Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 01 for the latest information.
People get scared when they hear the word Apocalypse, but here I mean
it in the good sense: a Revealing. An Apocalypse is supposed to reveal
good news to good people. (And if it also happens to reveal bad news
to bad people, so be it. Just don't be bad.)
What I will be revealing in these columns will be the design of Perl
6. Or more accurately, the beginnings of that design, since the design
process will certainly continue after I've had my initial say in the
matter. I'm not omniscient, rumors to the contrary notwithstanding.
This job of playing God is a little too big for me. Nevertheless,
someone has to do it, so I'll try my best to fake it. And I'll expect
all of you to help me out with the process of creating history. We all
have to do our bit with free will.
"If you look at the history of Perl 6 up to this point, you will see why this
column is subtitled The Ugly, the Bad, and the Good. The RFC process
of last year was ugly, in a good sense. It was a brainstorming process,
and that means it was deliberately ugly—not in the sense of incivility,
since the RFC process was in fact surprisingly civil, but in the sense that
there was little coherent design to the suggestions in the RFCs. Frankly,
the RFCs are all over the map, without actually covering the map. There are contradictory RFCs, and there are missing RFCs.
Many of the RFCs propose real problems but go off at funny angles in
trying to propose solutions. Many of them patch symptoms without
curing the underlying ailments.
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| Larry Wall will give his annual entertaining talk on the
state of the Perl world, covering both Perl 5 and
Perl 6 at this Year's Open Source Convention.
Don't miss this rare opportunity to hear the creator of
Perl, patch, and run share his insights.
|
| |
I also discovered Larry's First Law of Language Redesign: Everyone
wants the colon.
That was the Ugly part. The Bad part was that I was supposed to take
these RFCs and produce a coherent design in two weeks. I starting out
thinking I could just classify the RFCs into the good, bad, and ugly
categories, but somehow most of them ended up in the ugly category,
because the good ones typically had something wrong with them, and the
even the bad ones typically indicated a problem that could use some
thought, even if the solution was totally bogus.
It is now five months later, and I've been mulling over coherence the
whole time, for some definition of mulling. Many of you know what
happens when the size of your Perl process exceeds the size of your
physical memory—you start thrashing. Well, that's basically what
happened to me. I couldn't get enough of the problem into my head at
once to make good progress, and I'm not actually very good at
subdividing problems. My forte is synthesis, not analysis. It didn't
help that I had a number of distractions in my life, some of them
self-inflicted, and some of them not. I won't go into all that. Save
it for my unauthorized autobiography.
|
Programming Perl, 3rd Edition
Larry Wall, Tom Christiansen & Jon Orwant
3rd Edition July 2000
0-596-00027-8, Order Number: 0278
1092 pages, $49.95
|
|
But now we come to the Good part. (I hope.) After thinking lots and
lots about many of the individual RFCs, and not knowing how to start
thinking about them as a whole, it occurred to me (finally!) that the
proper order to think about things was, more or less, the order of the
chapters in the Camel Book. That is, the Camel Book's order is
designed to minimize forward references in the explanation of Perl, so
considering Perl 6 in roughly the same order will tend to reduce the
number of things that I have to decide before I've decided them.
So I've merrily classified all the RFCs by chapter number, and they
look much more manageable now. (I also restructured my email so that I
can look at a slice of all the messages that ever talked about a
particular RFC, regardless of which mailing list the message was on.
That's also a big help.) I intend to produce one Apocalypse for each
Chapter, so Apocalypse 1 corresponds to Chapter 1: An Overview of
Perl. (Of course, in the book, the Overview is more like a small
tutorial, not really a complete analysis of the philosophical
underpinnings of Perl. Nevertheless, it was a convenient place to
classify those RFCs that talk about Perl 6 on that level.)
So today I'm talking about the following RFCs:
RFC PSA Title
--- --- -----
16 bdb Keep default Perl free of constraints such as warnings and
strict.
26 ccb Named operators versus functions
28 acc Perl should stay Perl.
73 adb All Perl core functions should return objects
141 abr This Is The Last Major Revision
The PSA rating stands for ``Problem, Solution, Acceptance''.
The problem
and solution are graded on an a-f scale, and very often you'll find I
grade the problem higher than the solution. The acceptance rating is
one of
a Accepted wholeheartedly
b Accepted with a few "buts"
c Accepted with some major caveats
r Rejected
I might at some point add a ``d'' for Deferred, if I really think it's
too
soon to decide something.
I was initially inclined to accept this RFC, but decided to reject it
on theological grounds. In apocalyptic literature, 7 is the number
representing perfection, while 6 is the number representing
imperfection. In fact, we probably wouldn't end up converging on a
version number of 2*PI as the RFC suggests, but rather on
6.6.6, which
would be rather unfortunate.
