《Let's Build A Simple Interpreter》之 Golang 版
一直以来对编译器/解释器等都较有兴趣。我非科班出身，当初还在大学时，只是马马虎虎看完了《编译原理》之类教材，上机非常少，对龙书之类圣经也只是浅尝辄止而已。工作至今，基本已将编译原理相关知识忘记得差不多了，可能也就还对譬如预处理词法分析语法分析 AST 生成等基础性的概念还有点印象罢。
约 1 年多前，我也有想法搞一套基于简化的 Pascal 语法的带类型的脚本语言“编译器”（PaxCompiler 之类可能太复杂了），并将此脚本语言编写的脚本与 Golang 交互起来。当然这只是我个人的业余兴趣而已，至于是否会付诸行动、能搞成怎样都是未知。而选择 Pascal 作为参考改编语言的原因，其一我比较喜欢它的语言设计，其二它曾是我某段时间内的工作语言所以感情成分使然，其三较之诸如 Python、Lua 我更喜欢带类型的脚本语言（TypeScript？我不太喜欢 JavaScript 的语法...），当然，Pascal 的语法形式也确实比较方便为之开发编译器/解释器。
而短期内，个人恐怕没有太多精力去啃龙书之类，于是索性，看点基础资料且按此系列教程之类慢慢温习并从 tokenizer 开始一步步实现自己的 EcoPascal——即便最终，它只是个玩具脚本语言而已。
近 2 天趁有空，粗略看了前文所述教程的前两章，并以 Golang 重写了这两章里的解释程序（代码写得有些粗放）。
第一章：
package interpreter
import (
"fmt"
"unicode"
)
// Token types
//
// EOF (end-of-file) token is used to indicate that
// there is no more input left for lexical analysis
type TokenType int
const (
cTokenTypeOfNone TokenType = iota
cTokenTypeOfInteger
cTokenTypeOfPlusSign
cTokenTypeOfEOF
)
type token struct {
t TokenType // token type: INTEGER, PLUS, or EOF
v interface{} // token value: 0, 1, 2. 3, 4, 5, 6, 7, 8, 9, '+', or None
}
func newToken(t TokenType, v interface{}) token {
return token{
t: t,
v: v,
}
}
type Interpreter struct {
text []rune // client string input, e.g. "3+5"
pos int // an index into text
currToken token // current token instance
}
func New() *Interpreter {
return &Interpreter{
text: []rune(""),
pos: 0,
currToken: newToken(cTokenTypeOfNone, nil),
}
}
func convToDigit(c rune) (int, bool) {
if unicode.IsDigit(c) {
return int(c - '0'), true
}
return 0, false
}
// Lexical analyzer (also known as scanner or tokenizer)
//
// This method is responsible for breaking a sentence apart into tokens.
// One token at a time.
func (self *Interpreter) getNextToken() token {
text := self.text
// is self.pos index past the end of the self.text ?
// if so, then return EOF token because there is no more
// input left to convert into tokens
if self.pos > len(text)-1 {
return newToken(cTokenTypeOfEOF, nil)
}
// get a character at the position self.pos and decide
// what token to create based on the single character
// var currChar interface{} = text[self.pos]
currChar := text[self.pos]
// if the character is a digit then convert it to
// integer, create an INTEGER token, increment self.pos
// index to point to the next character after the digit,
// and return the INTEGER token
if v, ok := convToDigit(text[self.pos]); ok {
self.pos += 1
return newToken(cTokenTypeOfInteger, v)
}
if currChar == '+' {
self.pos += 1
return newToken(cTokenTypeOfPlusSign, '+')
}
panic(fmt.Sprintf("Error parsing input: %s", string(self.text)))
}
// compare the current token type with the passed token type
// and if they match then "eat" the current token
// and assign the next token to the self.currToken,
// otherwise raise an exception.
func (self *Interpreter) eat(tokenType TokenType) {
if self.currToken.t == tokenType {
self.currToken = self.getNextToken()
return
}
panic(fmt.Sprintf("Error parsing input: %s", self.text))
}
// parse "INTEGER PLUS INTEGER"
func (self *Interpreter) Parse(s string) int {
self.text = []rune(s)
self.pos = 0
// set current token to the first token taken from the input
self.currToken = self.getNextToken()
// we expect the current token to be a single-digit integer
left := self.currToken
self.eat(cTokenTypeOfInteger)
// we expect the current token to be a '+' token
// op := self.currToken
self.eat(cTokenTypeOfPlusSign)
// we expect the current token to be a single-digit integer
right := self.currToken
self.eat(cTokenTypeOfInteger)
// after the above call the self.current_token is set to EOF token.
