• nobody <nobody mailinator.com> Oct 29 2007
• Don Clugston <dac nospam.com.au> Oct 29 2007
• nobody <nobody mailinator.com> Oct 29 2007
• Matti Niemenmaa <see_signature for.real.address> Oct 29 2007

nobody <nobody mailinator.com> writes:

I do not think it should ever be the case that this block:

   if(false)
     if(true)
     {
       assert(false);
     }
   else
   {
     assert(true);
   }

should ever be functionally different from this block:


   if(false)
   {
     if(true)
     {
       assert(false);
     }
   }
   else
   {
     assert(true);
   }

The compiler agrees with me in this simple case. However, I have 
found a spot in my code where the second block results in 
correct behaviour and the first block does not. This is surely a 
compiler (v1.015) bug? Here is the big picture:

   struct S
   {
     void f()
     {
       while()
       {
         foreach()
         {
           if(...)
           {
             if(...)
             {

             }

             else
               if(...)
               {

               }
           }

           else
           {
             if(...)
             { // need brackets or next else is not attached
               if(...)
               {

               }
             }
             else
             {
               if(...)
               {

               }

               else if(...)
               {

               }

               else if(...)
               {

               }

               else if(...)
               {

               }
             }
           }
         }
       }
     }
   }

Don Clugston <dac nospam.com.au> writes:

nobody wrote:
 I do not think it should ever be the case that this block:
 
   if(false)
     if(true)
     {
       assert(false);
     }
   else
   {
     assert(true);
   }
 
 should ever be functionally different from this block:
 
 
   if(false)
   {
     if(true)
     {
       assert(false);
     }
   }
   else
   {
     assert(true);
   }


The else always wants to join to the most recent if.
The first snippet is equivalent to:

if (false) {
   if (true) {
     assert(false);
   } else {
     assert(true);
   }
}

So I can't see a bug in the bigger example.

nobody <nobody mailinator.com> writes:

Don Clugston wrote:
 nobody wrote:
 I do not think it should ever be the case that this block:

   if(false)
     if(true)
     {
       assert(false);
     }
   else
   {
     assert(true);
   }

 should ever be functionally different from this block:


   if(false)
   {
     if(true)
     {
       assert(false);
     }
   }
   else
   {
     assert(true);
   }


 The else always wants to join to the most recent if.
 The first snippet is equivalent to:
 
 if (false) {
   if (true) {
     assert(false);
   } else {
     assert(true);
   }
 }
 
 So I can't see a bug in the bigger example.


You are right. I was wrong in thinking that the elimination in 
the case of single statements applied here, which it does not.

Matti Niemenmaa <see_signature for.real.address> writes:

nobody wrote:
 I do not think it should ever be the case that this block:
 
   if(false)
     if(true)
     {
       assert(false);
     }
   else
   {
     assert(true);
   }
 
 should ever be functionally different from this block:
 
 
   if(false)
   {
     if(true)
     {
       assert(false);
     }
   }
   else
   {
     assert(true);
   }
 
 The compiler agrees with me in this simple case.


No, you've got it wrong. It's:

if (false) {
	if (true) {
		assert (false);
	} else {
		assert (true);
	}
}

Which compiles down to nothing since if (false) is always false.

The compiler appears to agree because both blocks actually do nothing. Compile
and run this:

import std.stdio;

void main() {
	writefln("%d", f      (false,true));
	writefln("%d", f_wrong(false,true));
	writefln("%d", f_right(false,true));
}

int f(bool a, bool b) {
	if (a)
		if (b) {
			return 0;
		} else {
			return 1;
		}
	return 2;
}
int f_wrong(bool a, bool b) {
	if (a) {
		if (b) {
			return 0;
		}
	} else {
		return 1;
	}
	return 2;
}
int f_right(bool a, bool b) {
	if (a) {
		if (b) {
			return 0;
		} else {
			return 1;
		}
	}
	return 2;
}

-- 
E-mail address: matti.niemenmaa+news, domain is iki (DOT) fi