write out a webpage that said, "Hello, Friend", and it would be a list of 2 elements.
示例代码：

1. def interpret(trees): # Hello, friend
   
2.     for tree in trees: # Hello,
   
3.         # ("word-element","Hello")
   
4.         nodetype=tree[0] # "word-element"
   
5.         if nodetype == "word-element":
   
6.             graphics.word(tree[1])
   
7.         elif nodetype == "tag-element":
   
8.             # <b>Strong text</b>
   
9.             tagname = tree[1] # b
   
10.             tagargs = tree[2] # []
    
11.             subtrees = tree[3] # ...Strong Text!...
    
12.             closetagname = tree[4] # b
    
13.   
    
14.             if(tagname <> closetagname):
    
15.                 graphics.warning()
    
16.             else:
    
17.                 graphics.begintag(tagname, tagargs)
    
18.                 interpret(subtrees)
    
19.                 graphics.endtag()
    

复制代码

2. 变量与环境
There is a special global environment that can hold variable values. Other environments have parent pointers to keep track of nesting or scoping. Environments hold storage locations and map variables to values.
关于chained environment：遇见变量名先查自己的scope，再依照chain查找parent environment。

3. 函数调用
interpreter发现有function statement被调用时，需要创建一个新环境（全局变量加上函数参数）。然后，再递归地interpret该函数体。


如果你希望在本机运行样例程序，坏消息：本章部分程序需要graphics库，需要2.5G
http://forums.udacity.com/cs262/questions/5712/unit-5-9-graphics-library

编程作业题：

---

I Could Wile Away The Hours

给出的代码可以完成“解释javascript的if-then结构和函数调用”的功能，你要做的是添加实现循环语句的功能。

For this assignment, you should write a procedure:
       eval_while(while_statement, evnironment)
Your procedure can (and should!) call those other procedures.


Higher Order Function

Suppose that we know that we will be filtering many lists down to only their odd

elements. Then we might want something like this:

# filter_odds = filter_maker(lambda n : n % 2 == 1)
# odds = filter_odds(numbers)

In this example, "filter_maker()" is a function that takes a function as # an argument and returns a function as its result. We say that filter_maker is a *higher order function*.

觉得programming language  一定要用funcional programming讲。。 scheme，LISP一类。。

joshua.cong 发表于 2012-8-10 17:59
觉得programming language  一定要用funcional programming讲。。 scheme，LISP一类。。

恩 这门课还专门讲了lamda 毕竟python和javascript都是支持lamda的