void main()
{
  bool b;

  if (b = true) {}
}

ok, bad form I know, but the message is a bit.. odd.. right?

Regan

The real-world example read something like

function() {
  foreach() {
    if (!(r = function_call(a,b,c)) break;
  }
  return r;
}

-- 
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

Regan Heath wrote:
> void main()
> {
>   bool b;
> 
>   if (b = true) {}

Did you mean...

if (b == true) {}

or did the point of this code fly right over my head?

-- 
Justin (a/k/a jcc7)
http://jcc_7.tripod.com/d/

On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese wrote:

> Regan Heath wrote:
>> void main()
>> {
>>   bool b;
>> 
>>   if (b = true) {}
> 
> Did you mean...
> 
> if (b == true) {}
> 
> or did the point of this code fly right over my head?

Maybe, but the error message is

"test.d(5): '=' does not give a boolean result"

The message is "odd" because the variable in question is a bool ;-)

-- 
Derek
Melbourne, Australia
4/10/2004 2:56:56 PM

> void main()
> {
>    bool b;
>
>    if (b = true) {}
> }
>
> ok, bad form I know, but the message is a bit.. odd.. right?
>
> Regan

It's not a bug, it's a feature described in the specs : "Assignments do not yield boolean results."

On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese <jcc7@cox.net> wrote:
> Regan Heath wrote:
>> void main()
>> {
>>   bool b;
>>
>>   if (b = true) {}
>
> Did you mean...
>
> if (b == true) {}
>
> or did the point of this code fly right over my head?

It was intended to be an assignment, the real-world code looks something like:

bool function() {
  foreach() {
    if (!(r = function_call(a,b,c)) break;
  }
  return r;
}

I just found it really odd that assigning '=' to a 'bool' doesn't give a boolean result.

Regan

-- 
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

On Mon, 4 Oct 2004 14:58:42 +1000, Derek Parnell <derek@psych.ward> wrote:
> On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese wrote:
>
>> Regan Heath wrote:
>>> void main()
>>> {
>>>   bool b;
>>>
>>>   if (b = true) {}
>>
>> Did you mean...
>>
>> if (b == true) {}
>>
>> or did the point of this code fly right over my head?
>
> Maybe, but the error message is
>
> "test.d(5): '=' does not give a boolean result"
>
> The message is "odd" because the variable in question is a bool ;-)

Yeah that's what I mean't. Assigning to a bool should give a boolean result, yes?
eg.

bool a,b;

a = (b = true);

if (b = true) wasn't a bool, then the above would give an error, right? But it doesn't, instead this

bool r;
bool fn();

if (r = fn()) break;

gives an error.

Basically something seems amiss to me.

Regan

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Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

On Mon, 4 Oct 2004 23:36:53 +0200, Valéry Croizier <valery@freesurf.fr> wrote:
>> void main()
>> {
>>    bool b;
>>
>>    if (b = true) {}
>> }
>>
>> ok, bad form I know, but the message is a bit.. odd.. right?
>>
>> Regan
>
> It's not a bug, it's a feature described in the specs : "Assignments do not yield boolean results."

Well I'll be.. I didn't see that. :)

While I can understand why this 'feature' is a good idea, it stops mistakes like "if (a=b)" where you meant "if (a==b)" it is a little weird when applied to boolean assignments.

I guess I'll have to re-write:

bool function() {
  foreach() {
    if (!(r = function_call(a,b,c))) break;
  }
  return r;
}

as:

bool function() {
  foreach() {
    r = function_call(a,b,c);
    if (!r) break;
  }
  return r;
}

which is probably a GoodThing(TM) anyway.

Regan

-- 
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

On Tue, 05 Oct 2004 10:38:19 +1300, Regan Heath wrote:

> On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese <jcc7@cox.net> wrote:
>> Regan Heath wrote:
>>> void main()
>>> {
>>>   bool b;
>>>
>>>   if (b = true) {}
>>
>> Did you mean...
>>
>> if (b == true) {}
>>
>> or did the point of this code fly right over my head?
> 
> It was intended to be an assignment, the real-world code looks something like:
> 
> bool function() {
>    foreach() {
>      if (!(r = function_call(a,b,c)) break;
>    }
>    return r;
> }
> 
> I just found it really odd that assigning '=' to a 'bool' doesn't give a boolean result.
> 

This syntax format is a special case that D checks for. Its designed to prevent a common source of bugs in C/C++. The more correct way to code what you want is ...

 bool function() {
    foreach() {
      if ( (r = function_call(a,b,c)) == true) break;
    }
    return r;
 }

In general, the syntax form ...

   if ( a = b )

needs to be coded as ...

   if ( (a = b) != 0 )

The reason is that *most* of the time, what people mean when they code "if (a = b)" is really "if (a == b)". Walter decided to prevent this common coding error. So now if you really really mean to do an assignment /and/ test the result of that assignment, as if it was a boolean test, you must explicitly code the test part of it.

-- 
Derek
Melbourne, Australia

On Tue, 5 Oct 2004 07:56:19 +1000, Derek wrote:

> On Tue, 05 Oct 2004 10:38:19 +1300, Regan Heath wrote:
> 
>> On Sun, 03 Oct 2004 23:34:42 -0500, J C Calvarese <jcc7@cox.net> wrote:
>>> Regan Heath wrote:
>>>> void main()
>>>> {
>>>>   bool b;
>>>>
>>>>   if (b = true) {}
>>>
>>> Did you mean...
>>>
>>> if (b == true) {}
>>>
>>> or did the point of this code fly right over my head?
>> 
>> It was intended to be an assignment, the real-world code looks something like:
>> 
>> bool function() {
>>    foreach() {
>>      if (!(r = function_call(a,b,c)) break;
>>    }
>>    return r;
>> }
>> 
>> I just found it really odd that assigning '=' to a 'bool' doesn't give a boolean result.
>> 
> 
> This syntax format is a special case that D checks for. Its designed to prevent a common source of bugs in C/C++. The more correct way to code what you want is ...
> 
>  bool function() {
>     foreach() {
>       if ( (r = function_call(a,b,c)) == true) break;
>     }
>     return r;
>  }
> 
> In general, the syntax form ...
> 
>    if ( a = b )
> 
> needs to be coded as ...
> 
>    if ( (a = b) != 0 )
> 
> The reason is that *most* of the time, what people mean when they code "if (a = b)" is really "if (a == b)". Walter decided to prevent this common coding error. So now if you really really mean to do an assignment /and/ test the result of that assignment, as if it was a boolean test, you must explicitly code the test part of it.

Well I suppose it could also be written out thus ...

   a = b;
   if (a) ...

so ...

 bool function() {
     foreach() {
       r = function_call(a,b,c);
       if (r) break;
     }
     return r;
  }

-- 
Derek
Melbourne, Australia
5/10/2004 10:00:29 AM