Rust beginner, Implementing TryFrom

help
1

Hi everyone,
I have started learning rust like 10 days ago, I came across this while I was on conversions by documents.

I have implemented TryFrom on a struct, can I have two functions in an implementation like this?

use std::convert::TryFrom;

#[derive(Debug, PartialEq)]
struct EvenNumber(i32);

impl TryFrom<i32> for EvenNumber {
    type Error = ();

    fn try_from(value: i32) -> Result<Self, Self::Error> {
        if value % 2 == 0 {
            println!("even number!");
            Ok(EvenNumber(value))
        } else {
            println!("Not an even number: {}", value);
            Err(())
        }
    }

    fn try_from2(value: i32) -> Result<Self, Self::Error> {
        if value % 2 == 0 {
            Ok(EvenNumber(value))
        } else {
            Err(())
        }
    }
}

fn main() {
    let num1 = EvenNumber::try_from(8);
    let num2 = EvenNumber::try_from2(8);
    println!("{:?}", num1); // Output: even number!\nOk(EvenNumber(8))
    println!("{:?}", num2); // Output: Ok(EvenNumber(8))
  let even_number: EvenNumber = match EvenNumber::try_from2(8) {
        Ok(dffs) => dffs,
        Err(()) => {
            println!("Not an even number!");
            return;
        }
    };
    println!("Even number: {:?}", even_number);
}outputs Even number: 8

(The first function is taken from rust by example, so its not me who gave first function try_form name as it is only accepting try_from), two functions are same except for the println difference .
In one case I wanted to have a println line in impl function and in other i wanted to print from main function, that was my intention or what i want to achieve.

In this case, the compiler is telling you that the method try_from2 is not part of the TryFrom trait, therefore you can't add it to the impl block where you implemented the trait for your struct. this is what complier is sayin, yes but how do i know what methods are a part of TryFrom inorder to only use them, when i go into TryFrom fn try_from(value: T) -> Result<Self, Self::Error>; , which means it only has one method as part to it?

2

One of the most important things that you need to learn when learning any programming language, and specially for Rust, is to read carefully the error messages.

In this case, the compiler is telling you that the method try_from2 is not part of the TryFrom trait, therefore you can't add it to the impl block where you implemented the trait for your struct.

You can of course implement a trait such as TryFrom multiple times if you need to convert multiple types into your struct, but given your code example I'm sure this is not exactly what you want. In the computer programming world it's common to refer to these cases as XY problems, because the original problem is lost in translation when the person asking for help was transcribing the situation in question.

That being said, it's better if you explicitly express what's what you want to achieve rather than how you are trying to achieve it. This will help people understand your case and offer solutions for it.

3

It's not clear what you are trying to do. The two functions look identical (except for the debug-printing, which shouldn't be in such a function anyway).

4

Yes, but you can't put things into a trait impl block that aren't part of that trait.

From its documentation: TryFrom in std::convert - Rust

If you want EvenNumber to have a method try_from2, then write it in a non-trait impl block:

impl EvenNumber {
    fn try_from2(value: i32) -> Result<Self, ()> {
        if value % 2 == 0 {
            Ok(EvenNumber(value))
        } else {
            Err(())
        }
    }
}
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