(This is a report of what I believe is a "new" bug in the D compiler.  I'm not sure whether this newsgroup is the appropriate place to report such things, but it is the best I have been able to find.)

While experimenting with scope guard statements, I came across some unexpected behavior.  This snippet:

{
 if( true ){ scope(exit) writef("hello"); }
 writefln(" world" );
}

Produces the output "hello world", as I expected.  However, if I remove the if statement's braces, like so:

{
 if( true ) scope(exit) writef("hello");
 writefln( " world" );
}

... then the first writef call is skipped entirely, and the output is " world".  No errors or compiler warnings are generated.

I'm using the dmd compiler v2.010 on Linux.

Hi Steve,

The place to report issues is here: http://d.puremagic.com/issues/ . You need to register a bugzilla account, then click "New" on the bottom bar, then "D", and enter a world of happiness and bug reporting.

I would guess that in both cases, that should produce the output:

 world
hello

Since the writefln("hello") is within a scope-exit, so should only fire as part of the function exit condition.

Steve Jackson wrote:
> (This is a report of what I believe is a "new" bug in the D compiler.  I'm not sure whether this newsgroup is the appropriate place to report such things, but it is the best I have been able to find.)
> 
> While experimenting with scope guard statements, I came across some unexpected behavior.  This snippet:
> 
> {
>  if( true ){ scope(exit) writef("hello"); }
>  writefln(" world" );
> }
> 
> Produces the output "hello world", as I expected.  However, if I remove the if statement's braces, like so:
> 
> {
>  if( true ) scope(exit) writef("hello");
>  writefln( " world" );
> }
> 
> ... then the first writef call is skipped entirely, and the output is " world".  No errors or compiler warnings are generated.
> 
> I'm using the dmd compiler v2.010 on Linux.

On Wed, 05 Mar 2008 23:54:35 +0100, Robert Fraser <fraserofthenight@gmail.com> wrote:

> Hi Steve,
>
> The place to report issues is here: http://d.puremagic.com/issues/ . You need to register a bugzilla account, then click "New" on the bottom bar, then "D", and enter a world of happiness and bug reporting.
>
> I would guess that in both cases, that should produce the output:
>
>   world
> hello
>
> Since the writefln("hello") is within a scope-exit, so should only fire as part of the function exit condition.

Actually, "hello world" is indeed correct output, as the scope(exit) is executed when it exits the scope of the if{}.

The latter does indeed look like a bug, though.

-- Simen

On Wed, 05 Mar 2008 16:26:27 -0500, Steve Jackson <sjackso@cs.wisc.edu> wrote:

>(This is a report of what I believe is a "new" bug in the D compiler.  I'm not sure whether this newsgroup is the appropriate place to report such things, but it is the best I have been able to find.)
>
>While experimenting with scope guard statements, I came across some unexpected behavior.  This snippet:
>
>{
> if( true ){ scope(exit) writef("hello"); }
> writefln(" world" );
>}
>
>Produces the output "hello world", as I expected.  However, if I remove the if statement's braces, like so:
>
>{
> if( true ) scope(exit) writef("hello");
> writefln( " world" );
>}
>
>... then the first writef call is skipped entirely, and the output is " world".  No errors or compiler warnings are generated.
>
>I'm using the dmd compiler v2.010 on Linux.

Looks similar to this bug.

http://d.puremagic.com/issues/show_bug.cgi?id=1765

Gide

On Wed, 05 Mar 2008 16:26:27 -0500, Steve Jackson <sjackso@cs.wisc.edu> wrote:

>(This is a report of what I believe is a "new" bug in the D compiler.  I'm not sure whether this newsgroup is the appropriate place to report such things, but it is the best I have been able to find.)
>
>While experimenting with scope guard statements, I came across some unexpected behavior.  This snippet:
>
>{
> if( true ){ scope(exit) writef("hello"); }
> writefln(" world" );
>}
>
>Produces the output "hello world", as I expected.  However, if I remove the if statement's braces, like so:
>
>{
> if( true ) scope(exit) writef("hello");
> writefln( " world" );
>}
>
>... then the first writef call is skipped entirely, and the output is " world".  No errors or compiler warnings are generated.
>
>I'm using the dmd compiler v2.010 on Linux.

Looks like a bug but makes no sense in a real application anyway?

if( true ) scope(exit) writef("hello");
is semantically the same as simply
if( true ) writef("hello");

Do you have any use case for that?

On 06/03/2008, Max Samukha <samukha@voliacable.com.removethis> wrote:
>  if( true ) scope(exit) writef("hello");
>
> is semantically the same as simply
>  if( true ) writef("hello");

For that matter,

    if (true) anything

is the same as

    anything

But I assume "if (condition)" what what was really intended. It seems to me that one could instead write:

    scope(exit) { if (condition) writef("hello") }

That is, reverse the order of "scope" and "if".

