nom 2.2.1

A byte-oriented, zero-copy, parser combinators library
Documentation 
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#[macro_use]
extern crate nom;

use nom::{IResult,digit};

use std::str;
use std::str::FromStr;

named!(unsigned_float <f32>, map_res!(
  map_res!(
    recognize!(
      alt!(
        delimited!(digit, tag!("."), opt!(complete!(digit))) |
        delimited!(opt!(digit), tag!("."), digit)
      )
    ),
    str::from_utf8
  ),
  FromStr::from_str
));

named!(float <f32>, map!(
  pair!(
    opt!(alt!(tag!("+") | tag!("-"))),
    unsigned_float
  ),
  |(sign, value): (Option<&[u8]>, f32)| {
    sign.and_then(|s| if s[0] == ('-' as u8) { Some(-1f32) } else { None }).unwrap_or(1f32) * value
  }
));

#[test]
fn unsigned_float_test() {
  assert_eq!(unsigned_float(&b"123.456"[..]), IResult::Done(&b""[..], 123.456));
  assert_eq!(unsigned_float(&b"0.123"[..]),   IResult::Done(&b""[..], 0.123));
  assert_eq!(unsigned_float(&b"123.0"[..]),   IResult::Done(&b""[..], 123.0));
  assert_eq!(unsigned_float(&b"123."[..]),    IResult::Done(&b""[..], 123.0));
  assert_eq!(unsigned_float(&b".123"[..]),    IResult::Done(&b""[..], 0.123));
}

#[test]
fn float_test() {
  assert_eq!(float(&b"123.456"[..]),  IResult::Done(&b""[..], 123.456));
  assert_eq!(float(&b"+123.456"[..]), IResult::Done(&b""[..], 123.456));
  assert_eq!(float(&b"-123.456"[..]), IResult::Done(&b""[..], -123.456));
}