[estefan]

Nice example. I was learning Rust over Christmas so I see how it works. But one issue I have is that from the outside it's not obvious that your `close()` method will take ownership of `self`. I'd probably prefer it if there was some (compiler-enforced) convention that made it obvious to clients that `close()` would consume `self`.

As it stands, Rust code can break the principle of least surprise, since what happens in a method can affect the associated instance variable (in this case `open_socket`). A user could easily write several lines of code and only discover that `open_socket` has been moved into a method at compile-time.

I found myself having very small coding-compile iterations when writing in Rust to catch issues like this (but that was also undoubtedly due to the fact I was learning).

Is there a pro tip to avoid this issue?

[tom_mellior]

> it's not obvious that your `close()` method will take ownership of `self`

I'm in the middle of reading the Rust book right now, so this may be dumb, but... Isn't this exactly the point of using a naked `self` instead of the reference `&self`? If it's not a reference, `self` is consumed by the method. At least that's my current understanding

If you mean that the one-character difference is very small for such an important semantic distinction, I might agree.

As for the state machine example, I like it a lot as well. It's very similar to the idea of "making illegal states unrepresentable" from the OCaml world: http://fsharpforfunandprofit.com/posts/designing-with-types-... (This particular post is F#, but the idea is general.)

[estefan]

> Isn't this exactly the point of using a naked `self` instead of the reference `&self`?

Yes it is, but you only see it's a naked self if you look into the method. From the outside there's nothing in the name to indicate that it doesn't take a reference. So potentially it means that for every method call you need to check that it takes a reference to `self` instead of ownership. This would be especially annoying with third-party libraries.

[dbaupp]

The alternative in other languages is the error (using a socket that has been closed/resource that has been semantically moved away) is only found at runtime. The compiler flagging a mistake like this exactly the point of having the compiler and is a great example of how Rust helps defend against mistakes. Compiling early and compiling often is a great way to develop, IME.

In Rust, I personally don't think about moving vs. not until the compiler tells me I have to, because it usually doesn't matter. Rust previously required moves to be explicitly marked, but it wasn't worth it in practice: http://smallcultfollowing.com/babysteps/blog/2012/10/01/move... (see "Wait, doesn’t this make it hard to know what your program does?" in particular.)