Go词法语法分析

  |   0 评论   |   0 浏览

词法分析

下列一段代码

package main

import (
	"fmt"
)

func main() {
	fmt.Println("Hello")
}

通过 lex 规则最终解释成

PACKAGE  IDENT

IMPORT  QUOTE IDENT QUOTE

IDENT  IDENT LPAREN RPAREN  LBRACE
IDENT DOT IDENT LPAREN QUOTE IDENT QUOTE RPAREN
RBRACE

这样将源码翻译成 好多的 token(字符) 这一过程可以理解成词法分析

go 的词法分析最终生成的 Token 列表 src/cmd/compile/internal/syntax/tokens.go

// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package syntax

type token uint

//go:generate stringer -type token -linecomment

const (
	_    token = iota
	_EOF       // EOF

	// names and literals
	_Name    // name
	_Literal // literal

	// operators and operations
	// _Operator is excluding '*' (_Star)
	_Operator // op
	_AssignOp // op=
	_IncOp    // opop
	_Assign   // =
	_Define   // :=
	_Arrow    // <-
	_Star     // *

	// delimiters
	_Lparen    // (
	_Lbrack    // [
	_Lbrace    // {
	_Rparen    // )
	_Rbrack    // ]
	_Rbrace    // }
	_Comma     // ,
	_Semi      // ;
	_Colon     // :
	_Dot       // .
	_DotDotDot // ...

	// keywords
	_Break       // break
	_Case        // case
	_Chan        // chan
	_Const       // const
	_Continue    // continue
	_Default     // default
	_Defer       // defer
	_Else        // else
	_Fallthrough // fallthrough
	_For         // for
	_Func        // func
	_Go          // go
	_Goto        // goto
	_If          // if
	_Import      // import
	_Interface   // interface
	_Map         // map
	_Package     // package
	_Range       // range
	_Return      // return
	_Select      // select
	_Struct      // struct
	_Switch      // switch
	_Type        // type
	_Var         // var

	// empty line comment to exclude it from .String
	tokenCount //
)

const (
	// for BranchStmt
	Break       = _Break
	Continue    = _Continue
	Fallthrough = _Fallthrough
	Goto        = _Goto

	// for CallStmt
	Go    = _Go
	Defer = _Defer
)

// Make sure we have at most 64 tokens so we can use them in a set.
const _ uint64 = 1 << (tokenCount - 1)

// contains reports whether tok is in tokset.
func contains(tokset uint64, tok token) bool {
	return tokset&(1<<tok) != 0
}

type LitKind uint8

// TODO(gri) With the 'i' (imaginary) suffix now permitted on integer
//           and floating-point numbers, having a single ImagLit does
//           not represent the literal kind well anymore. Remove it?
const (
	IntLit LitKind = iota
	FloatLit
	ImagLit
	RuneLit
	StringLit
)

type Operator uint

//go:generate stringer -type Operator -linecomment

const (
	_ Operator = iota

	// Def is the : in :=
	Def  // :
	Not  // !
	Recv // <-

	// precOrOr
	OrOr // ||

	// precAndAnd
	AndAnd // &&

	// precCmp
	Eql // ==
	Neq // !=
	Lss // <
	Leq // <=
	Gtr // >
	Geq // >=

	// precAdd
	Add // +
	Sub // -
	Or  // |
	Xor // ^

	// precMul
	Mul    // *
	Div    // /
	Rem    // %
	And    // &
	AndNot // &^
	Shl    // <<
	Shr    // >>
)

// Operator precedences
const (
	_ = iota
	precOrOr
	precAndAnd
	precCmp
	precAdd
	precMul
)

以上便是词法分析需要保留的最终结果

语法分析比较重要的一个结构体 scanner 位于 src/cmd/compile/internal/syntax/scanner.go

type scanner struct {
	// 源
	source
	// 模式
	mode uint
	// 当遇到EOF/换行时 转换成 ;
	nlsemi bool // if set '\n' and EOF translate to ';'

	// 行列
	line, col uint
	// 空行
	blank bool // line is blank up to col
	tok   token
	lit   string   // valid if tok is _Name, _Literal, or _Semi ("semicolon", "newline", or "EOF"); may be malformed if bad is true
	bad   bool     // valid if tok is _Literal, true if a syntax error occurred, lit may be malformed
	kind  LitKind  // valid if tok is _Literal
	op    Operator // valid if tok is _Operator, _AssignOp, or _IncOp
	prec  int      // valid if tok is _Operator, _AssignOp, or _IncOp
}

scanner 的 next 函数扫描下一个字符

func (s *scanner) next() {
	nlsemi := s.nlsemi
	s.nlsemi = false

redo:
	// skip white space
	s.stop()
	startLine, startCol := s.pos()
	for s.ch == ' ' || s.ch == '\t' || s.ch == '\n' && !nlsemi || s.ch == '\r' {
		s.nextch()
	}

	// token start
	s.line, s.col = s.pos()
	s.blank = s.line > startLine || startCol == colbase
	s.start()
	if isLetter(s.ch) || s.ch >= utf8.RuneSelf && s.atIdentChar(true) {
		s.nextch()
		s.ident()
		return
	}

	switch s.ch {
	case -1:
		if nlsemi {
			s.lit = "EOF"
			s.tok = _Semi
			break
		}
		s.tok = _EOF

	case '\n':
		s.nextch()
		s.lit = "newline"
		s.tok = _Semi

	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		s.number(false)

	case '"':
		s.stdString()

	case '`':
		s.rawString()

	case '\'':
		s.rune()

	case '(':
		s.nextch()
		s.tok = _Lparen

	case '[':
		s.nextch()
		s.tok = _Lbrack

...

