Support comment syntax

[wakame-tech]

overview

• support comment syntax (// comment, /* comment */)
  • need 2-pass parse?

[hamadakafu]

https://github.com/Geal/nom/blob/main/doc/nom_recipes.md#comments

• ok: many0(alt((..., multispace1)))

• bad: many0(alt((..., multispace0)))

• bad: many0(alt(multispace0, ...)

• bad: many1(alt((..., multispace0)))

• bad: many1(alt((multispace0, ...)))

• ok: FnMut(&'a str) ...

• bad: Fn(&'a str)

/// multispace0の代わりにparse_commentを使うべき
pub fn ws<'a, F: 'a, O, E: ParseError<&'a str>>(
    inner: F,
) -> impl FnMut(&'a str) -> IResult<&'a str, O, E>
where
    F: FnMut(&'a str) -> IResult<&'a str, O, E>,
{
    delimited(
        many0(alt((parse_comments, value((), multispace1)))),
        inner,
        many0(alt((parse_comments, value((), multispace1)))),
    )
}

fn parse_comments<'a, E: ParseError<&'a str>>(s: &'a str) -> IResult<&str, (), E> {
    alt((parse_comment_oneline, parse_comment_multiline))(s)
}

// ファイルの末尾に改行なしのコメントが書けるので
// 末尾の改行は消費しないことに注意する
fn parse_comment_oneline<'a, E: ParseError<&'a str>>(s: &'a str) -> IResult<&str, (), E> {
    value((), pair(tag("//"), is_not("\n")))(s)
}

fn parse_comment_multiline<'a, E: ParseError<&'a str>>(s: &'a str) -> IResult<&str, (), E> {
    value(
        (), // Output is thrown away.
        tuple((tag("/*"), take_until("*/"), tag("*/"))),
    )(s)
}