Parsing & Printing

Scheme, having an incredibly simple grammar, lends itself well to a hand-written lexer and recursive descent parser.

Marwood's parser is composed of three separate components:

• the Cell object, which represents a Scheme sexpr in Rust

• the lexer (or scanner or tokenizer), which is responsible for turning plain-text scheme into a list of tokens to be consumed by Marwood's parser.

• the parser, which is responsible for constructing an aggregate structure called Cell from the lexer output.

Example

This example demonstrates the manual use of Marwood's lexer, parser, and Cell types. It also shows use of Marwood's printer. The alternative printing syntax {:#} provides additional escaping, and is the printer used for Scheme's write procedure.

    let scheme_code = r#"
        (quote (#\x 20 puppies))
    "#;

    let tokens: Vec<Token> = lex::scan(scheme_code).unwrap();
    let cell = parse::parse(scheme_code, &mut tokens.iter().peekable()).unwrap();

    println!("{}", cell);
    println!("{:#}", cell);

outputs:

'(x 20 puppies)
'(#\x 20 puppies)