# Syntax of block-like expressions in match arms **URL:** \ **Category:** language design\ **Created:** [March 31, 2017, 11:16am UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025 "2017-03-31T11:16:41Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![matklad](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/matklad/32/12266_2.png) [@matklad](https://internals.rust-lang.org/u/matklad)\ **Post date:** [March 31, 2017, 11:16am UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/1 "2017-03-31T11:16:41Z")
Hi! I’ve noticed that `rustc` treats block-like expressions differently for determining if the `;` or `,` is needed: ```rust fn stmt() { # no need for semicolons { () } if true { () } else { () } loop {} while true {} } fn match_arm(n: i32) { match n { 1 => { () } # can omit comma here 2 => if true { () } else { () }, # but all other cases do need semicolons. 3 => loop { }, 4 => while true {}, _ => () } } ``` This seems just weird to me… Is there a reason you really need `,` after an `if` in match? Perhaps this is a bug that needs to be fixed? ---
**Author:** ![kornel](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/kornel/32/2711_2.png) [@kornel](https://internals.rust-lang.org/u/kornel)\ **Post date:** [March 31, 2017, 11:59am UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/2 "2017-03-31T11:59:22Z")
I wrangle with this too. In my case it’s: ```rust match n { 1 => foo() // not legal 2 => foo(); // not legal either }; { foo(); // fine foo() // fine too } ``` ---
**Author:** ![Enamex](https://avatars.discourse-cdn.com/v4/letter/e/41988e/32.png) [@Enamex](https://internals.rust-lang.org/u/Enamex)\ **Post date:** [March 31, 2017, 6:11pm UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/3 "2017-03-31T18:11:25Z")
Seems like at one point all right hand blocks/statements of match arms were meant to end with a comma to separate them from the next arms. I think that makes more sense than extending the ability to omit the comma to other block-like right hands. ---
**Author:** ![withoutboats](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/withoutboats/32/4560_2.png) [@withoutboats](https://internals.rust-lang.org/u/withoutboats)\ **Post date:** [March 31, 2017, 7:01pm UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/4 "2017-03-31T19:01:03Z")
I think it makes sense to make these consistent. I’ve definitely found the need for a comma after an inner `match` annoying (you missed this one but its the same). ---
**Author:** ![harpocrates](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/harpocrates/32/3097_2.png) [@harpocrates](https://internals.rust-lang.org/u/harpocrates)\ **Post date:** [March 31, 2017, 9:42pm UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/5 "2017-03-31T21:42:11Z")
I came here to figure out what conflicts this would introduce from a parsing perspective and, armed with an [LALR grammar I recently wrote](https://github.com/harpocrates/language-rust/blob/master/src/Language/Rust/Parser/Internal.y), I tried loosening the need for a comma after block-like expressions (`if`, `loop`, `for`, `while`, `match`, macros with braces, and unsafe blocks) to see what conflicts that would introduce. To my surprise, it didn’t introduce any. Of course, it could be my grammar that is broken. 🙂 Naturally, we still _do_ need commas after struct literals even if they end in a block… ---
**Author:** ![matklad](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/matklad/32/12266_2.png) [@matklad](https://internals.rust-lang.org/u/matklad)\ **Post date:** [March 31, 2017, 9:56pm UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/6 "2017-03-31T21:56:00Z")
A problematic case might be ```rust let x: &i32 = unimplemented!(); match x { &1 => if true { 1 } else { 2 } &3 // is &3 a pattern or a bitwise and? } ``` But looks like this is already forbidden in match arms (in blocks, &3 is treated as a separate expression statement). ---
**Author:** ![harpocrates](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/harpocrates/32/3097_2.png) [@harpocrates](https://internals.rust-lang.org/u/harpocrates)\ **Post date:** [April 1, 2017, 1:02am UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/7 "2017-04-01T01:02:01Z")
`if true { 1 } else { 2 } &3` _isn’t a block-like expression_, so it isn’t even in the running. `&3` cannot be anything but a pattern. The problem you are describing is an ambiguity that one might see happening in expression statements. To deal with _that_ Rust expects the LHS of a binary expression to not be block-like for it to be turned into a statement. In any case, this poses no problems in `match` blocks. ---
**Author:** ![matklad](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/matklad/32/12266_2.png) [@matklad](https://internals.rust-lang.org/u/matklad)\ **Post date:** [April 1, 2017, 6:46am UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/8 "2017-04-01T06:46:12Z")
@harpocrates are you aware of [https://github.com/rust-lang/rust/issues/30942](https://github.com/rust-lang/rust/issues/30942)? I think yours is the most complete LALR style Rust grammar so far, might be useful to advertise it on that issue 🙂 ---
**Author:** ![matklad](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/matklad/32/12266_2.png) [@matklad](https://internals.rust-lang.org/u/matklad)\ **Post date:** [April 1, 2017, 8:16am UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/9 "2017-04-01T08:16:10Z")
PR: [https://github.com/rust-lang/rust/pull/40989](https://github.com/rust-lang/rust/pull/40989) ---
**Author:** ![system](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/system/32/14092_2.png) [@system](https://internals.rust-lang.org/u/system)\ **Post date:** [March 25, 2019, 8:28am UTC](https://internals.rust-lang.org/t/syntax-of-block-like-expressions-in-match-arms/5025/10 "2019-03-25T08:28:16Z")
This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.