A Case Against Separators

[mitsuhiko]

There are two places where rust currently users separators instead of terminators where the constructs are usually line terminated: struct fields and match arms:

struct Foo {
   left: int,
   right: int
}

let x = match foo() {
  None => 0,
  Some(num) => num
}

Here is why I think this is a bad idea: diffs. At the moment if you add a field to the struct at the end most diff tools will show two modified lines which causes conflicts if two developers add an entry. This is pretty annoying. For match there is currently a way to avoid that by wrapping the arms in {} in which case there is no terminator at all and the diff succeeds. The struct however at the moment requires the commas. I am not sure why, but I think it's because of the initializer syntax.

I understand that an argument against that could be to always add a terminating comma but history has shown that developers will not do that because they don't see the benefit of it (and it also looks unexpected).

[mitsuhiko]

Example diff showing the problem:

$ diff -u a.rs b.rs
--- a.rs 2012-10-18 02:31:09.000000000 +0100
+++ b.rs 2012-10-18 02:31:18.000000000 +0100
@@ -1,3 +1,4 @@
 struct Foo {
- field_1: int
+ field_1: int,
+ field_2: int
 }

[Blei]

Note that you can terminate each field/match clause with a ,. I.e., the following is legal:

struct Foo {
   a: int,
   b: int,
}

fn bar(c: uint) {
    match c {
        0 => io::println("0"),
        1 => io::println("1"),
        2 => io::println("2"),
        _ => io::println("many"),
    }
}

[mitsuhiko]

That is mentioned in the comment. My point is: programmers optimize towards laziness.

[bblum]

I think the optional comma at the end is useful precisely for this reason. Programmers who care about diffs can use them; if you're running a project and you think it's important, you can make it part of the house style guide.

It's important for it to be optional, too, though, especially for concise one-liners. foo { f: 1, g: 2, } and match bar() { Some(x) => x, None => fail, } just look bad.

[catamorphism]

Insufficient interest, closing.