How to have a separator in nom with an optional terminating separator?

Question

I'd like to parse both of these with nom:

[
   a, 
   b,  
   c
]

[
   a, 
   b,  
   c,
]

Currently I have this code which parses the first but not the second (the first function is a recipe from the nom docs which just parses whitespace):

// https://github.com/Geal/nom/blob/main/doc/nom_recipes.md#wrapper-combinators-that-eat-whitespace-before-and-after-a-parser
fn ws<'a, F: 'a, O, E: ParseError<&'a str>>(
    inner: F,
) -> impl FnMut(&'a str) -> IResult<&'a str, O, E>
where
    F: Fn(&'a str) -> IResult<&'a str, O, E>,
{
    delimited(multispace0, inner, multispace0)
}

pub fn parse_list(input: &str) -> IResult<&str, Vec<&str>> {
    delimited(
        ws(tag("[")),
        separated_list0(
            ws(tag(",")),
            take_while1(|x| char::is_alphabetic(x) || x == '_'),
        ),
        ws(tag("]")),
    )(input)
}

I'm new to nom, and don't have any allegiance to the current code, so fine to tell me I'm doing it all wrong...

Thanks!

Spirketing

Answer 1

Here's one (of possibly many solutions).

Just use terminated along with opt:

pub fn parse_list(input: &str) -> IResult<&str, Vec<&str>> {
    delimited(
        ws(tag("[")),
        terminated(
            separated_list0(
                ws(tag(",")),
                take_while1(|x| char::is_alphabetic(x) || x == '_'),
            ),
            opt(ws(tag(","))),
        ),
        ws(tag("]")),
    )(input)
}

Choirmaster

Note that this accepts a single separator with no items, e.g. [,] – Ripon

@Ripon You're kind of right. I used separated_list0 because OP did as well. Of course, you can simply use seperated_list1 in case you need at least one element. – Choirmaster

Answer 2

An adaptation of hellow's answer that disallows [,]:

pub fn parse_list(input: &str) -> IResult<&str, Vec<&str>> {
    delimited(
        ws(tag("[")),
        opt(
            terminated(
                separated_list1(
                    ws(tag(",")),
                    take_while1(|x| char::is_alphabetic(x) || x == '_'),
                ),
                opt(ws(tag(","))),
            )
        ).map(Option::unwrap_or_default),
        ws(tag("]")),
    )(input)
}

Playground link

Ripon