earthquake changes of std.regexp to come

[Andrei Alexandrescu]

Bill Baxter wrote:
> On Wed, Feb 18, 2009 at 3:36 AM, Andrei Alexandrescu
> <SeeWebsiteForEmail at erdani.org> wrote:
> 
>> Besides std.regexp only works with (narrow) strings and we want it to work
>> on streams of all widths and structures. One pet complaint I have is that
>> std.regexp puts a class around it all as if everybody's favorite pastime
>> would be to inherit Regexp and override some random function in it.
> 
> So what do you think it should be, a struct?

Yes.

> That would imply to me that everybody's favorite pastime is making
> value copies of regex structures, when in fact nobody does that.

Well you'd be surprised. The RegEx class saves the state of the last 
search, which is a sensible thing to do. But then consider a simple 
range Splitter that, when iterated, nicely gives you...

string a = ",a,  bcd, def,gh,";
foreach (e; splitter(a, pattern(", *"))
     writeln("[", e, "]");

writes

[]
[a]
[bcd]
[def]
[gh]

This is similar to the function std.regex.split with the notable 
difference that no extra memory is allocated. Now Splitter is an input 
range. This means you wouldn't expect that you copy a Splitter and then 
have iterating the original value affect the copy. Well, that's exactly 
what happens when you use the "good" reference semantics of the RegEx 
stored inside splitter. Worse, RegExp has no cloning primitive, so I 
need to resort to storing the pattern and recompiling it from scratch at 
every copy of Splitter. So essentially the "good" semantics of RegEx are 
useless when it comes to composing it in larger objects.

> Regex is a class in order to give it reference semantics and provide
> encapsulation of some re-usable state.  Maybe it should be a final
> class, but my impression is "final class" doesn't really work in D.

Re-usable state is provided by structs too. In addition they can choose 
value vs. reference semantics with ease.


Andrei