Add section about diverging functions #960

msehnout · 2017-11-21T16:04:32Z

msehnout · 2017-11-22T15:21:03Z

Strange, the fail seems to be in this code:

fn main() {
    fn some_fn() -> () {
        return ()
    }

    let a: () = some_fn();
    println!("This functions returns and you can see this line.")
}

but it works for me with rustc 1.21, any ideas?

steveklabnik · 2017-11-22T15:33:56Z

That is...... very odd. Let me restart it, maybe it's flaky or something? No idea.

msehnout · 2017-11-23T16:37:37Z

For some reason works like this:

fn some_fn() {
    ()
}

fn main() {
    let a: () = some_fn();
    println!("This functions returns and you can see this line.")
}

projektir · 2018-01-14T02:26:26Z

This is a really cool example and I love seeing this! Given that I don't actually understand diverging functions I gave it a read to try to, well, understand them, so I have a few comments:

The never type and it's relationship to the unit type strike me as fairly advanced topics. [also, ! is not yet a type in stable Rust], and it seems the terms used here are not necessarily very precise (and I'm not sure how necessary it is to get into all these details in an RBE example). I.e., ! is considered "uninhabited" as opposed to "empty", it seems, and also it's not the only "uninhabited" type out there.

Diverging functions never return. Their return type in Rust is denoted by !, which is an empty type.

I feel this is tough to read. You say it never returns, but then it has a return type. I get what this means, but maybe something like this would read better:

Diverging functions never return. They are specified/marked by using ! in place of the return type.

Maybe provide a fn foo() -> ! example along with it.

I had trouble understanding the main example used. I imagine this is the relevant piece of code:

let addition: u32 = match i%2 == 1 {
   true => i,
   false => continue,
};

Is it the false => continue portion? I think it's important to mention that not just functions, but also expressions, can be divergent, especially since the final example you use uses expressions. I think this should be very clearly specified. What does that actually do? What gets added to acc? I think it would be very helpful to lay out all these small details very explicitly because this may be the first time someone's seeing divergent functions.

msehnout · 2018-01-15T10:20:08Z

Thanks for comments. I will look into them soon.

btw this is the source of information I used: https://github.com/rust-lang/rfcs/blob/master/text/1216-bang-type.md

steveklabnik · 2018-03-15T13:04:08Z

Any update on this PR?

msehnout · 2018-03-16T13:37:34Z

Sorry, I totally forgot about this PR. I submitted a modified version based on the comment above, is it better now?

steveklabnik

This looks great! Thanks so much. No worries about forgetting, it happens to all of us.

I have some small formatting things, and then this is good to go!

steveklabnik · 2018-03-16T13:39:34Z

src/macro/syntax.md

@@ -0,0 +1 @@
+# Syntax


looks like this was accidentally included

steveklabnik · 2018-03-16T13:39:42Z

src/fn/diverging.md

+It is also the return type of functions that loop forever (e.g. `loop {}`) like network servers or functions that terminates the process (e.g. `exit()`).
+
+
+


could you remove these extra blank lines please?

steveklabnik · 2018-03-16T13:39:50Z

src/fn/diverging.md

+}
+```
+
+Although this might seem like an abstract concept, it is in fact very useful and often handy. The main advantage of this type is that it can be cast to any other one and therefore used at places where an exact type is required, for instance in `match` branches. This allows us to write code like this:


could you wrap this to 80 columns please?

steveklabnik · 2018-03-16T13:40:15Z

src/fn/diverging.md

+```
+
+As opposed to this function, which will never return the control back to the caller.
+```rust


could you put whitespace above this line, to separate the paragraph from the code please?

steveklabnik · 2018-03-16T13:40:18Z

src/fn/diverging.md

+As opposed to all the other types, this one cannot be instantiated, because the set of all possible values this type can have is empty. Note, that it is different from the `()` type, which has exactly one possible value.
+
+For example, this functions returns as usual, although there is no information in the return value.
+```rust


could you put whitespace above this line, to separate the paragraph from the code please?

steveklabnik · 2018-03-16T13:40:26Z

src/fn/diverging.md

+}
+```
+
+As opposed to all the other types, this one cannot be instantiated, because the set of all possible values this type can have is empty. Note, that it is different from the `()` type, which has exactly one possible value.


could you wrap this to 80 columns please?

msehnout · 2018-03-16T13:54:36Z

All requested changes should be fixed now.

msehnout · 2018-03-16T17:58:11Z

I'm starting to dislike Travis. Previous checks went just fine a the last one failed without changing the code. I tried to remove the #feature[...] line as it is nightly only, but it is still failing, this time because the feature is not stable yet. Was there any change in the CI?

frewsxcv · 2018-03-18T11:26:00Z

src/fn/diverging.md

+
+As opposed to this function, which will never return the control back to the caller.
+
+```rust


you can change this to rust,ignore so this block doesn't get doc tested, since it uses an unstable feature (and travis ci uses stable rust right now)

and maybe add that #[feature(...)] line back in so readers know this is an unstable featuree

Thanks, I didn't know about the ignore option. :-)

marioidival · 2018-03-18T21:04:58Z

@msehnout Thank you!

msehnout force-pushed the master branch 2 times, most recently from 86b8f3f to 74419e9 Compare November 23, 2017 16:28

msehnout force-pushed the master branch from 74419e9 to 81013a7 Compare March 16, 2018 12:26

steveklabnik suggested changes Mar 16, 2018

View reviewed changes

msehnout force-pushed the master branch from 81013a7 to d2abd34 Compare March 16, 2018 13:50

msehnout force-pushed the master branch from d2abd34 to 164082b Compare March 16, 2018 14:30

frewsxcv reviewed Mar 18, 2018

View reviewed changes

Add section about diverging functions

a1737ff

msehnout force-pushed the master branch from 164082b to a1737ff Compare March 18, 2018 20:38

marioidival merged commit d5ec87e into rust-lang:master Mar 18, 2018

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Add section about diverging functions #960

Add section about diverging functions #960

msehnout commented Nov 21, 2017

msehnout commented Nov 22, 2017

steveklabnik commented Nov 22, 2017

msehnout commented Nov 23, 2017

projektir commented Jan 14, 2018

msehnout commented Jan 15, 2018

steveklabnik commented Mar 15, 2018

msehnout commented Mar 16, 2018

steveklabnik left a comment

steveklabnik Mar 16, 2018

steveklabnik Mar 16, 2018

steveklabnik Mar 16, 2018

steveklabnik Mar 16, 2018

steveklabnik Mar 16, 2018

steveklabnik Mar 16, 2018

msehnout commented Mar 16, 2018

msehnout commented Mar 16, 2018

frewsxcv Mar 18, 2018

frewsxcv Mar 18, 2018

msehnout Mar 18, 2018

marioidival commented Mar 18, 2018

		It is also the return type of functions that loop forever (e.g. `loop {}`) like network servers or functions that terminates the process (e.g. `exit()`).


		As opposed to this function, which will never return the control back to the caller.

		```rust

Add section about diverging functions #960

Add section about diverging functions #960

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msehnout commented Nov 21, 2017

msehnout commented Nov 22, 2017

steveklabnik commented Nov 22, 2017

msehnout commented Nov 23, 2017

projektir commented Jan 14, 2018

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steveklabnik commented Mar 15, 2018

msehnout commented Mar 16, 2018

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msehnout commented Mar 16, 2018

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marioidival commented Mar 18, 2018