Guide: Enums chapter contains unexplained match keyword

[lgrahl]

See: https://github.com/rust-lang/rust/blob/master/src/doc/guide.md#enums

The match keyword hasn't been introduced until that point but it's used in the code example which is confusing.

I believe an if/else solution would be more explanatory, especially because the reader probably wants to see how to unpack the Value variant of enum OptionalInt. The same enum could then be used with the match keyword in the beginning of the next chapter (Match).

[steveklabnik]

You are correct. Nice catch!

[gamazeps]

Are you ok with simply reordering them ?
Because introducing sum type without precedent knowledge of pattern matching is a bit pointles (or at least misses the fact that they are juste AWESOME for pattern matching).
If you're ok with this I'll PR in a few hours :)

[steveklabnik]

Maybe, if it flows okay, yeah. I do want to put SOME kind of mention of it first.

[lgrahl]

It's probably easy to find out what match is doing by simply looking at it. This part of the tutorial is just a bit confusing and should be fixed. However, my main concern here is that the reader is left clueless on how to solve this problem with if/else:

let x = Value(5);
let y = Missing;

match x {
    Value(n) => println!("x is {}", n),
    Missing  => println!("x is missing!"),
}

In fact, I really don't know how the equivalent code without match would look like. I've briefly tried to figure it out and gave up.

[gamazeps]

With an Option you coul unwrap I guess.

By the way @steveklabnik why did you write a fake option enum ?
Because that's basically what is done with this, it might be better to give out the real name

[steveklabnik]

Because we haven't talked about generics yet.

[lgrahl]

Nicely done. However, my other concern, how this example could be done without match, remains unresolved. I'm certain that I'm not the only one who wants to know how it works with if/else before continue reading about match. Should I open another issue for that matter?

[gamazeps]

@lgrahl I'm honestly not sure that it is possible with an arbitrary enum...
You can test for some arbitrary values but i guess but that's it.

Bt it's more an issue of sum types in general rather than a Rust issue (I can't find ways to do that in haskell nor in scala)

[lgrahl]

@gamazeps Alright, I expected something like this as I was unable to figure out how to write it without match. I don't have much experience with statically typed languages, therefore the enum concept was new to me. If you are right, could we mention in the tutorial that it is actually not possible to write an equivalent solution with if/else? Certainly, I can't be the only one who was a bit puzzled. Cheers.

[gamazeps]

@lgrahl Thanks a lot for the remark, it's exactly the kind of comment needed, if you see others don't hesitate to open new issues :)

I'll try to add this tonight or tomorrow.