How do I use nom to parse a string with sign into an i32?

Question

Using the nom crate, I'm trying to write a parser that can recognize signed i32 number from a String, i.e. can transform the string -42 into the i32 representation.

So far I've come up with the following, but I'm failing to parse negative numbers:

use nom::types::CompleteStr;
use std::str::FromStr;

named!(
    i32_parser<CompleteStr, i32>,
    map_res!(nom::digit, |CompleteStr(s)| i32::from_str(s))
);

#[test]
fn parse_i32_positive() {
    assert_eq!(
        i32_parser(CompleteStr::from("42")),
        Ok((CompleteStr::from(""), 42))
    );
}

#[test]
fn parse_i32_negative() {
    assert_eq!(
        i32_parser(CompleteStr::from("-42")),
        Ok((CompleteStr::from(""), -42))
    );
}

I've also tried the following, but with a cryptic compilation error:

named!(
     i32_parser<CompleteStr, i32>,
     map_res!(alt!(char!('-') | nom::digit), |CompleteStr(s)| i32::from_str(s))
 );
^ expected char, found struct `nom::types::CompleteStr`

Any suggestion on how to fix it? Or a simpler way to achieve that with nom?

I'm explicitly looking to implement this with nom because I'm trying to parse a more complex structure. i32::from_str(s) works for simple strings but it's not what I'm looking for.

Answer 1

The recognize! macro can help you. It returns the parsed input string instead of the parser output, which can then be converted as usual. For example:

named!(i32_parser<&str, i32>,
    map_res!(
        recognize!(tuple!(opt!(char!('-')), digit)),
        FromStr::from_str)
);