DMD 0.170 release (foreach_reverse)

[Chris Nicholson-Sauls]

Bill Baxter wrote:
> Walter Bright wrote:
> 
>> The saving grace about foreach_reverse is:
>>
>> 1) Only a small (5% ?) of loops will be reverse, so you won't see it 
>> or need to type it that often.
> 
> 
> A good reason why it doesn't deserve its own keyword.
> 
>> 2) It's very clear what it does.
> 
> 
> And  'foreach(i; list.reversed)' is pretty clear as well.

Sure, and primitive arrays already have the .reverse property; however, that means extra 
temporaries that hit performance and memory usage.  Both these hits are canceled out by 
adding foreach_reverse.  The presence of opApplyReverse is primarily for completeness, and 
for generalization within templates.

>> 3) It isn't likely to conflict with other names.
> 
> 
> Because it's a 15 letter keyword with an underscore!  While we're at it, 
> let's make a for_i_equals_one_to_ten keyword, too.  I loop from 1 to 10 
> a lot in my code (well, not that much really <5%, but at least it's a 
> name that's not likely to conflict with other names).
> 
>> 4) It is analogous with C++'s reverse_iterator names.
> 
> 
> But those aren't keywords.  And even more analogous is STL's 
> std::for_each in the standard header 'algorithm'.
>    http://www.sgi.com/tech/stl/for_each.html
> Note that STL does not have a std::for_each_reverse.  You get that 
> behavior, as you might expect, by using a reverse iterator instead of a 
> forward iterator as the argument.
> 
> 
> And in another message Walter Bright wrote:
> 
>  > foreach_reverse was pretty trivial to implement. All the machinery 
> was > already there.
> 
> That should be setting off warning bells in your head, too.  "The 
> machinery is already there" is just another way of saying "this feature 
> is mostly redundant". The foreach machinery already does the job. 
> foreach_reverse is more or less just a copy-paste of that code with all 
> references to "opApply" replaced with "opApplyReverse".  So of course 
> it's trivial to implement.
> 
> I'll repeat a line from the "zen of python" one more time, at the risk 
> of being told that 'D isn't Python', because I think it's true 
> regardless of the language:
> 
>   "Special cases aren't special enough to break the rules."
> 
> As I understand it, the only real reason for foreach_reverse, when it 
> comes down to it, is because of a strong desire to have a special case 
> fast reverse iteration for arrays and strings.  But it really should be 
> possible to hide that special case from the user and just detect some 
> particular property e.g. "some_array.reversed" in a foreach statement. 
> It may be a little more work (and maybe you just punt on the 
> optimization till 2.0), but in the long run, the language and all it's 
> users will benefit from removing that special case.
> 
> --bb

Mostly agreeable.  However, neither does the language nor its users suffer from having the 
foreach_reverse statement.

-- Chris Nicholson-Sauls