Order of Rules Affects Parsing

[lambdaknight]

Describe the bug
BNF grammar should be independent of the order of the rules as long as it is otherwise well-formed.

To Reproduce
Take the dna_left_recursive test and reverse the order of the rules. The BNF parser can no longer successfully parse the input string.

#[test]
fn dna_left_recursive() {
    let grammar: Grammar = "<base> ::= 'A' | 'C' | 'G' | 'T'
        <dna> ::= <base> | <dna> <base>"
        .parse()
        .unwrap();

    let input = "GATTACA";

    let parses: Vec<_> = grammar.parse_input(input).map(|a| a.to_string()).collect();
    assert_snapshot!(parses.join("\n"));
}

[CrockAgile]

Thanks for bringing this up! I am not sure yet, but I have a guess.

The current bnf::Grammar::parse doesn't have a way to designate the "starting" term. So as is, it assumes the first rule begins with the "starting" term.

In this example, this means the "starting" term is <base>, which cannot parse "GATTACA".

Sorry if this turns out to be the reason! I meant to mark this strange implicit assumption in the API documentation, but I must have forgotten.

[lambdaknight]

That's what I figured was happening, but I figured I'd file a bug so you have a nice reminder to add it to the documentation or whatever you end up doing. :) Thanks!