Throughout "The Rust Programming Language", trailing commas are used in the examples, seemingly anywhere there is a comma-separated list... i.e.: enum Message { Quit, Move { x: i32, y: i32 }, Write(String), ChangeColor(i32, i32, i32), // <-- Trailing comma } Are those necessary? Do…

They are optional, but considered good style. No one will call you out on not using them, though :).

[image] vozeldr: Are those necessary? No. [image] vozeldr: Do they change the functionality similar to having a trailing semi-colon versus leaving it off changes from a statement to an expression? No. [image] vozeldr: Or is this just a preference to make it easier to add items on…

Just to add to the other comments in this thread with a possible rationale. Adding a trailing comma can ease refactoring. Rust doesn't (yet) have amazing IDE support and forgetting to add a comma when adding a new field can feel a bit like a papercut.

More importantly, line-by-line diffs (like GitHub uses) will mark fewer lines as changed when you use trailing commas.

Just one thing: If you use them religiously, you will frequently run into the fact that they cannot be used with: panic!() assert_eq!() ...and quite frankly, in my experience, any other macro not written by me. Except for the json! macro , in some bizarre twist of irony . (I mean, c'mon, really! …

[image] ExpHP: ExpHP’s magical guide to supporting a trailing comma I usually just put a $(,)* at the end.

I secretly do that everywhere for internal macros, but for anything user-facing I'd probably want it to give an error on two commas.

I posted a comprehensive report on all unsupported trailing commas in std macros to internals. I'm glad to report that assert_eq! has apparently been fixed in the upcoming release!

Trailing Commas

tutorials

vozeldr November 21, 2017, 6:23pm 4

Thanks @skade and @notriddle!

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Trailing Commas

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