Add docs for "!" Never type (rfc 1216) #35286

dns2utf8 · 2016-08-04T09:29:17Z

Pull Request: rust-lang/rfcs#1216
Tracking Issue: #35121

rust-highfive · 2016-08-04T09:29:26Z

r? @steveklabnik

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dns2utf8 · 2016-08-04T09:32:08Z

r? @nikomatsakis

apasel422 · 2016-08-04T12:23:28Z

src/doc/book/syntax-index.md

@@ -162,6 +162,7 @@
 <!-- Various things involving parens and tuples -->

 * `()`: empty tuple (*a.k.a.* unit), both literal and type.
+* `!`: empty type (*a.k.a.* unit), both literal and type.


I think this should be something other than "unit".

petrochenkov · 2016-08-04T13:15:28Z

rust-lang/rfcs#1216 doesn't contain !-expressions.
"* !: empty type" seems to be the only change in this PR that needs to stay.

dns2utf8 · 2016-08-04T13:19:16Z

Good point. Changed it

nikomatsakis · 2016-08-05T17:28:10Z

Seems OK. Do we want to wait for the ! implementation to land?

nikomatsakis · 2016-08-05T17:28:26Z

Also, we might want to change the grammar for fn items, which probably specifies ! explicitly.

dns2utf8 · 2016-08-05T18:58:02Z

Currently it has a special entry with a link:
https://github.com/rust-lang/rust/blame/master/src/doc/book/syntax-index.md#L64

Since it is a very special case, what would you change?

canndrew · 2016-08-09T14:03:25Z

I think there's some confusion here. ! is only a type, not a literal or an expression. You can't write let x = !; for example.

nikomatsakis · 2016-08-10T20:11:00Z

@dns2utf8

Since it is a very special case, what would you change?

I would remove the -> ! business and just have an entry for ! itself, which is a type.

canndrew · 2016-08-11T06:36:19Z

Also ! should probably be referred to as "never type" instead of "empty type" if that's the nomenclature we've decided to go with.

dns2utf8 · 2016-08-15T09:25:33Z

I would leave the empty word in the doc because comming from type theory/haskell, I would search for empty type

canndrew · 2016-08-15T09:59:00Z

src/doc/grammar.md

+### Never expressions
+
+```antlr
+never_expr : "!" ;


This still isn't right. ! isn't an expression, just a type (although the sections for primitive types is nothing but FIXMEs at the moment).

bors · 2016-08-18T11:45:13Z

☔ The latest upstream changes (presumably #35769) made this pull request unmergeable. Please resolve the merge conflicts.

nikomatsakis · 2016-08-18T19:09:18Z

I agree with @canndrew's edits, though if the type is called never -- isn't it? I forget -- let's just call it that.

dns2utf8 · 2016-08-20T20:13:27Z

I added the word "always" and capitalized the never to prevent the missreading @apasel422 mentioned, but I am not sure if it enhances the readability over all.

I agree with @nikomatsakis, it is no longer a special case but I kept the reference to the Diverging Functions

nikomatsakis · 2016-08-22T22:02:23Z

src/doc/grammar.md

@@ -764,6 +764,13 @@ bound-list := bound | bound '+' bound-list
 bound := path | lifetime
 ```

+### Never type
+An empty type


Better, though I feel like we could say a lot more here. :) For example, "A type representing the result of a function which never returns -- for example because it panics or goes into an infinite loop."

nikomatsakis · 2016-09-28T19:13:51Z

Well, this has been inactive for a while. I'm going to just merge it since it seems like an improvement, even if I think we could expand the text.

nikomatsakis · 2016-09-28T19:13:56Z

@bors r+ rollup

bors · 2016-09-28T19:13:57Z

📌 Commit d9bd4d5 has been approved by nikomatsakis

…komatsakis Add docs for "!" Never type (rfc 1216) Pull Request: rust-lang/rfcs#1216 Tracking Issue: rust-lang#35121

Rollup of 12 pull requests - Successful merges: #35286, #35892, #36460, #36704, #36741, #36760, #36787, #36789, #36794, #36803, #36811, #36813 - Failed merges:

rust-highfive assigned steveklabnik Aug 4, 2016

rust-highfive assigned nikomatsakis and unassigned steveklabnik Aug 4, 2016

apasel422 reviewed Aug 4, 2016
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dns2utf8 force-pushed the doc_never_expression branch from 85b0966 to 0ff8841 Compare August 4, 2016 13:18

dns2utf8 force-pushed the doc_never_expression branch from 0ff8841 to 43b98f7 Compare August 15, 2016 09:23

canndrew reviewed Aug 15, 2016
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dns2utf8 force-pushed the doc_never_expression branch 4 times, most recently from 0929661 to 2632691 Compare August 20, 2016 20:03

Add docs for "!" empty Never type (rfc 1216)

d9bd4d5

dns2utf8 force-pushed the doc_never_expression branch from 2632691 to d9bd4d5 Compare August 20, 2016 20:13

dns2utf8 changed the title ~~Add docs for "!" Never type/expression (rfc 1216)~~ Add docs for "!" Never type (rfc 1216) Aug 21, 2016

nikomatsakis reviewed Aug 22, 2016
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sophiajt mentioned this pull request Sep 29, 2016

Rollup of 12 pull requests #36818

Merged

sophiajt pushed a commit to sophiajt/rust that referenced this pull request Sep 29, 2016

Rollup merge of rust-lang#35286 - dns2utf8:doc_never_expression, r=ni…

83c54dc

…komatsakis Add docs for "!" Never type (rfc 1216) Pull Request: rust-lang/rfcs#1216 Tracking Issue: rust-lang#35121

bors merged commit d9bd4d5 into rust-lang:master Sep 29, 2016

dns2utf8 deleted the doc_never_expression branch September 29, 2016 11:54

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Add docs for "!" Never type (rfc 1216) #35286

Add docs for "!" Never type (rfc 1216) #35286

dns2utf8 commented Aug 4, 2016

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Add docs for "!" Never type (rfc 1216) #35286

Add docs for "!" Never type (rfc 1216) #35286

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