Diff for "compiler/Let's build a compiler/ch1"

申明不会采用典型教科书上关于编译器的教学顺序，比如：

• o An introductory chapter describing what a compiler is. o A chapter or two on syntax equations, using Backus-Naur Form
  • (BNF).
  o A chapter or two on lexical scanning, with emphasis on
  • deterministic and non-deterministic finite automata.
  o Several chapters on parsing theory, beginning with top-down
  • recursive descent, and ending with LALR parsers.
  o A chapter on intermediate languages, with emphasis on P-code
  • and similar reverse polish representations.
  o Many chapters on alternative ways to handle subroutines and
  • parameter passing, type declarations, and such.
  o A chapter toward the end on code generation, usually for some
  • imaginary CPU with a simple instruction set. Most readers (and in fact, most college classes) never make it this far.
  o A final chapter or two on optimization. This chapter often
  • goes unread, too.

作者不会遵照这些，而是带着你直接上手写一个能工作的编译器，采用他的方式：

1. KISS原则
   • 所有的Identifier用single character来代替，不考虑空格（即词法规定不允许用户写空格）。作者认为如果他能正确处理I-T-L，那么他也能让IF-THEN-ELSE工作
   • 只处理实时I/O，不处理File I/O。
   • 忽略error correction/recovery。遇到第一个error即停机，但这不是崩溃。
2. 只讨论一种情形的解析：top-down、recursive descent parsing
   • 自顶向下、递归下降的情形，作者认为这是唯一一种支持手写编译器的技术选型
3. 从源代码直接生成汇编代码。不需要中间码
4. 延迟对module的构建，直至真的需要它的时候