Return from function upon condition met in map

Question

Is there a way I can use map such that if a certain condition is met I can return an error? The reason is that i'm trying to implement the trait FromStr for my struct BigNum:

impl FromStr for BigNum { fn from_str(s: &str) -> Result<BigNum, BigNum::Err> { let filter_vec = t_num.chars(). map(|a| match a.to_digit(10) { Some(x) => { x }, None => { /* return from function with Err */ } }); } }

Is this possible to do? Or should I simply iterate over the chars() myself and return an error upon reaching a None? Just wondering if there's an easy way to break out of map and return from the function

Did you try return Err(..whatever..)?

Shepmaster
– Shepmaster

2015-07-19 23:47:24 +00:00
Commented Jul 19, 2015 at 23:47 — Shepmaster
– Shepmaster, Commented Jul 19, 2015 at 23:47

Shepmaster · Accepted Answer · 2015-07-20 13:47:28Z

Two key pieces of information will be useful here:

Iterators operate one item at a time, so iterator adaptors will only be executed while something is consuming the iterator.
FromIterator (which powers Iterator::collect) is implemented for Result as well.

Since you didn't provide a MCVE, I made up sample code that can actually compile:

fn foo(input: &[u8]) -> Result<Vec<u8>, ()> { input.iter().map(|&i| { if i > 127 { Err(()) } else { Ok(i + 1) } }).collect() } fn main() { let a = foo(&[1,2,3,4]); let b = foo(&[200,1,2,3]); println!("{:?}", a); println!("{:?}", b); }

If you stick a println! in the map body, you can see that no numbers are processed after the 200.

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