switch-case-default over switch-when-else

[satyr]

Before

switch a
  when b
    foo()
  when c, d
    bar()
  else
    baz()

After

switch a
case b
  foo()
case c, d
  bar()
default
  baz()

• Less indentation
• Mirrors JS keywords
• No ambiguity against if-else (switch..when..then..if..then on one line not working #562)

[StanAngeloff]

Oh bummer, there was another ticket for this which Jeremy closed. I'll try and dig it up.

EDIT: #697.

[satyr]

On a related note, how about changing the subjectless compilation:

switch (true) {
  case !!a:
    b;
    break;
}

into:

switch (false) {
  case !a:
    b;
    break;
}

[TrevorBurnham]

This makes sense to me. What was the original justification for using when-else instead of JavaScript's case-default?

[zmthy]

Because it matches the when of loops and else of if.

[TrevorBurnham]

OK, on further reflection, I like the way it is now. Reasons:

1. The when-else syntax is consistent with Ruby. Also as in Ruby, there's no fall-through.
2. Because there's no fall-through, a switch statement from JavaScript may not be easily ported to CoffeeScript. The different keywords help to signal this (and preserve sanity if you're alternately working on CoffeeScript and JavaScript code).
3. As to the indentation, it's true that the standard style in Ruby is for case and when to line up, but it feels more consistent with CoffeeScript to keep it: If you were to remove the switch a line, you would get a syntax error because the case and default lines are dependent on it; the indentation clearly signals this.

[satyr]

1. CoffeeScript isnt Ruby

2. for is also quite different but we keep the keyword.

3. Would you rather write:

    try
      catch
      finally #?
   

[zmthy]

Coffeescript isn't Javascript, either, at least not in syntax. The switch doesn't work the same as in Javascript, so the syntax doesn't require consistency.

[TrevorBurnham]

OK, #3 is a good point. I'm fine with either keeping the mandatory indentation or adopting the Ruby standard, as long as there's a standard. The important thing is that we don't wind up in a situation where indentation is optional and half of folks do it one way and half do it another way.

[StanAngeloff]

The switch doesn't work the same as in Javascript, so the syntax doesn't require consistency.

This isn't true as of 0.9.3.

switch cond
  when true then false
  when false then true

into

switch (cond) {
  case true:
    false;
    break;
  case false:
    true;
    break;
}

No more ifs.

I prefer to keep the whens indented as they follow the switch. Perhaps the else can be placed one level out:

switch a
  when b
    foo()
  when c, d
    bar()
else
  baz()

[TrevorBurnham]

That looks like the worst of both worlds to me... if there's an else clause, it should be at the same level of indentation as the case clauses.

[zmthy]

This isn't true as of 0.9.3.

I was referring to the fact that we insert break into each case.

[jashkenas]

Let's leave it the way it is. I see the appeal of mirroring JS syntax, but it's not as compelling as it could be, because case and default are not used anywhere else in JavaScript. The general readability of switch ... when, the semantic consistency between the use of when and else here and elsewhere in CoffeeScript -- all tip the scales in favor of the current style.

[satyr]

the semantic consistency between the use of when

I don't think so--this when can take multiple condition, while others can't. And the ambiguity issue holds. We shouldn't use when/else there.

[satyr]

I guess I can use this indentation for now:

switch a
 when b, c
  while foo()
    bar()
 else
  baz()