Here’s to Andy

Guile 1.9.0, the first in a series of alpha releases leading to 2.0 by the end of this year, has just been released. The number of improvements listed in the NEWS is impressive: you may well find that most, if not all, of your pet peeves against Guile are a thing of the past, and also relish one or three features that are hard to find in other schemes. As for me, after working with the development version for some months now, i’m sold. Here are some of my favourites goodies:

The new compiler and virtual machine:

** Guile now can compile Scheme to bytecode for a custom virtual machine.

Compiled code loads much faster than Scheme source code, and runs around
3 or 4 times as fast, generating much less garbage in the process.

which comes with automatic caching of object files, and provides a language tower that is beginning to fulfill Guile’s foundational goals:

** New language: ECMAScript

Guile now ships with one other high-level language supported,
ECMAScript. The goal is to support all of version 3.1 of the standard,
but not all of the libraries are there yet. This support is not yet
documented; ask on the mailing list if you are interested.

Yes, Guile has now multi-language support and, what’s more important, a clearly defined way of adding new languages to the mix (full Elisp support comming).

What must be the most infamous Guile misfeature is over:

** The psyntax expander is now hygienic with respect to modules.

Free variables in a macro are scoped in the module that the macro was
defined in, not in the module the macro is used in. For example, code
like this works now:

   (define-module (foo) #:export (bar))
   (define (helper x) ...)
   (define-syntax bar
     (syntax-rules () ((_ x) (helper x))))

   (define-module (baz) #:use-module (foo))
   (bar qux)

It used to be you had to export `helper' from `(foo)' as well.
Thankfully, this has been fixed.

thanks (thanks, thanks!) to the new psyntax:

** psyntax is now the default expander

Scheme code is now expanded by default by the psyntax hygienic macro
expander. Expansion is performed completely before compilation or

Notably, syntax errors will be signalled before interpretation begins.
In the past, many syntax errors were only detected at runtime if the
code in question was memoized.

with such additional niceties as allowing docstrings in macros (docstrings is problably the Guile extension over standard scheme that i like most). Note also that now Guile knows about syntax-case and syntax-rules by default, no need to import additional modules to get that.

And, to finish my laundry list, some new modules for good measure:

* New modules (see the manual for details)

** `(srfi srfi-18)', more sophisticated multithreading support
** `(ice-9 i18n)', internationalization support
** `(rnrs bytevector)', the R6RS bytevector API
** `(rnrs io ports)', a subset of the R6RS I/O port API
** `(system xref)', a cross-referencing facility

The last one, in particular, being put to good use in Geiser. And, by the way, ‘multithreading’ here means native multithreading.

While it’s true that there are still some bugs and rough edges to be smoothed, the future looks bright for Guile, and 2.0 will be a landmark in its history. All these goodies are the result of the combined effort of several hackers, but, without meaning to belittle any of them, i must bow to the amazing hacking powers of Andy Wingo. During this last year, almost single-handedly, he has brought us the compiler and virtual machine, the ECMAScript language, and the psyntax expander. I must remember to introduce myself as “Andy’s coworker” more often!

Happy hacking!

Posted in Scheme. 2 Comments »

A merry gang

Last Wednesday, FLIB’s kick-off meeting took place at Oblong’s Barcelona lab, with a dozen deliciously crazy people attending.

Due to lack of time and seriousness on my side, the main talk was given by Jos, who gave us an overview of his work with PLT Redex to model lambda calculus. Redex provides an embedded DSL to create context-sensitive term-rewriting systems, if you’ll pardon my buzzwording. In a hand-waving nutshell, term-rewriting systems are syntax-rules on steroids: one specifies a set of rules for transforming (rewriting, or reducing) terms to other terms according to their structure, possibly depending on context. Jos has a very nice example of such a system taken from GEB (Best. Book. Ever.), the MIU formal system, whose formal rules can be expressed in Redex as:

  (--> (‹symbol›  ... I) (‹symbol› ... I U))
  (--> (M ‹symbol› ..) (M ‹symbol› ... ‹symbol› ...))
  (--> (‹symbol›_0 ... I I I ‹symbol›_1 ...) (‹symbol›_0 ... U ‹symbol›_1 ...))

that is, if you find a trailing I, you can append U; if you find M, you can duplicate the rest of the string; and three consecutive Is can be reduced to a single U. Now, you can start with a given string (or axiom) and apply the rules to produce new ones (theorems). Note how the rules are contextual, and how there’s in general more than one that is applicable. Redex will do that for you, creating a tree with all possible reductions.

MIU reductions in Redex

Of course, there’s more to Redex than this simple example. For instance, it’s been used to provide an operational semantics for R6RS. Jos’ work is somewhere in the middle: while the reduction rules in lambda calculus are even simpler than in MIU, issues of scope quickly complicate things; moreover, Jos explores classical topics in lambda calculus, such as reduction to normal form, fixed point combinators or Church numerals to name a few, always using Redex (the staggering conceptual richness embodied by the humble premises of lambda calculus always amazes me). All in all, a beautiful 32-pages long paper, with accompanying code, that serves as a nice hands-on introduction to both lambda calculus and Redex, and which you can get at Jos homepage.

After the presentation, we devoted some time to talk about the future of FLIB. Monthly presentations, lightning talks on demand and a reading group. I like the latter a lot, because having a physical meeting among readers every month is an excellent way of keeping reading groups alive. We’re still deciding on our first book, but PLAI followed by LiSP seems to be gaining momentum right now. Another nice thing about the reading group is that it opens the possibility for people not able to come to the meetings to participate: just subscribe to our mailing list and join the discussions about the book du jour.

And then we just sat down around a table with some beer and snacks, and start talking about life and programming languages. I found it very stimulating because of the varied people’s backgrounds: we had guys from academia and industry; ones just starting their graduate courses, others with twenty years of teaching under their belts; people from several different countries; schemers obsessed with call/cc, smalltalkers, python experts, C++ loathers and programmers who secretly enjoy it, perlmongers and ruby or haskell aficionados. But, they all, people with a passion for programming: i think everybody was happy to have found a bunch of keen souls.

Perhaps it was inevitable that much of the discussion gravitated around our frustrations as programmers and teachers, given the sad state of computer science in both industry and academia, and the insurmountable barriers for adoption faced by the kind of languages we like. But, with that out of the way, here’s hope (as expressed by Andy after the meeting) that future meetings will concentrate on brighter fields.

A great evening, and no mistake. I hope you’ll join the fun next July 22nd!