What happens in Rust using match if nothing matches?

Question

I study Rust, and it looks very interesting. I am still not very familiar with the “match”, but it looks pretty solid. I used the following code (below) to convert String to i64, which included the numbered string "None" instead of the next string "_". I wondered what happened in the event of a mismatch without underlining or “No” might be a trick. The call code requires a positive i64, so a negative result leads to an invalid input (in this case). I am not sure if it is possible to indicate an error in the return using this example, except possibly using a struct.

• Without underlining as a matching element, "None" will catch everything and can it be used instead of underlining?

• Is it possible to return an error in such a function without using struct as the return value?

• In general, is a match possible using a match, and if so, what happens?

• In the example below, is there a difference between using underscores and using No?

Code example:

fn fParseI64(sVal: &str) -> i64 { match from_str::<i64>(sVal) { Some(iVal) => iVal, // None => -1 _ => -1 } } 

Answer 1

• Without underlining as a matching element, "None" will catch everything and can it be used instead of underlining?

In this case, yes. For Option<T> only 2 cases: Some<T> or None . You are already matching the Some case for all values, None is the only case that remains, so None will behave exactly like _ .

2. Is it possible to return an error in such a function without using struct as a return value?

I would advise you to read this detailed guide to understand all the possible ways to handle errors - http://static.rust-lang.org/doc/master/tutorial-conditions.html

Using parameters and results will require a structure return.

3. In general, is match possible using match, and if so, what happens?

Rust will not compile match if it cannot guarantee that all possible cases will be handled. You will need to have a suggestion _ if you do not meet all the possibilities.

4. In the example below, is there a difference between using underscores and using No?

No. See Answer No. 1.

Here are three ways you can write your function:

 // Returns -1 on wrong input. fn alt_1(s: &str) -> i64 { match from_str::<i64>(s) { Some(i) => if i >= 0 { i } else { -1 }, None => -1 } } // Returns None on invalid input. fn alt_2(s: &str) -> Option<i64> { match from_str::<i64>(s) { Some(i) => if i >= 0 { Some(i) } else { None }, None => None } } // Returns a nice message describing the problem with the input, if any. fn alt_3(s: &str) -> Result<i64, ~str> { match from_str::<i64>(s) { Some(i) => if i >= 0 { Ok(i) } else { Err(~"It less than 0!") }, None => Err(~"It not even a valid integer!") } } fn main() { println!("{:?}", alt_1("123")); println!("{:?}", alt_1("-123")); println!("{:?}", alt_1("abc")); println!("{:?}", alt_2("123")); println!("{:?}", alt_2("-123")); println!("{:?}", alt_2("abc")); println!("{:?}", alt_3("123")); println!("{:?}", alt_3("-123")); println!("{:?}", alt_3("abc")); } 

Output:

 123i64 -1i64 -1i64 Some(123i64) None None Ok(123i64) Err(~"It less than 0!") Err(~"It not even a valid integer!")