So Perl 7 will be the last major revision. In fact, Perl 7 will be
so perfect, it will need no revision at all. Perl 6 is merely
the prototype
for Perl 7. :-)
Actually, I agree with the underlying sentiment of the RFC—I only
rejected
it for the entertainment value. I want Perl to be a language that can
continue to evolve to better fit the problems people want to
solve with it.
To that end, I have several design goals that will tend to be obscured if
you just peruse the RFCs.
First, Perl will support multiple syntaxes that map onto a single
semantic model. Second, that single semantic model will in turn map to
multiple platforms.
Multiple syntaxes sound like an evil thing, but they're really
necessary for the evolution of the language. To some extent we already
have a multi-syntax model in Perl 5; every time you use a pragma or
module, you are warping the language you're using. As long as it's
clear from the declarations at the top of the module which version of
the language you're using, this causes little problem.
A particularly strong example of how support of multiple syntaxes
will allow continued evolution is the migration from Perl 5 to Perl 6
itself. See the discussion of RFC 16 below.
Multiple backends are a necessity of the world we live in
today. Perl 6
must
not be limited to running only on platforms that can be
programmed in C. It
must be able to run in other kinds of virtual machines, such as those
supported
by Java and C#.
It is my fond hope that those who are fond of Perl 5 will be
fonder still
of Perl 6. That being said, it's also my hope that Perl will continue
trying to be all things to all people, because that's part of
Perl too.
While I accept the RFC in principle (that is, I don't intend to go
raving mad), I have some major caveats with it, because I think it is
needlessly fearful that any of several programming paradigms will ``take
over'' the design. This is not going to happen. Part of what makes
Perl Perl is that it is intentionally multi-paradigmatic. You might
say that Perl allows you to be paradigmatic without being
``paradogmatic''.
The essence of Perl is really context sensitivity, not just to
syntactic
context, but also to semantic, pragmatic, and cultural context. This
overall philosophy is not going to change in Perl 6, although specific
context sensitivities may come and go. Some of the current context
sensitivities actually prevent us from doing a better job of it in
other areas. By intentionally breaking a few things, we can make Perl
understand what we mean even better than it does now.
As a specific example, there are various ways things could improve if
we muster the courage to break the ``weird'' relationship between
@foo
and $foo[]. True, we'd lose the current slice
notation (it can
be
replaced with something better, I expect). But by consistently treating
@foo
as an utterance that in scalar context returns an array reference, we
can make subscripts always take an array reference, which among
other
things fixes the botch that in Perl 5 requires us to distinguish
$foo[] from $foo->[]. There will be more
discussion of this in
Apocalypse 2, when we'll dissect ideas like RFC 9: Highlander Variable
Types.
I am of two minds about this debate—there are good arguments for
both sides. And if you read through the discussions, all those arguments
were forcefully made, repeatedly. The specific discussion centered around
the issue of strictness, of course, but the title of the RFC claims a
more general philosophical position, and so it ended up in this
Apocalypse.
I'll talk about strictness and warnings in a moment, and I'll also talk
about constraints in general, but I'd like to take a detour through
some more esoteric design issues first. To my mind, this RFC (and the
ones it is reacting against), are examples of why some language
designer like me has to be the one to judge them, because they're all
right, and they're all wrong, simultaneously. Many of the RFCs stake
out polar positions and defend them ably, but fail to point out
possible areas of compromise. To be sure, it is right for an RFC to
focus in on a particular area and not try to do everything. But because
all these RFCs are written with (mostly) the design of Perl 5 in mind,
they cannot synthesize compromise even where the design of Perl 6 will
make it mandatory.
To me, one of the overriding issues is whether it's possible to
translate Perl 5 code into Perl 6 code. One particular place of
concern is in the many one-liners embedded in shell scripts here and
there. There's no really good way to translate those invocations, so
requiring a new command line switch to set ``no strict'' is not going to
fly.
A closely related question is how Perl is going to recognize when it
has accidentally been fed Perl 5 code rather than Perl 6 code. It
would be rather bad to suddenly give working code a brand new set of
semantics. The answer, I believe, is that it has to be impossible by
definition to accidentally feed Perl 5 code to Perl 6. That is, Perl 6
must assume it is being fed Perl 5 code until it knows otherwise. And
that implies that we must have some declaration that unambiguously
declares the code to be Perl 6.
Now, there are right ways to do this, and wrong ways. I was peeved by
the approach taken by DEC when they upgraded BASIC/PLUS to handle long
variable names. Their solution was to require every program using long
variable names to use the command EXTEND at the top. So
henceforth
and forevermore, every BASIC/PLUS program had EXTEND
at the top
of
it. I don't know whether to call it Bad or Ugly, but it certainly
wasn't Good.
A better approach is to modify something that would have to be there
anyway. If you go out to CPAN and look at every single module out
there, what do you see at the top? Answer: a ``package''
declaration. So we break that.
