Larry's approaching perl6 through the Programming Perl book (the Camel).
He's going chapter by chapter through the Camel, writing documents about
the perl6 equivalent concepts. These missives are known as "Apocalypses",
for reasons best known to Larry. :-)
He's churning through the RFCs, looking through them to deeper issues.
He hopes to emit Apocalypses more-or-less weekly, although some chapters
have fewer RFCs and issues than others.
Enjoy!
Nat
-----
Apocalypse 1: The Ugly, the Bad, and the Good
by Larry Wall
Apr. 2, 2001
Table of Contents
o RFC 141: This Is The Last Major Revision
o RFC 28: Perl should stay Perl
o RFC 16: Keep default Perl free of constraints such as warnings
and strict
o RFC 73: All Perl core functions should return objects
o RFC 26: Named operators versus functions
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.
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.
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.
[22]RFC 141: This Is The Last Major Revision
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#.
[23]RFC 28: Perl should stay Perl.
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.
[24]RFC 16: Keep default Perl free of constraints such as warnings and
strict.
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.
[25]RFC 73: All Perl core functions should return objects
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.
[26]RFC 26: Named operators versus functions
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.)
_________________________________________________________________
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.
References
22. http://dev.perl.org/rfc/141.html
23. http://dev.perl.org/rfc/28.html
24. http://dev.perl.org/rfc/16.html
25. http://dev.perl.org/rfc/73.html
26. http://dev.perl.org/rfc/26.html
