从现在起正式把《编程语言设计模式》(中文版和英文电子版)加为参考书目。
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对于一个parser，规范的命名是LL(k)，LR(k)之类。LL(k)的含义：第一个L只按从左到有顺序解析输入内容，第二个L表示自顶向下解析时按照从左到右顺序遍历子节点，k表示forward的token为k个。而LL(k)中的r表示的是自底向上解析时按照从右向左推导(从左向右规约)。
LL分析的时候先看到的是产生式左值，然后确定其是由什么右值构成的；于是要做的事就是迭代地匹配产生式，这涉及到了不断地猜测、回溯，通常将A的产生式分为first(A)和follow(A)（和lisp中的car/cdr看起来有些像哈>o<）。这里的“顶”应该是从语法树上看的。LR正好相反，是从产生式的右值开始分析的，开始拿到的是推导式右值中的一个文法(grammar)符号；也可以说是从语法树的叶子节点开始分析。
上下文无关文法(context free grammar)是指没有二义性的文法。一个具有二义性的语法可以参照C++：
T(a)
在只知道T和a是token的情况下，它可能指下面几种情况(或其中一部分)：
当然在C++手册中规定遇到这种情况声明(a)比表达式(b)(c)(d)优先级较高，所以在构造解析规则的时候通常隐含(用了if-else等分支结构)对解析选项进行了排序，例如：
stat:(declaration)=>declaration
|expression
;
也就是说只要看起来像声明了，就认为是声明；确定不是声明那种货色的了，才考虑它匹配表达式。但问题在于仍然没有办法区分(b)(c)(d)的情况。如果用LL(k)的话，因为在对象初始化前还有一个token（如Fraction），是可以区分(b)和(c)(d)。然而，仅凭这些还是无法区分(c)(d)。事实上这两者的CFG是一样的：
expr:INTEGER
|ID '(' expr ')'//bar(-67)
|ID '(' expr ')'//float(11);
;
这时就需要采用语义信息了，通常LL(k)是采用形如这样的谓词判断：
void expr() {
if ( LA(1)==INTEGER) match(INTEGER);
else if ( LA(1)==ID && isFunction(LT(1).text) ) «match-function-call »
else if ( LA(1)==ID && isType(LT(1).text) ) «match-typecast »
else «error »
}
在编写解析不同同种语言的解析器是也会用到谓词逻辑；比如gcc的那100+个坑爹的选项，比如java的各个兼容模式……
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