lexer

package
v0.1.0 Latest Latest
Warning

This package is not in the latest version of its module.

Go to latest
Published: Sep 11, 2018 License: MIT Imports: 11 Imported by: 0

README

Participle Lexers

Stateful lexer

Example: string interpolation.

hello ${first + "Surname: ${last}"}

This can be represented by the following stateful lexer grammar:

Enter = "${" Push(Interpolated) .
Text = any .

Interpolated {
  Leave = "}" Pop() .
  Ident = alpha { alpha | number } .
  Number = number { number } .
  Whitespace = "\n" | "\r" | "\t" | " " .
  Operator = "+" | "-" | "*" | "/"
  String = "\"" { "\\" ${" Push(Interpolated) | } "\"" .
}

alpha = "a"…"z" | "A"…"Z" | "_" .
number = "0"…"9" .
any = "\u0000" … "\uffff" .

EBNF

Regexp

text/scanner based lexer

Documentation

Overview

Package lexer defines interfaces and implementations used by Participle to perform lexing.

The primary interfaces are Definition and Lexer. There are three implementations of these interfaces:

TextScannerLexer is based on text/scanner. This is the fastest, but least flexible, in that tokens are restricted to those supported by that package. It can scan about 5M tokens/second on a late 2013 15" MacBook Pro.

The second lexer is constructed via the Regexp() function, mapping regexp capture groups to tokens. The complete input source is read into memory, so it is unsuitable for large inputs.

The final lexer provided accepts a lexical grammar in EBNF. Each capitalised production is a lexical token supported by the resulting Lexer. This is very flexible, but a bit slower, scanning around 730K tokens/second on the same machine, though it is currently completely unoptimised. This could/should be converted to a table-based lexer.

Lexer implementations must use Panic/Panicf to report errors.

Index

Constants

View Source
const (
	// EOF represents an end of file.
	EOF rune = -(iota + 1)
)

Variables

This section is empty.

Functions

func MakeSymbolTable

func MakeSymbolTable(def Definition, types ...string) (map[rune]bool, error)

MakeSymbolTable builds a lookup table for checking token ID existence.

For each symbolic name in "types", the returned map will contain the corresponding token ID as a key.

func NameOfReader

func NameOfReader(r io.Reader) string

NameOfReader attempts to retrieve the filename of a reader.

func Panic

func Panic(pos Position, message string)

Panic throws a lexer error. Lexers should use this to report errors.

func Panicf

func Panicf(pos Position, format string, args ...interface{})

Panicf throws an *Error while parsing.

func SymbolsByRune

func SymbolsByRune(def Definition) map[rune]string

Types

type Definition

type Definition interface {
	// Lex an io.Reader.
	Lex(io.Reader) Lexer
	// Symbols returns a map of symbolic names to the corresponding pseudo-runes for those symbols.
	// This is the same approach as used by text/scanner. For example, "EOF" might have the rune
	// value of -1, "Ident" might be -2, and so on.
	Symbols() map[string]rune
}

Definition provides the parser with metadata for a lexer.

var (
	TextScannerLexer Definition = &defaultDefinition{}

	// DefaultDefinition defines properties for the default lexer.
	DefaultDefinition = TextScannerLexer
)

TextScannerLexer is a lexer that uses the text/scanner module.

func EBNF

func EBNF(grammar string) (Definition, error)

EBNF creates a Lexer from an EBNF grammar.

The EBNF grammar syntax is as defined by "golang.org/x/exp/ebnf". Upper-case productions are exported as symbols. All productions are lexical.

Here's an example grammar for parsing whitespace and identifiers:

Identifier = alpha { alpha | number } .
Whitespace = "\n" | "\r" | "\t" | " " .
alpha = "a"…"z" | "A"…"Z" | "_" .
number = "0"…"9" .

func Must

func Must(def Definition, err error) Definition

Must takes the result of a Definition constructor call and returns the definition, but panics if it errors

eg.

lex = lexer.Must(lexer.Build(`Symbol = "symbol" .`))

func Regexp

func Regexp(pattern string) (Definition, error)

Regexp creates a lexer definition from a regular expression.

Each named sub-expression in the regular expression matches a token. Anonymous sub-expressions will be matched and discarded.

eg.

def, err := Regexp(`(?P<Ident>[a-z]+)|(\s+)|(?P<Number>\d+)`)

type Error

type Error struct {
	Message string
	Pos     Position
}

Error represents an error while parsing.

func Errorf

func Errorf(pos Position, format string, args ...interface{}) *Error

Errorf creats a new Error at the given position.

func (*Error) Error

func (e *Error) Error() string

Error complies with the error interface and reports the position of an error.

type Lexer

type Lexer interface {
	// Next consumes and returns the next token.
	Next() Token
}

A Lexer returns tokens from a source.

Errors are reported via panic, with the panic value being an instance of Error.

func Lex

func Lex(r io.Reader) Lexer

Lex an io.Reader with text/scanner.Scanner.

This provides very fast lexing of source code compatibile with Go tokens.

Note that this differs from text/scanner.Scanner in that string tokens will be unquoted.

func LexBytes

func LexBytes(b []byte) Lexer

LexBytes returns a new default lexer over bytes.

func LexString

func LexString(s string) Lexer

LexString returns a new default lexer over a string.

func LexWithScanner

func LexWithScanner(r io.Reader, scan *scanner.Scanner) Lexer

LexWithScanner creates a Lexer from a user-provided scanner.Scanner.

Useful if you need to customise the Scanner.

type PeekingLexer

type PeekingLexer interface {
	Lexer
	// Peek at the next token.
	Peek(n int) Token
}

A PeekingLexer returns tokens from a source and allows peeking.

Errors are reported via panic, with the panic value being an instance of Error.

func Upgrade

func Upgrade(lexer Lexer) PeekingLexer

Upgrade a Lexer to a PeekingLexer with arbitrary lookahead.

type Position

type Position struct {
	Filename string
	Offset   int
	Line     int
	Column   int
}

Position of a token.

func (Position) GoString

func (p Position) GoString() string

func (Position) String

func (p Position) String() string

type Stateful

type Stateful struct{}

A Stateful lexer.

A lexer where tokens are defined as an extension to EBNF that supports multiple lexer states.

type Token

type Token struct {
	// Type of token. This is the value keyed by symbol as returned by Definition.Symbols().
	Type  rune
	Value string
	Pos   Position
}

A Token returned by a Lexer.

func ConsumeAll

func ConsumeAll(lexer Lexer) (tokens []Token, err error)

ConsumeAll reads all tokens from a Lexer.

func EOFToken

func EOFToken(pos Position) Token

EOFToken creates a new EOF token at the given position.

func RuneToken

func RuneToken(r rune) Token

RuneToken represents a rune as a Token.

func (Token) EOF

func (t Token) EOF() bool

EOF returns true if this Token is an EOF token.

func (Token) GoString

func (t Token) GoString() string

func (Token) String

func (t Token) String() string

Jump to

Keyboard shortcuts

? : This menu
/ : Search site
f or F : Jump to
y or Y : Canonical URL