• bearophile <bearophileHUGS lycos.com> Nov 13 2010
• Dmitry Olshansky <dmitry.olsh gmail.com> Nov 13 2010
• "Daniel Murphy" <yebblies nospamgmail.com> Nov 13 2010
• bearophile <bearophileHUGS lycos.com> Nov 13 2010
• Stanislav Blinov <stanislav.blinov gmail.com> Nov 13 2010
• BCS <anon anon.com> Nov 14 2010

bearophile <bearophileHUGS lycos.com> writes:

In a not-ranged cases body, like in the program below (that doesn't compile),
the switch variable is a compile-time constant, so why doesn't the compile see
x as constant there?


template Foo(uint x) {
    static if (x <= 1)
        enum Foo = 1;
    else
        enum Foo = x * Foo!(x - 1);
}

int bar(uint x) {
    switch (x) {
        case 0: return Foo!x;
        case 1: return Foo!x;
        case 2: return Foo!x;
        case 3: return Foo!x;
        case 4: return Foo!x;
        default: return -1;
    }
}

void main() {
    assert(bar(4) == 24);
}


That code works if I replace lines like:
case 2: return Foo!x;

With:
case 2: return Foo!2;

But when the code isn't DRY bugs may happen...
(There are ten different better ways to write that program, but this is not the
point).

Bye,
bearophile

Dmitry Olshansky <dmitry.olsh gmail.com> writes:

On 14.11.2010 1:21, bearophile wrote:
 In a not-ranged cases body, like in the program below (that doesn't compile),
the switch variable is a compile-time constant, so why doesn't the compile see
x as constant there?


In essence "case x:" is nothing but a glorified local label.
 template Foo(uint x) {
      static if (x<= 1)
          enum Foo = 1;
      else
          enum Foo = x * Foo!(x - 1);
 }

 int bar(uint x) {
      switch (x) {
          case 0: return Foo!x;
          case 1: return Foo!x;
          case 2: return Foo!x;
          case 3: return Foo!x;
          case 4: return Foo!x;
          default: return -1;
      }
 }

 void main() {
      assert(bar(4) == 24);
 }


 That code works if I replace lines like:
 case 2: return Foo!x;

 With:
 case 2: return Foo!2;

 But when the code isn't DRY bugs may happen...
 (There are ten different better ways to write that program, but this is not
the point).

 Bye,
 bearophile



-- 
Dmitry Olshansky

"Daniel Murphy" <yebblies nospamgmail.com> writes:

"bearophile" <bearophileHUGS lycos.com> wrote in message 
news:ibn320$2ucs$1 digitalmars.com...
 In a not-ranged cases body, like in the program below (that doesn't 
 compile), the switch variable is a compile-time constant, so why doesn't 
 the compile see x as constant there?


In switch statements, you can do stuff like:

switch(x)
{
case 0:
case 1:
   // what is x here?
  break;
}

switch(x)
{
case 0:
   break; // what is x here?
case 1:
   goto case 0:
}

goto label1;
switch(x)
{
case 0:
label1:
  break; // what is x here?
}

switch(x)
{
case 0:
  x = y;
  break; // what is x here?
}

As far as I know, NONE of the constructs in d allow you to treat a run-time 
variable as if it was compile-time constant.
I doubt this would be possible without flow analysis.

You can however do something like this (if you must)

template Foo(uint x) {
    static if (x <= 1)
        enum Foo = 1;
    else
        enum Foo = x * Foo!(x - 1);
}

int getv(int x)
{
   switch(x)
   {
      foreach(i; TypeTuple!(0, 1, 2, 3, 4, 5, 6))
      {
         case i: return Foo!i;
      }
   }
   assert(0);
}

where the switch expands out to

   switch(x)
   {
         case 0: return Foo!0;
         case 1: return Foo!1;
         case 2: return Foo!2;
         case 3: return Foo!3;
         case 4: return Foo!4;
         case 5: return Foo!5;
         case 6: return Foo!6;
   }

Is this DRY enough for you? 

bearophile <bearophileHUGS lycos.com> writes:

Daniel Murphy:

 switch(x)
 {
 case 0:
    break; // what is x here?
 case 1:
    goto case 0:
 }
 etc


You are right. Thank you for all the answers.

Bye,
bearophile

Stanislav Blinov <stanislav.blinov gmail.com> writes:

bearophile wrote:
 In a not-ranged cases body, like in the program below (that doesn't compile),
the switch variable is a compile-time constant, so why doesn't the compile see
x as constant there?
 
 
 template Foo(uint x) {
     static if (x <= 1)
         enum Foo = 1;
     else
         enum Foo = x * Foo!(x - 1);
 }
 
 int bar(uint x) {
     switch (x) {
         case 0: return Foo!x;
         case 1: return Foo!x;
         case 2: return Foo!x;
         case 3: return Foo!x;
         case 4: return Foo!x;
         default: return -1;
     }
 }
 
 void main() {
     assert(bar(4) == 24);
 }
 
 
 That code works if I replace lines like:
 case 2: return Foo!x;
 
 With:
 case 2: return Foo!2;
 
 But when the code isn't DRY bugs may happen...
 (There are ten different better ways to write that program, but this is not
the point).
 
 Bye,
 bearophile


I would say that while bar may be CTFE'd, it is nevertheless a function 
that can be called at runtime, in which case x may no longer be a 
compile-time constant. So there is little compiler can do except for 
refusing such code.

BCS <anon anon.com> writes:

Hello bearophile,

 In a not-ranged cases body, like in the program below (that doesn't
 compile), the switch variable is a compile-time constant, so why
 doesn't the compile see x as constant there?
 
 template Foo(uint x) {
 static if (x <= 1)
 enum Foo = 1;
 else
 enum Foo = x * Foo!(x - 1);
 }
 int bar(uint x) {
 switch (x) {
 case 0: return Foo!x;
 case 1: return Foo!x;
 case 2: return Foo!x;
 case 3: return Foo!x;
 case 4: return Foo!x;
 default: return -1;
 }
 }


If you want exactly that:

switch(x) {
  foreach(X; Tuple!(0,1,2,3,4)) {
    case X: return Foo!X;
  }
}