www.digitalmars.com         C & C++   DMDScript  

D - [Bug](low) dmd assumes that '=' always yields a non boolean

reply Manfred Nowak <svv1999 hotmail.com> writes:
void main()
{
  bit b1, b2, b3;
  b1=b2=!b3;
  if(b1) printf("true\n"); // prints 'true'
  // if(b1=!b3) printf("true\n");
  //error: '=' does not give a boolean result
}

In this case `=' yields a boolean.

So long.
Feb 10 2004
next sibling parent reply "C" <dont respond.com> writes:
I think = always returns a boolean result , it always returns true that the
assignment succeeded right ?

I use assingment in conditional expressions alot this is one I miss too.

C

"Manfred Nowak" <svv1999 hotmail.com> wrote in message
news:c0bebm$28bb$1 digitaldaemon.com...

 void main()
 {
   bit b1, b2, b3;
   b1=b2=!b3;
   if(b1) printf("true\n"); // prints 'true'
   // if(b1=!b3) printf("true\n");
   //error: '=' does not give a boolean result
 }

 In this case `=' yields a boolean.

 So long.
Feb 10 2004
parent "Ben Hinkle" <bhinkle4 juno.com> writes:
"C" <dont respond.com> wrote in message news:c0bf0d$29h8$1 digitaldaemon.com...
| I think = always returns a boolean result , it always returns true that the
| assignment succeeded right ?

Assignment returns the lvalue for chaining - same as in C.

| I use assingment in conditional expressions alot this is one I miss too.
|
| C
|
| "Manfred Nowak" <svv1999 hotmail.com> wrote in message
| news:c0bebm$28bb$1 digitaldaemon.com...
| > void main()
| > {
| >   bit b1, b2, b3;
| >   b1=b2=!b3;
| >   if(b1) printf("true\n"); // prints 'true'
| >   // if(b1=!b3) printf("true\n");
| >   //error: '=' does not give a boolean result
| > }
| >
| > In this case `=' yields a boolean.
| >
| > So long.
| >
|
|
Feb 10 2004
prev sibling next sibling parent Vathix <vathix dprogramming.com> writes:
Manfred Nowak wrote:


 void main()
 {
   bit b1, b2, b3;
   b1=b2=!b3;
   if(b1) printf("true\n"); // prints 'true'
   // if(b1=!b3) printf("true\n");
   //error: '=' does not give a boolean result
 }
 
 In this case `=' yields a boolean.
 
 So long.
 


Perhaps if the error message said that it wasn't a comparison instead of 
boolean, it would be more clear. It's to catch the common mistake of 
writing = when you actually meant to use ==. It caught me doing it a 
couple times (and I've written a LOT of D code), not very common, but 
could save a lot of frustration. Not to say that I'm for or against it; 
I don't mind either way.


-- 
Christopher E. Miller
www.dprogramming.com
Feb 10 2004
prev sibling next sibling parent reply Ilya Minkov <minkov cs.tum.edu> writes:
Manfred Nowak wrote:

 In this case `=' yields a boolean.


This is an intentional plug, since any (human) code reviewer would flag 
assignment in an if as a probable bug. And it is one more often than 
not. The = unstead of == bug is really a plague, very hard to see and to 
debug once it's there.

-eye
Feb 10 2004
next sibling parent reply Sean Kelly <sean ffwd.cx> writes:
Ilya Minkov wrote:

 
 This is an intentional plug, since any (human) code reviewer would flag 
 assignment in an if as a probable bug. And it is one more often than 
 not. The = unstead of == bug is really a plague, very hard to see and to 
 debug once it's there.


I'd rather this one be treated as a warning and have the phrasing 
changed a bit.  Assignments within if statements are quite common.

I don't suppose D supports declaration/assignments within if's?

if( int i = 5 ) {
     ...
}


Sean
Feb 10 2004
parent "C" <dont respond.com> writes:
 I don't suppose D supports declaration/assignments within if's?


No not supported.

"Sean Kelly" <sean ffwd.cx> wrote in message
news:c0biik$2fbj$1 digitaldaemon.com...

 Ilya Minkov wrote:

 This is an intentional plug, since any (human) code reviewer would flag
 assignment in an if as a probable bug. And it is one more often than
 not. The = unstead of == bug is really a plague, very hard to see and to
 debug once it's there.


 I'd rather this one be treated as a warning and have the phrasing
 changed a bit.  Assignments within if statements are quite common.

 I don't suppose D supports declaration/assignments within if's?

 if( int i = 5 ) {
      ...
 }


 Sean
Feb 10 2004
prev sibling parent larry cowan <larry_member pathlink.com> writes:
if (!(fp=fopen("filename","r")) { errmsg...; exit(1); }

is a very common C usage, and a few similar checks on returned values, but most
of those will probably disappear in D, in favor of streams and better exception
handling.  I don't see nearly as many reasonable usages in D. 

-larry

In article <c0bhh0$2drt$1 digitaldaemon.com>, Ilya Minkov says...

Manfred Nowak wrote:

 In this case `=' yields a boolean.


This is an intentional plug, since any (human) code reviewer would flag 
assignment in an if as a probable bug. And it is one more often than 
not. The = unstead of == bug is really a plague, very hard to see and to 
debug once it's there.

-eye
Feb 10 2004
prev sibling parent reply "BERO" <berobero users.sourceforge.net> writes:
I think it is not bug but specification.

if ( foo=bar)
may be typo of
if ( foo==bar)

If typo, it is bug in your code but difficult to find.
So, some C compiler such as GCC cause warning.
D cause error.

If you really want check lvalue(foo) is zero,write clearly
if ((foo=bar)!=0)
This way  is good even if C.

"Manfred Nowak" <svv1999 hotmail.com> wrote in message
news:c0bebm$28bb$1 digitaldaemon.com...

 void main()
 {
   bit b1, b2, b3;
   b1=b2=!b3;
   if(b1) printf("true\n"); // prints 'true'
   // if(b1=!b3) printf("true\n");
   //error: '=' does not give a boolean result
 }

 In this case `=' yields a boolean.

 So long.
Feb 10 2004
parent Manfred Nowak <svv1999 hotmail.com> writes:
BERO wrote:

[...]

 if ( foo=bar)
 may be typo of
 if ( foo==bar)


[...]

 If you really want check lvalue(foo) is zero,write clearly
 if ((foo=bar)!=0)


It is real fun to see how people try to circumvent bad language design
once they have noticed it.

Because `foo=bar' might be a typo of `foo==bar' the solution would be to
introduce something like `:=' for the assignment, then there would be no
fear, that `foo:=bar' could be a typo of `foo==bar'.

Why does `if((foo=bar))' or `if(!(foo=bar))' still result in the error
`'=' does not give a boolean result', whereas `if((foo=bar)==true)' as
well as your recommendation compiles?

And why is it good to compare a boolean with an integer number?

According to my education holding assignment to and query of a variable
distinct is the better choice anyway. So to me clear writing would be
`foo:= bar; if(foo)' instead of `if((foo= bar) != 0)'.

And if `if(foo=bar)' produces an error because of the possibility of a
typo, then the question raises why the statement `foo==bar;' does not
produce an error: `foo==bar;' might be a typo of `foo=bar;'.

To me the answer for the raised question is very simple: the definition of
the semantic of `=' in C, and therefore in D also, contradicts its
semantic people learn in early ages: to be the comparison operator. So
getting it wrong in an `if' statement and not finding it whenever it is
there have a common reason.

So long.
Feb 10 2004