example

Lexical and Parser user-defined context data

For advanced use cases, it is sometimes helpful to be able to have additional data available in either the parsing context, such as sdt actions, or the lexical context, on the lexer or the tokens it produces.

Distinction between Lexical and Parser Context

Both parser and token define Context as interface{}.

Gocc does not care if the parser and lexical contexts are the same, or whether you use one and not the other.

Lexical Context

When the lexer.Lexer object is configured with a non-nil Context value, the value will be assigned to the Pos.Context property of every token the lexer generates.

If you are parsing multiple files, this could allow you to inform the user that tokenA is redeclared in file2.txt after being previously declared in file1.txt, rather than simply "I've seen tokenA before".

Assigning:

	// assigning
	l := lexer.NewLexer()
	l.Context = &MyContext{ /* populate fields */ }
	// or just
	l.Context = filename

Use (assumes MyContext has method 'Source()' that returns filename)

	func duplicate(newToken, oldToken *token.Token) error {
		newPos, oldPos := newToken.Pos, oldToken.Pos

		return fmt.Errorf("%s:%d:%d: error: '%s' redefined.\n%s:%d:%d: previous definition",
			newPos.Context.(*MyContext).Source(), newPos.Line, newPos.Column,
			string(newToken.Lit),
			oldPos.Context.(*MyContext).Source(), oldPos.Line, oldPos.Column)

Parser Context

This allows you to surface contextual information when invoking SDT actions. It corresponds to the Context property of the parser.Parser class, and can be accessed with the SDT token $Context.

Example:

bnf

	Identifier: identifier << ast.NewIdentifier($0, $Context);

ast/main.go

	func NewIdentifier(nameAttr Attrib, context interface{}) (string, error) {
		name := string(nameAttr.(*token.Token).Lit)
		if previous, exists := context.(*ParseContext).Identifiers[name]; exists {
			...