Forbid dynamic arrays in boolean evaluation contexts

[Steven Schveighoffer]

On Mon, 25 Mar 2013 10:18:11 -0400, Daniel Murphy  
<yebblies at nospamgmail.com> wrote:

> "bearophile" <bearophileHUGS at lycos.com> wrote in message
> news:bwgnbflygowctlisistg at forum.dlang.org...
>> So my proposal of Issue 4733 is to forbid (with the usual
>> warning/deprecation intermediate steps) the use of dynamic arrays in a
>> boolean context:
>
> I am in favour of deprecating/removing this, then bringing it back so  
> that
> if (arr)
> is the same as
> if (arr.length)

I would favor just changing the behavior.  The existing behavior is most  
often a bug when it is used, because quite often arrays are null (that is  
the default value), and quite often, people THINK they are checking if the  
array is empty instead of if the pointer is null.  Because most empty  
arrays are null, and null arrays have zero length, their tests do not  
expose this bug.  It's actually very difficult to generate a non-null  
empty array for testing. [] does not work, it returns a null array!

Only in rare cases do people actually want to check for null, and in most  
people's code it is an error when a non-null but empty array is checked  
with if(arr).  If people are interested in the pointer, they usually check  
if(arr.ptr) or if(arr !is null).

I would posit that for every knowledgeable person who uses this "shortcut"  
to check for null, 100 people use it expecting it to be measuring the  
array length.

If we go through a deprecation cycle, it makes people who do if(arr) for  
length have to change there code to if(arr.length) then back to if(arr)  
once the deprecation is over.  The switch to if(arr.length) will be  
simple, the compiler will show all the places if(arr) is used, but the  
switch back will be more difficult since if(arr.length) will not be  
deprecated.

What about a switch that identifies all places where if(arr) is done?   
Then people who use if(arr) for testing if(arr.ptr) can find and fix their  
cases.  Or even a separate tool to do that.

-Steve