[Issue 11988] Add __switch symbol to allow retrieval of switch statement value

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=11988


monkeyworks12 at hotmail.com changed:

           What    |Removed                     |Added
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                 CC|                            |monkeyworks12 at hotmail.com


--- Comment #1 from monkeyworks12 at hotmail.com 2014-01-24 13:30:21 PST ---
(In reply to comment #0)
> Often you would want to use the value of the switch statement, but the only way
> to do this is to store the value to a variable first. This has the unfortunate
> effect of invading the outer scope:
> 
> -----
> import std.string;
> 
> int get() { return 0; }
> 
> void a(int) { }
> void b(int) { }
> void c(int) { }
> 
> void main()
> {
>     int v = get();
>     switch (v)
>     {
>         case 1: a(v); break;
>         case 2: b(v); break;
>         case 3: c(v); break;
> 
>         default:
>             assert(0, format("Unhandled case %s", v));
>     }
> 
>     // problem: v is still visible here
> }
> -----
> 
> To avoid invading the outer scope, but at the same time avoiding introduction
> of arbitrary language features, it might be useful to introduce another
> compiler-reserved symbol "__switch". The above code would then look like:
> 
> -----
> import std.string;
> 
> int get() { return 0; }
> 
> void a(int) { }
> void b(int) { }
> void c(int) { }
> 
> void main()
> {
>     switch (get())
>     {
>         case 1: a(__switch); break;
>         case 2: b(__switch); break;
>         case 3: c(__switch); break;
> 
>         default:
>             assert(0, format("Unhandled case %s", __switch));
>     }
> }
> -----
> 
> The default diagnostic is where I've encountered a need for this feature, very
> frequently too.

Why don't we just allow `auto var = <expression>`, like in an if-statement?

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