// at this point INTEGER PLUS INTEGER sequence of tokens
// has been successfully found and the method can just
// return the result of adding two integers, thus
// effectively interpreting client input
return left.v.(int) + right.v.(int)
}
第二章：
package interpreter
import (
"fmt"
"unicode"
"github.com/ecofast/rtl/sysutils"
)
// Token types
//
// EOF (end-of-file) token is used to indicate that
// there is no more input left for lexical analysis
type TokenType int
const (
cTokenTypeOfNone TokenType = iota
cTokenTypeOfInteger
cTokenTypeOfPlusSign
cTokenTypeOfMinusSign
cTokenTypeOfEOF
)
type token struct {
t TokenType // token type: INTEGER, PLUS, MINUS, or EOF
v interface{} // token value: non-negative integer value, '+', '-', or None
}
func newToken(t TokenType, v interface{}) token {
return token{
t: t,
v: v,
}
}
type Interpreter struct {
text []rune // client string input, e.g. "3 + 5", "12 - 5", etc
pos int // an index into text
currToken token // current token instance
currChar rune
}
func New() *Interpreter {
return &Interpreter{
text: []rune(""),
pos: 0,
currToken: newToken(cTokenTypeOfNone, nil),
currChar: 0,
}
}
// Advance the 'pos' pointer and set the 'currChar' variable
func (self *Interpreter) advance() {
self.pos += 1
if self.pos > len(self.text)-1 {
self.currChar = 0
} else {
self.currChar = self.text[self.pos]
}
}
func (self *Interpreter) skipWhiteSpace() {
for self.currChar != 0 && unicode.IsSpace(self.currChar) {
self.advance()
}
}
// Return a (multidigit) integer consumed from the input
func (self *Interpreter) integer() int {
ret := ""
for self.currChar != 0 && unicode.IsDigit(self.currChar) {
ret += string(self.currChar)
self.advance()
}
return sysutils.StrToInt(ret)
}
// Lexical analyzer (also known as scanner or tokenizer)
//
// This method is responsible for breaking a sentence apart into tokens.
func (self *Interpreter) getNextToken() token {
for self.currChar != 0 {
if unicode.IsSpace(self.currChar) {
self.skipWhiteSpace()
continue
}
if unicode.IsDigit(self.currChar) {
return newToken(cTokenTypeOfInteger, self.integer())
}
if self.currChar == '+' {
self.advance()
return newToken(cTokenTypeOfPlusSign, '+')
}
if self.currChar == '-' {
self.advance()
return newToken(cTokenTypeOfMinusSign, '-')
}
panic(fmt.Sprintf("Error parsing input: %s", string(self.text)))
}
return newToken(cTokenTypeOfEOF, nil)
}
// compare the current token type with the passed token type
// and if they match then "eat" the current token
// and assign the next token to the self.currToken,
// otherwise raise an exception.
func (self *Interpreter) eat(tokenType TokenType) {
if self.currToken.t == tokenType {
self.currToken = self.getNextToken()
return
}
panic(fmt.Sprintf("Error parsing input: %s", self.text))
}
// parse "INTEGER PLUS INTEGER" or "INTEGER MINUS INTEGER"
func (self *Interpreter) Parse(s string) int {
self.text = []rune(s)
self.pos = 0
self.currChar = self.text[self.pos]
// set current token to the first token taken from the input
self.currToken = self.getNextToken()
// we expect the current token to be an integer
left := self.currToken
self.eat(cTokenTypeOfInteger)
// we expect the current token to be either a '+' or '-'
op := self.currToken
if op.t == cTokenTypeOfPlusSign {
self.eat(cTokenTypeOfPlusSign)
} else {
self.eat(cTokenTypeOfMinusSign)
}
// we expect the current token to be an integer
right := self.currToken
self.eat(cTokenTypeOfInteger)
// after the above call the self.current_token is set to EOF token.
// at this point either the INTEGER PLUS INTEGER or
// the INTEGER MINUS INTEGER sequence of tokens
// has been successfully found and the method can just
// return the result of adding or subtracting two integers, thus
// effectively interpreting client input
if op.t == cTokenTypeOfPlusSign {
return left.v.(int) + right.v.(int)
}
return left.v.(int) - right.v.(int)
}
有了“核心”解释程序，使用起来就很简单了：
// part2 project main.go
package main
import (
"bufio"
"fmt"
"os"
"part2/interpreter"
"strings"
)
func main() {
fmt.Println("Let's Build A Simple Interpreter - Part 2")
parser := interpreter.New()
reader := bufio.NewReader(os.Stdin)
for {
if s, err := reader.ReadString('\n'); err == nil {
fmt.Println(parser.Parse(strings.TrimSpace(s)))
continue
}
break
}
}
本兴趣项目已托管至 Github，比较可能会不定期慢慢更新。