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
- func MakeSymbolTable(def Definition, types ...string) map[rune]bool
- func NameOfReader(r io.Reader) string
- func Panic(pos Position, message string)
- func Panicf(pos Position, format string, args ...interface{})
- type Definition
- func EBNF(grammar string) (Definition, error)
- func Elide(def Definition, types ...string) Definition
- func Map(def Definition, f MapFunc) Definition
- func Must(def Definition, err error) Definition
- func Regexp(pattern string) (Definition, error)
- func Unquote(def Definition, types ...string) Definition
- func Upper(def Definition, types ...string) Definition
- type Error
- type Lexer
- type MapFunc
- type Position
- type Token
Constants ¶
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
MakeSymbolTable is a useful helper function for Definition decorator types.
func NameOfReader ¶
NameOfReader attempts to retrieve the filename of a reader.
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 ( // EOFToken is a Token representing EOF. EOFToken = Token{Type: EOF, Value: "<<EOF>>"} // DefaultDefinition defines properties for the default lexer. DefaultDefinition Definition = &defaultDefinition{} )
var TextScannerLexer Definition = &defaultDefinition{}
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 Elide ¶
func Elide(def Definition, types ...string) Definition
Elide wraps a Lexer, removing tokens matching the given types.
func Map ¶
func Map(def Definition, f MapFunc) Definition
Map is a Lexer that applies a mapping function to a Lexer's tokens.
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+)`)
func Unquote ¶
func Unquote(def Definition, types ...string) Definition
Unquote applies strconv.Unquote() to tokens of the given types.
Tokens of type "String" will be unquoted if no other types are provided.
func Upper ¶
func Upper(def Definition, types ...string) Definition
Upper case all tokens of the given type. Useful for case normalisation.
type Error ¶
Error represents an error while parsing.
type Lexer ¶
type Lexer interface {
// Peek at the next token.
Peek() Token
// 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.
Source Files