语法分析
top-down parsing
Recursive descent Parsing
a general form of top-down parsing
may require backtracing to find the correct production to be applied.
Predictive parsing - LL(k)
- a special case of recursive descent parsing
- no backtracking is required
chooses the correct production by looking ahead at the input a fixed number of symbols (typically only 1)
需要提取左因子（Left Factoring），否则会遇到多个可选项
FIRST & FOLLOW & NULLABLE
LL(1) grammar: if a predictive parsing table constructed this way contains no duplicate entries, the grammar is called LL(1).
Left-to-right parse, left-most derivation, 1 symbol lookahead
Based on this parsing table, we can build a recursive predictive parser
We can also build a non-recursive predictive parser by maintaining a stack explicitly.
左递归
Left Recursion cause duplicate entries in the LL(1) parsing table
Predictive Parsing– Error Recovery
语法器如果查到了表中的一个空项，就说明遇到语法错误了
方法一：直接报错
用户体验不好，能不能只编译一次就找出程序的全部错误
方法二：插入正确的 token，假装无事发生
Errors can be recovered by deleting, replacing, or inserting tokens
可能会导致错误不断发生，不断插入 token，陷入死循环
删除/跳过 token 会更安全，因为不会无限进行
方法三：跳到下一个 FOLLOW 里的符号
skipp tokens until a token in the FOLLOW set is reached
