http://d.puremagic.com/issues/show_bug.cgi?id=596

           Summary: Support array, arrayliteral and struct in switch and
                    case
           Product: D
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: lovesyao@hotmail.com


struct Test{
  int x;
  int y;
  int z;
}

void main(){
  Test* t = new Test;
  const static Test c1 = {0,0,0};
  switch(*t){
    case c1://struct
      break;
    default:
      assert(0);
  }
  ubyte[] t2 = [0,1,2];
  switch(t2){
    case [0,1,2]://arrayliteral
      break;
    default:
  }
}


-- 

d-bugmail@puremagic.com schrieb am 2006-11-25:
> http://d.puremagic.com/issues/show_bug.cgi?id=596

> void main(){

<snip>

>   ubyte[] t2 = [0,1,2];
>   switch(t2){
>     case [0,1,2]://arrayliteral
>       break;
>     default:
>   }
> }

Added to DStress as http://dstress.kuehne.cn/run/c/case_05_A.d http://dstress.kuehne.cn/run/c/case_05_B.d http://dstress.kuehne.cn/run/c/case_05_C.d http://dstress.kuehne.cn/run/s/switch_23_A.d http://dstress.kuehne.cn/run/s/switch_23_B.d http://dstress.kuehne.cn/run/s/switch_23_C.d

Thomas

http://d.puremagic.com/issues/show_bug.cgi?id=596


Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |andrei@metalanguage.com
         AssignedTo|nobody@puremagic.com        |bugzilla@digitalmars.com


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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs@eml.cc


--- Comment #2 from bearophile_hugs@eml.cc 2010-11-26 12:37:59 PST ---
If structs become supported by switch, then an interesting use case is to support Tuples of typecons too.

(A possible enhancement is to support class references too (comparing the dynamic type), but similar switches on objects is considered a bad practice in OO code. So maybe it's better to not support class references).

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--- Comment #3 from bearophile_hugs@eml.cc 2011-01-07 05:13:09 PST ---
A possible improvement for the switch on structs is to support wildcards in some way:

struct Foo { int x, y; }
void main() {
    auto f = Foo(1, 2);
    int f;
    switch (f) {
        case Foo(1, _): break; // all structs where x==1, y == don't care
        default: break;
    }
}

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--- Comment #4 from bearophile_hugs@eml.cc 2011-01-30 06:37:06 PST ---
(In reply to comment #3)

> struct Foo { int x, y; }
> void main() {
>     auto f = Foo(1, 2);
>     int f;
>     switch (f) {
>         case Foo(1, _): break; // all structs where x==1, y == don't care
>         default: break;
>     }
> }

The idea of wildcards is very useful, it turns the D switch into something useful to partially match Tuples too. This is a very commonly used operation in functional languages. But probably the "_" (underscore) is not right as wildcard, because while it's nice and clear, it's a legal variable name. Possible alternatives are a single &, or @ or %:

case Foo(1, &): break;
case Foo(1, @): break;
case Foo(1, %): break;

Or maybe just "void", that is longer, but adds no new tokens and is more clear than an arbitrary symbol:


import std.typecons: Tuple;
alias Tuple!(int, "x", int, "y") Foo;
void main() {
    auto f = Foo(1, 2);
    switch (f) {
        case Foo(1, void): break; // any Foo with x==1, don't care y
        default: break;
    }
}

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--- Comment #5 from bearophile_hugs@eml.cc 2011-03-21 18:24:53 PDT ---
If structs too gets supported by switch, then BigInts too become allowed:


import std.bigint;
void main() {
    auto x = BigInt(3);
    switch (x) {
        case BigInt(0): break;
        default: break;
    }
}

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--- Comment #6 from bearophile_hugs@eml.cc 2011-07-23 12:15:36 PDT ---
Regarding tuple unpacking see also issue 6365

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--- Comment #7 from bearophile_hugs@eml.cc 2011-08-15 16:44:27 PDT ---
One more useful use is with OOP (this is GUI code):


import core.stdc.stdio;
class Control {}
class Slider : Control {}
class Button : Control {}
void main() {
    Control c = new Button;
    switch (typeof(c)) {
        case Slider: printf("A"); break;
        case Button: printf("B"); break;
        default: // probably a Control
    }
}



That is similar to this (but faster):

import core.stdc.stdio;
class Control {}
class Slider : Control {}
class Button : Control {}
void main() {
    Control c = new Button;
         if (cast(Slider)c)  { printf("A"); }
    else if (cast(Button)c)  { printf("B"); }
}

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--- Comment #8 from bearophile_hugs@eml.cc 2011-09-28 19:13:23 PDT ---
Supporting something like this will be very useful (this is done very commonly in functional languages):


import std.variant: Algebraic;
alias Algebraic!(int, float) A;
void main() {
    A a = 1.5;
    final switch (a.type()) {
        case typeid(int): ...
        case typeid(float): ...
    }
}

But if possible I suggest to introduce a compiler optimization specific for this usage, to avoid actually allocating and using TypeInfo class instances.

The advantage of using a final switch instead of a sequence of if statements:
- The code is more readable, the various cases are shown in a more ordered way;
- The "final" of switch makes sure all types of the Algebraic are taken into
account;
- The compiler has ways to optimize this code better (sometimes avoiding many
runtime tests).

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