根据扫描到的 不同的 字符 翻译成不同的 token 并且存储到 scanner 的 缓冲区内(tok)中,实际类型是 01234.... iota 正数类型

词法解析的过程都是惰性的,只有在上层的解析器需要时才会调用 next 获取最新的 Token

语法分析

  • 词法分析的 输出 是 语法分析的 输入 但是 词法分析和语法分析是同时进行的

主要结构体,里面存入了文件,scanner 语法分析器,和一些错误行数信息

type parser struct {
	file  *PosBase
	errh  ErrorHandler
	mode  Mode
	pragh PragmaHandler
	// parser 中存入了 语法分析的scanner
	scanner

	base   *PosBase // current position base
	first  error    // first error encountered
	errcnt int      // number of errors encountered
	pragma Pragma   // pragmas

	fnest  int    // function nesting level (for error handling)
	xnest  int    // expression nesting level (for complit ambiguity resolution)
	indent []byte // tracing support
}

通过 fileOrNil 来同时进行词法语法转译


func (p *parser) fileOrNil() *File {
	if trace {
		defer p.trace("file")()
	}

	f := new(File)
	f.pos = p.pos()

	// PackageClause
	// 检测 是否存有 _Package token对应的为 语法需要分析的 package
	if !p.got(_Package) {
		p.syntaxError("package statement must be first")
		return nil
	}
	// 存入参数和 package name
	f.Pragma = p.takePragma()
	f.PkgName = p.name()
	// 然后用 ; 分隔开Package
	p.want(_Semi)

	// don't bother continuing if package clause has errors
	if p.first != nil {
		return nil
	}
	// 词法分析的 import 语法分析成 ImportDecl
	// { ImportDecl ";" }
	for p.got(_Import) {
		f.DeclList = p.appendGroup(f.DeclList, p.importDecl)
		p.want(_Semi)
	}
	// 顶层代码: var const type 分别转成 varDecl constDecl typeDecl
	// 顶层代码都是通过 appendGroup来进行分析
	// { TopLevelDecl ";" }
	for p.tok != _EOF {
		switch p.tok {
		case _Const:
			// 语法分析词法分析同时进行的标志,由于结构体中存有scanner,这里调用的是scanner.next来进行下一个字符的词法分析,分析出的_Import等token继续流入这里进行语法分析
			p.next()
			f.DeclList = p.appendGroup(f.DeclList, p.constDecl)

		case _Type:
			p.next()
			f.DeclList = p.appendGroup(f.DeclList, p.typeDecl)

		case _Var:
			p.next()
			f.DeclList = p.appendGroup(f.DeclList, p.varDecl)

		case _Func:
			p.next()
			// 函数略特殊
			// 函数会转成 Funcdel 存有属性类型参数函数体的 语法分析结构
			// 函数的主体其实就是一个 Stmt 数组,Stmt 是一个接口,实现该接口的类型其实也非常多,总共有 14 种不同类型的 Stmt 实现: 例如:IfStmt ForStmt SwitchStmt等
			if d := p.funcDeclOrNil(); d != nil {
				f.DeclList = append(f.DeclList, d)
			}

		default:
			if p.tok == _Lbrace && len(f.DeclList) > 0 && isEmptyFuncDecl(f.DeclList[len(f.DeclList)-1]) {
				// opening { of function declaration on next line
				p.syntaxError("unexpected semicolon or newline before {")
			} else {
				p.syntaxError("non-declaration statement outside function body")
			}
			p.advance(_Const, _Type, _Var, _Func)
			continue
		}

		// Reset p.pragma BEFORE advancing to the next token (consuming ';')
		// since comments before may set pragmas for the next function decl.
		p.clearPragma()

		if p.tok != _EOF && !p.got(_Semi) {
			p.syntaxError("after top level declaration")
			p.advance(_Const, _Type, _Var, _Func)
		}
	}
	// p.tok == _EOF

	p.clearPragma()
	f.Lines = p.line

	return f
}

最后解析为

ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .

TypeDecl  = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec  = AliasDecl | TypeDef .
AliasDecl = identifier "=" Type .
TypeDef   = identifier Type .

VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .

FunctionDecl = "func" FunctionName Signature [ FunctionBody ] .
FunctionName = identifier .
FunctionBody = Block .

MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
Receiver   = Parameters .

Block = "{" StatementList "}" .
StatementList = { Statement ";" } .

Statement =
	Declaration | LabeledStmt | SimpleStmt |
	GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
	FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
	DeferStmt .

SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .


对应的类型形成语法树

"json.go": SourceFile {
    PackageName: "json",
    ImportDecl: []Import{
        "io",
    },
    TopLevelDecl: ...
}

生成语法树后就是 使用语法树 生成中间代码

参考资料

draveness


标题:Go词法语法分析
作者:devcui
地址:https://blog.eiyouhe.com/articles/2020/08/26/1598449762305.html