Allow leading pipe for rules with >1 production

[ammkrn]

It's not uncommon for EBNF (or just patterns using | for alternation)
to allow a leading pipe. Ungram currently does not allow this; the
current behavior is to fail and return a generic parse error.

This change allows the use of a leading pipe for rules that have
more than one production/RHS. For example, this would now be valid:

A ::=
  | 'tok1'
  | 'tok2'

[ammkrn]

A side effect is that

AssocTypeArg =
  NameRef ( | ':' TypeBoundList | '=' Type)

would also be allowed. I think this is fairly benign, but preventing is an easy fix if this is undesirable.

[matklad]

I'd rather not do this -- one of the (informal) design values for ungrammar is to be tight rather than loose. For example, we use a very tight definition for whitespace and idents:

fn is_whitespace(c: char) -> bool {
    matches!(c, ' ' | '\t' | '\n')
}
fn is_ident_char(c: char) -> bool {
    matches!(c, 'a'..='z' | 'A'..='Z' | '_')
}

So, having "optional" elements doesn't sit well with the current design. It's not that there's something objectively wrong with allowing leading | -- it's fine! It's just that, overall, the current philosophy is that there should be only one way to do it.

[bjorn3]

Can we require the leading pipe then?

[matklad]

That creates another false choice: a and | a mean the same thing. More practically, leading | was allowed in Rust at some point, but I've almost never seen people actually taking advantage of that

[ammkrn]

I'll settle for a more detailed error message:

fn rule(p: &mut Parser) -> Result<Rule> {
    if let Some(lexer::Token { kind: TokenKind::Pipe, loc }) = p.peek() {
        bail!(
            *loc, 
            "The first element in a sequence of productions or alternatives \
            must not have a leading pipe (`|`)"
        );
    }
   ...
}

[matklad]

That would be lovely!