D port of dmd: Lexer, Parser, AND CodeGenerator fully operational

Jonathan M Davis jmdavisProg at gmx.com
Wed Mar 7 23:48:28 PST 2012


On Thursday, March 08, 2012 08:21:17 Zach the Mystic wrote:
> On Thursday, 8 March 2012 at 04:56:07 UTC, Jonathan M Davis wrote:
> > If you took it from ddmd, then it's definitely going to have to
> > be GPL.
> > 
> > Now, there is interest in having a D parser and lexer in
> > Phobos. I don't know
> > if your version will fit the bill (e.g. it must have a
> > range-based API), but we
> > need one at some point. The original idea was to more or less
> > directly port
> > dmd's lexer and parser with some adjustments to the API as
> > necessary
> > (primarily to make it range-based). But no one has had the time
> > to complete
> > such a project yet (I originally volunteered to do it, but I
> > just haven't had
> > the time).
> > 
> > When that project was proposed, Walter agreed to let that port
> > be Boost rather
> > than GPL (since he holds the copyright and the port would be
> > going in Phobos,
> > which uses boost).
> > 
> > The problem with what you have (even if the API and
> > implementation were
> > perfect) is that it comes from ddmd, which had other
> > contributors working on
> > it. So, you would have to get permission from not only Walter
> > but all of the
> > relevant ddmd contributors. If you were able to _that_, and it
> > could get
> > passed the review process, then what you've done could be put
> > into Phobos. But
> > that requires that you take the time and effort to take care of
> > getting the
> > appropriate permissions, making sure that the API and
> > implementation are
> > acceptable for Phobos, and putting it through the Phobos review
> > process. It
> > would be great if you could do that though.
> > 
> > - Jonathan M Davis
> 
> This is great news. I was really worried that the license was
> etched in stone. I'll need help finding out who owns the code,
> plus legal advice if the process is more than just getting a
> simple confirmation email from each of the original authors.
> 
> I have some comments I feel are very interesting regarding the
> lexer and pointers. There are no pointers in any of the code
> besides the lexer, so I think that will be very satisfying to
> you. Now I don't know everything about ranges, but if you simply
> mean dynamic arrays, then yes, everything except the lexer uses
> arrays when necessary, although there's simply a lot of code
> which doesn't need them because most of the objects are really
> just lists of members, many of which are not arrays.
> 
> About the lexer, one thing I realized about the Wild-West pointer
> style as I was porting it is that it must be blazing fast. To my
> understanding, to call p.popFront() requires two operations, ++p;
> followed by --p.length; plus possibly array bounds checking, I
> don't know.
> 
> ++p is all that the current lexer needs. It used to only check
> for EOF at each junction, but since I'm parsing little chunks of
> code instead of whole files now, it checks "if ( p >= endBuf )"
> at the beginning of each token scan, which gets pretty close to
> not going out of bounds, since most tokens aren't very long. That
> lexer is a tribute to very fast programming of an old school
> which will go away if it changes. Still, I can sense a tidal wave
> of RANGES coming, and I fear I'll just have to bid the little
> thing goodbye! :-(

A range is not necessarily a dynamic array, though a dynamic array is a range. 
The lexer is going to need to take  a range of dchar (which may or may not be 
an array), and it's probably going  to need to return a range of tokens. The 
parser would then take a range of  tokens and then output the AST in some form 
or other - it probably couldn't be  range, but I'm not sure. And while the 
lexer would need to operate on generic ranges of dchar, it would probably have 
to be special-cased for strings in a number of places in order to make it 
faster (e.g. checking the first char in a string rather than using front when 
it's known that the value being checked against is an ASCII character and will 
therefore fit in a single char - front has to decode the next character, which 
is less efficient).

So, if you're not familiar with ranges, you probably have a fair bit of 
learning ahead of you, and you're probably going to have to make a number of 
changes to your lexer and parser (though the majority of it will probably be 
able to stay intact). Unfortunately, a proper article and tutorial on them is 
currently lacking in spite of the fact that Phobos uses them heavily. 
Fortunately however, in a book that Ali Çehreli is writing on D, he has a 
chapter on ranges that should help get you started:

http://ddili.org/ders/d.en/ranges.html

But I'd suggest that you play around with ranges a fair bit (especially with 
strings) before trying to change what you have to use them. std.algorithm in 
particular makes heavy use of ranges. And it wouldn't surprise me at all if 
some portions of your lexer and parser really should be using some of Phobos' 
functions but isn't currently, because it's originally a port from C++. You 
should also make sure that you understand the basics of Unicode fairly well - 
especially with how they pertain to char, wchar, and dchar - since that will 
affect your ability to correctly translate code to use ranges as well as 
properly optimize them.

It would probably help if other D developers who are more familiar with ranges 
took a look at what you have and maybe even helped you start adjusting your 
code, but I don't know how many will both have the time and be interested. If 
I have time, I'll probably start poking at it, but I don't know that I'll have 
time any time soon, much as I'd like to.

Regardless, you need to familiarize yourself with ranges if you want to get 
the lexer and parser ready for inclusion in Phobos. And you really should 
familiarize yourself with them anyway, since they're heavily used in D code in 
general. Not being able to use ranges in D would be like not being able to use 
iterators in C++. You can program in it, but you'd be fairly crippled - 
particularly when dealing with the standard library.

- Jonathan M Davis


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