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

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

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.

Solution

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)
);