[Udacity CS 262] Programming Languages (Week #6)

defjex · 2012-07-24

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previously：
第五个unit  http://www.1point3acres.com/bbs/thread-37444-1-1.html
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

其实这门课重点的前几章都已经cover了: 编译任何语言需要的几个主要步骤：lexing, parsing和interpreting。复习一下：
Lexing was based on regular expressions. They specify sets of strings, and they can be implemented under the hood with finite state machines.
Parsing uses context-free grammars, which can capture behavior like balanced parentheses that regular expressions can't. And we saw how to implement those with dynamic programming, that is, writing ourselves little memos in a chart, and parse states.
interpreting refers to walking the abstract syntax tree recursively and computing the final value.

而这节课讲了以下一些知识点

1. Put them together to build a Web Browser
对于浏览器，输入是一个网页源代码文件，首先通过Lexical Analysis把那些plain text转化成一些列tokens.接着通过比照该语言的grammar生成一个AST树(parse tree)。html interpreter遍历这棵树，（同时调用graphics library在终端上渲染该网页），对于这棵AST树中javascript的部分，则要通过js interpreter去执行它。

2. function call
重新强调了每一次函数调用都应该新建一个frame同时父指针指向当前环境。而frame里的环境变量就是该function里声明的变量，其他变量则需要依循父指针向上层的frame里查询。最上层的frame是global environment。有个非常好的例子就是javascript里面的闭包，闭包里的可以访问外层环境的变量就是因为最临近的上层frame是外层函数。

3. Optimization
举了一些js interpreter优化的例子，比如 A * 0 直接得到0， A+0得到A。虽然这些优化规则很简单，但是能减少AST树中Node的数量总是好的，可以让interpreter处理更快一点。

4. the Living and the Dead
考虑如下代码

function myfun(a,b,c,d) {
# a = 1;
b = 2;
c = 3;
# d = 4;
a = 5;
d = c + b;
return (a + d);
}

复制代码

有两行可以去掉是因为他们根本没对结果产生任何影响，We say that a variable is LIVE if the value it holds may be needed in the future. More formally, a variable is LIVE if its value may be read before the next time it is overwritten. 从下往上分析，每一行我们都可以计算出当前哪些变量是live的，这样没有在live列表里面的变量就是不对结果产生影响的也即可以去掉的。这个思想可用在任何语言的IDE(比如eclipse)中作为检测dead code/ unused variable的方法。

编程作业题：

JavaScript's Big Bang

给interpreter中处理javascript的部分增加逻辑，使之可以处理javascript代码块中的html代码。

# In this assignment you will extend our web browser so that the string
# produced by JavaScript is not merely passed to the graphics library as a
# word, but is instead lexed, parsed and interpreted as HTML. (For the
# purposes of this assignment, if JavaScript creates HTML, it must created
# only well-balanced tags.)

举例：
# In practice, however, JavaScript output may include HTML tags and should
# be lexed, parsed and interpreted again. For example, on modern web
# browsers the following webpage ...
#
# <html>
# <script type="text/javascript">
# document.write("Tags in <i>my</i> output should be processed.");
# </script>
# </html>
#
# Does not output the literal string "Tags in <i>my</i> output should be
# processed." Instead, the <i> tags are lexed, parsed and interpreted
# again, and the web page contains "Tags in my output should be processed."
# with the word "my" drawn in italics.

zzwcsong

半年了...U站的第三张证书，估计也是最后一张了

[Udacity CS 262] Programming Languages (Week #6)

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