Hey, I work my way through the rust book and found

Question

Hey, I work my way through the rust book and found

something with the "_" Placeholder in "match" which confuses me...

This works:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=5c8ac044d6c720ae122f23517db714b8

but this is not working:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f1e7fc114e4f7fbfd3987267a8ead294

Compiler says:
match arms have incompatible types

I think the return value for the function is the reason for this behavior? And in my opinion the error message is also confusing, because the match arms have in both cases the same types?

But maybe I just overlook something?

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0

19.09.2020

5 ответов

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Peter Автор вопроса

so it's more or less not related to the _ Placeholder...but anyway I would like to know why this happens ... And is it possible to give back some kind of "empty" value when the return type of fn is i32 ?

0

19.09.2020

Artem Starikov

Peter
so it's more or less not related to the _ Placehol...

if you want to represent absence of value, use the Option type

0

19.09.2020

Artem Starikov

Artem Starikov
have in both cases the same types no, they don't:...

match x { 1 => {println!("{}",x+1);}, _ => (), } this works because this: { println!("{}", x + 1); } returns () as well

0

19.09.2020

swarnim

Peter
so it's more or less not related to the _ Placehol...

Put never type or unreachable!() instead of unit type in _

0

19.09.2020

Artem Starikov · Accepted Answer

Artem Starikov

have in both cases the same types no, they don't: match x { 1 => x+1, _ => (), } if x is 1, you return an i32, if it's something else, you return the unit type, (). these are different types

0

19.09.2020

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