On Thu, 6 Mar 2008 10:47:19 +0000, "Janice Caron" <caron800@googlemail.com> wrote:

>On 06/03/2008, Max Samukha <samukha@voliacable.com.removethis> wrote:
>>  if( true ) scope(exit) writef("hello");
>>
>> is semantically the same as simply
>>  if( true ) writef("hello");
>
>For that matter,
>
>    if (true) anything
>
>is the same as
>
>    anything
>

scope is redundant in your example because it is the only statement within if's scope. It executes a block of statements when the thread exits if's scope. you could as well omit it.

if (condition)
{
    scope(exit)
   {
		...
   }
}

or without braces (according to you, doesn't work as expected)

if (condition)
    scope(exit)
    {
		...
    }

should be equivalent to

if (condition)
{
	...
}

In case of static if, which doesn't create a new scope:

{
    static if (condition)
       scope (exit)
            writef("hello");

} // hello should be printed when control gets here (not tested)

>But I assume "if (condition)" what what was really intended. It seems to me that one could instead write:
>
>    scope(exit) { if (condition) writef("hello") }
>
>That is, reverse the order of "scope" and "if".

{
    bool condition = ...

    ...

    scope(exit)
    {
        if (condition) writef("hello");
    }

  ...
} // hello is printed at this point if condition is true
  // and the function didn't return or throw before
  // the 'scope'

On Thu, 06 Mar 2008 15:37:07 +0200, Max Samukha <samukha@voliacable.com.removethis> wrote:

>On Thu, 6 Mar 2008 10:47:19 +0000, "Janice Caron" <caron800@googlemail.com> wrote:
>
>>On 06/03/2008, Max Samukha <samukha@voliacable.com.removethis> wrote:
>>>  if( true ) scope(exit) writef("hello");
>>>
>>> is semantically the same as simply
>>>  if( true ) writef("hello");
>>
>>For that matter,
>>
>>    if (true) anything
>>
>>is the same as
>>
>>    anything
>>
>
>scope is redundant in your example because it is the only statement within if's scope. It executes a block of statements when the thread exits if's scope. you could as well omit it.
>
>if (condition)
>{
>    scope(exit)
>   {
>		...
>   }
>}
>
>or without braces (according to you, doesn't work as expected)
>
>if (condition)
>    scope(exit)
>    {
>		...
>    }
>
>should be equivalent to
>
>if (condition)
>{
>	...
>}
>
>In case of static if, which doesn't create a new scope:
>
>{
>    static if (condition)
>       scope (exit)
>            writef("hello");
>
>} // hello should be printed when control gets here (not tested)
>
>>But I assume "if (condition)" what what was really intended. It seems to me that one could instead write:
>>
>>    scope(exit) { if (condition) writef("hello") }
>>
>>That is, reverse the order of "scope" and "if".
>
>{
>    bool condition = ...
>
>    ...
>
>    scope(exit)
>    {
>        if (condition) writef("hello");
>    }
>
>  ...
>} // hello is printed at this point if condition is true
>  // and the function didn't return or throw before
>  // the 'scope'

Sorry, Janice, somehow my brain mistook you for Steve.
Please ignore

Max Samukha Wrote:
> Do you have any use case for that?

No.  I encountered this while experimenting with the behavior of scope(), not while building any particular application.  I agree that my example is a corner case.  But I thought it best to raise the issue in case it reflects some more subtle problem in the compiler.

I appreciate the prompt responses!

Max Samukha wrote:
> On Wed, 05 Mar 2008 16:26:27 -0500, Steve Jackson
> <sjackso@cs.wisc.edu> wrote:
> 
>> (This is a report of what I believe is a "new" bug in the D compiler.  I'm not sure whether this newsgroup is the appropriate place to report such things, but it is the best I have been able to find.)
>>
>> While experimenting with scope guard statements, I came across some unexpected behavior.  This snippet:
>>
>> {
>> if( true ){ scope(exit) writef("hello"); }
>> writefln(" world" );
>> }
>>
>> Produces the output "hello world", as I expected.  However, if I remove the if statement's braces, like so:
>>
>> {
>> if( true ) scope(exit) writef("hello");
>> writefln( " world" );
>> }
>>
>> ... then the first writef call is skipped entirely, and the output is " world".  No errors or compiler warnings are generated.
>>
>> I'm using the dmd compiler v2.010 on Linux.
> 
> Looks like a bug but makes no sense in a real application anyway?
> 
> if( true ) scope(exit) writef("hello");
> is semantically the same as simply
> if( true ) writef("hello");
> 
> Do you have any use case for that?

Compile-time values could evaluate to true, and compile-time code generation could very easily create code that would seem silly to write directly. So it's a valid problem.