I hereby declare that a package declaration at
the front of
a file
unambiguously indicates you are parsing Perl 5 code. If you want to
write a Perl 6 module or class, it'll start with the keyword
module
or class. I don't know yet what the exact syntax of a module
or a
class declaration will be, but one thing I do know is that it'll set
the current global namespace much like a package declaration
does.
Now with one fell swoop, much of the problem of programming in the
large can be dealt with simply by making modules and classes default to
strict, with warnings. But note that the default in the main program
(and in one liners) is Perl 5, which is non-strict by definition. We
still have to figure out how Perl 6 main programs should distinguish
themselves from Perl 5 (with a ``use 6.0'' maybe?),
and whether
Perl 6
main programs should default to strict or not (I think not), but you
can already see that a course instructor could threaten to flunk anyone
who doesn't put ``module Main'' at the front each
program, and
never
actually tell their pupils that they want that because it turns on
strictures and warnings.
Other approaches are possible, but that leads us to a deeper issue,
which is the issue of project policy and site policy. People are
always hankering for various files to be automatically read in from
various locations, and I've always steadfastly resisted that because it
makes scripts implicitly non-portable. However, explicit
non-portability is okay, so there's no reason our hypothetical class
instructor could not insist that programs start with a ``use
Policy;'' or
some such.
But now again we see how this leads to an even deeper language design
issue. The real problem is that it's difficult to write such a Policy
module in Perl 5, because it's really not a module but a meta-module.
It wants to do ``use strict'' and ``use
warnings''
on behalf of the
student, but it cannot do so. Therefore one thing we must implement in
Perl 6 is the ability to write meta-use statements that look like
ordinary use statements but turn around and declare other things on
behalf of the user, for the good of the user, or of the project, or of
the site. (Whatever. I'm not a policy wonk.)
So whether I agree with this RFC really depends on what it means by
``default''. And like Humpty Dumpty, I'll just make it mean whatever I
think is
most convenient. That's context sensitivity at work.
I also happen to agree with this RFC because it's my philosophical
position that morality works best when chosen, not when mandated.
Nevertheless, there are times when morality should be strongly suggested,
and I think modules and classes are a good place for that.
I'm not sure this belongs in the overview, but here it is nonetheless.
In principle, I agree with the RFC. Of course, if all Perl variables
are really objects underneath, this RFC is trivially true. But the
real question is how interesting of an object you can return for a
given level of performance. Perl 5's objects are relatively heavyweight,
and if all of Perl 6's objects are as heavy, things might bog down.
I'm thinking that the solution is better abstract type support for data
values that happen to be represented internally by C structs.
We get
bogged down when we try to translate a C struct such a
struct tm
into an actual hash value. On the other hand, it's rather efficient to
translate a struct tm to a struct tm,
since it's a
no-op. We can
make such a struct look like a Perl object, and access it
efficiently
with attribute methods as if it were a ``real'' object. And the typology
will (hopefully) mostly only impose an abstract overhead. The biggest
overhead will likely be memory management of a struct over an
int
(say), and that overhead could go away much of the time with some
amount of contextually aware optimization.
In any event, I just want to point out that nobody should panic when we
talk about making things return objects that didn't used to return
them. Remember that any object can define its stringify and
numify overloadings to do whatever the class likes,
so old code
that
looks like
print scalar localtime;
can continue to run unchanged, even though localtime might
be returning
an object in scalar context.
Here's another RFC that's here because I couldn't think of a better
place for it.
I find this RFC somewhat confusing because the abstract seems to
suggest something more radical than the description describes. If you
ignore the abstract, I pretty much agree with it. It's already the
case in Perl 5 that we distinguish operators from functions primarily
by how they are called, not by how they are defined. One place where
the RFC could be clarified is that Perl 5 distinguishes two classes of
named operators: named unary operators vs list operators. They are
distinguished because they have different precedence. We'll discuss
precedence reform under Apocalypse 3, but I doubt we'll combine the two
kinds of named operators. (As a teaser, I do see ways of simplifying
Perl's precedence table from 24 levels down to 18 levels, albeit with
some damage to C compatibility in the less frequently used ops. More
on that later.)
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Perl 6 Apocalypse |
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The rest of the "Apocalypse" series can be found here, as well as other articles by Larry Wall.
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Do you begin to see why my self-appointed job here is much larger than
just voting RFCs up or down? There are many big issues to face that
simply aren't covered by the RFCs. We have to decide how much of our
culture
is just baggage to be thrown overboard, and how much of it is
who we are. We have to smooth out the migration from Perl 5 to Perl 6 to
prevent people from using that as an excuse not to adopt Perl 6. And
we have to stare at all those deep issues until we see through
them down to
the underlying deeper issues, and the issues below that. And then in
our depths of understanding, we have to keep Perl simple enough for
anyone to pick up and start using to get their job done right now.
Stay tuned for Apocalypse 2, wherein we will attempt to
vary our variables, question our quotes, recontextualize our contexts,
and in general set the lexical stage for everything that follows.
