Extract errors

[OtaK]

Hey,

First of all, thanks for the amazing crate!
Is there any way to extract Err(T) in the else statement?

Thanks,
Mathieu

[lambda-fairy]

Hi Mathieu, that's good to hear!

What is the code that you're trying to write?

[OtaK]

I was wondering if it was possible to write something like that

if_chain! {
    if let Ok(thing) = result_thing;
    then { do stuff with thing; }
    else(my_error) { do stuff with the Err(e) aliased to my_error } 
}

The most bothering thing about if_chain is the fact that once the chain goes to else there's no way to know what/when it went wrong along the chain, making debugging quite hard.

[lambda-fairy]

In that case, you can use a normal match:

match result_thing {
    Ok(thing) => ... ,
    Err(my_error) => ... ,
}

That won't work if there are multiple steps of pattern matching involved (what if_chain was designed for), but I don't think there's an easy way to handle the else in that case.

[OtaK]

That would somewhat defeat the purpose of using the if_chain macro in the first place though...
Any idea on how it could be done by modifying it?

[lambda-fairy]

You tell me ;)

In this code snippet:

let result_thing: Result<String, MyError>;
if_chain! {
    if let Ok(thing) = result_thing;
    if thing.len() > 2;
    then { /* ... */ }
    else(foo) { /* ... */ }
}

What do you want the type of foo to be?

[OtaK]

I think it would be MyError; what so you think?

[lambda-fairy]

Then what do we do when result_thing is Ok, but thing.len() <= 2?

[OtaK]

Hmm, I see what you mean, then I guess it's simply not possible to do so 🤔
We might send a () but it'll break match arms return types.