Baby's First Parser Combinators

baby3.rs

enum ParseNode {
    NonTerminal(~[ParseNode]),
    Terminal(~str),
}

enum ParseResult {
    ParseOk(ParseNode, ~str),
    ParseEnd(ParseNode),
    NoParse,
}

fn match_a(s: &s/str) -> ParseResult {
    if s.starts_with("a") {
        let (s_begin, s_end) = (1u, s.len()-1);
        if s_end - s_begin < 0 {
            ParseEnd(Terminal(~"a"))
        } else {
            ParseOk(Terminal(~"a"), s.slice(s_begin, s_end))
        }
    }
    else { NoParse }
}

fn match_b(s: &s/str) -> ParseResult {
    if s.starts_with("b") {
        let (s_begin, s_end) = (1u, s.len()-1);
        if s_end - s_begin < 0 {
            ParseEnd(Terminal(~"b"))
        } else {
            ParseOk(Terminal(~"b"), s.slice(s_begin, s_end))
        }
    }
    else { NoParse }
}

fn match_ab(s: &s/str) -> ParseResult {
    match match_a(s) {
        ParseOk(a, tail_a) => {
            match match_b(tail_a) {
                ParseOk(b, tail_b)  => ParseOk(NonTerminal(~[a, b]), tail_b),
                ParseEnd(b)         => ParseEnd(NonTerminal(~[a, b])),
                NoParse             => NoParse
            }
        }
        ParseEnd(_) | NoParse => NoParse
    }
}

fn main() {
    io::println(fmt!("match_ab(\"ab\") == %?", match_a("ab")));
    io::println(fmt!("match_ab(\"ba\") == %?", match_a("ba")));
}

baby3_error

 % rustc baby3.rs
baby3.rs:40:69: 40:75 warning: implicitly copying a non-implicitly-copyable value
baby3.rs:40                 ParseOk(b, tail_b)  => ParseOk(NonTerminal(~[a, b]), tail_b),
                                                                                 ^~~~~~
baby3.rs:40:61: 40:62 warning: implicitly copying a non-implicitly-copyable value
baby3.rs:40                 ParseOk(b, tail_b)  => ParseOk(NonTerminal(~[a, b]), tail_b),
                                                                         ^
baby3.rs:40:64: 40:65 warning: implicitly copying a non-implicitly-copyable value
baby3.rs:40                 ParseOk(b, tail_b)  => ParseOk(NonTerminal(~[a, b]), tail_b),
                                                                            ^
baby3.rs:41:62: 41:63 warning: implicitly copying a non-implicitly-copyable value
baby3.rs:41                 ParseEnd(b)         => ParseEnd(NonTerminal(~[a, b])),
                                                                          ^
baby3.rs:41:65: 41:66 warning: implicitly copying a non-implicitly-copyable value
baby3.rs:41                 ParseEnd(b)         => ParseEnd(NonTerminal(~[a, b])),
                                                                             ^
warning: no debug symbols in executable (-arch x86_64)