A week of Lisp in Madrid

Jose Maria Cuellar

Geeks from all Lisps of life met in Madrid last week for the European Common Lisp Meeting and European Lisp Symposium 2013.

A lot of things happened, so I'll just recount the most memorable ones.

I enjoyed meeting Pascal Costanza and Charlotte Herzeel. Pascal shares my disdain for the aberration that is Lisp-1 and doesn't tire of telling Schemers so. I think he's a bit too opposed to all things Scheme though, and when somebody tells him about the niceness that is, e.g. syntax-parse, he goes all "nyah, nyah, nyah, I cannot hear you". Charlotte is one of the handful of people who understand 3-Lisp because she re-implemented it.

(Apparently, 3-Lisp is quite similar to Kernel: every operative receives not only its operand tree and the lexical environment in which it is called, but also its continuation. I still don't understand it. Why would you pass the continuation to an operative, when it can easily obtain it using e.g. call/cc? Apparently because 3-Lisp considers the continuation to exist on the next meta-level, not the current one.)

Erik Sandewall, who worked with John McCarthy, presented his document-editor/operating-system/database/agent-environment called Leonardo. We talked a lot about it, and I found out I'm working on a very similar system in my own next-gen OS efforts.

A very enjoyable talk was SISCOG: a story written in Lisp, by Tiago Maduro Dias and Luís Oliveira. SISCOG was started by two professor-level Lisp nerds in the eighties, because they wanted to apply the language to something useful. If you've ridden a train in Europe, chances are it was scheduled by one of SISCOG's apps. They employ a large number of Lisp programmers in Portugal, and reported how they view Lisp (most of them like it). Anyone who claims that dynamic languages can't be used for something other than prototyping I'd like to hit over the head with the SISCOG manual, which is probably heavy, given the highly complex stuff they work on.

One of the absolute over-the-top experiences, both geek-wise, food-wise, and otherwise-wise was the extended dinner with Ludovic Courtès (Guix and Guile), Andy Wingo (Guile and V8), Florian Loitsch (Hop and Dart), and Sam Tobin-Hochstadt (Racketeer). It doesn't get much better for a PL geek than such a barrage of PL-nerdery, funny background stories, and Spanish cakes.

Another memorable chat was with the very nice Luke Gorrie about dynlangs, networking, and immigration and property buying in the world's most agreeable country, Switzerland.

I also enjoyed the discussions between Ernst "I'm very male" van Waning and Venetian gentleman  Janusz Prodrazik, who presented OpusModus. OpusModus is a very polished tool for composers that uses S-expressions instead of scores for composition. Composers seem to enjoy it.

The last, but not least, three days I enjoyed the company of style icon Faré and Masatoshi Sano, who came all the way from Boston and Tokyo, respectively. Faré and I basically agree about all major points about how the next OS will look like, and I feel like I spiritually joined the TUNES project and made a friend. (Watch out, TUNES will be on your lap sooner than you imagine!)

Greetings to Piotr Kuchta with whom I immediately hit it off, who ported Wat to Python, and who I look forward to visiting in rainy GB.

All in all, a memorable and enjoyable week in the beautiful city of Madrid. Thanks to the organizers for the flawless execution and see you all again next time!

Wat: now in Perl, Python, and Ruby, too

I'm delighted somebody by the name of "shadowcat-mst" has taken my Wat interpreter and reimplemented it in Perl. That really seems fitting. Wat now covers JavaScript and Perl - think of the possibilities!

Update: Piotr Kuchta ported Wat to Python.
Update: Victor Hugo Borja ported Wat to Ruby.

Green threads in the browser in 20 lines of Wat

This page shows 5 independent, cooperatively scheduled Wat green threads (view source for full Wat code).

Each thread has an ID and is defined as a function that loops forever, repeatedly printing its ID, and then sleeping for a (randomly long) while.

(define (run-thread (id String))
  (loop
    (@appendChild (.body $document)
                  (@createTextNode $document (+ "Active thread: " id " ")))
    (sleep (* 1000 (@random $Math)))))

So, how can a Wat thread sleep inside a loop when JavaScript forbids sleeping? Why, with good ole delimited continuations:

To spawn a thread, I just wrap the call to RUN-THREAD in a prompt (which is just a unique object used as an identifier):

(define default-prompt 'default-prompt)

(define (spawn-thread (id String))
  (push-prompt default-prompt
    (run-thread id)))

Where it gets interesting is the SLEEP function which captures the continuation up to the prompt, and sets up a callback with JavaScript's setTimeout that will reinstall the continuation later:

(define (sleep (ms Number))
  (take-subcont default-prompt k
    (define (callback . #ignore)
      (push-prompt-subcont default-prompt k))
    ($setTimeout (js-callback callback) ms)))

So, first, SLEEP aborts up to and including the default prompt using TAKE-SUBCONT. It receives the continuation in the variable K. Once it has K, it defines a CALLBACK function, that will reinstall the default prompt with PUSH-PROMPT, and then continue with K again with PUSH-SUBCONT. All that's left is to give this callback to setTimeout.

Then I can spawn independent threads:

(spawn-thread "thread-1")
(spawn-thread "thread-2")
(spawn-thread "thread-3")
(spawn-thread "thread-4")
(spawn-thread "thread-5")

Wat is very new, but it's already practical for adding concurrency and metaprogramming to JavaScript. Deployment is very easy. Include the single wat.js file, put some Lisp code in a <script> tag, and run it.

A new low in programming language design and implementation

The new Wat is the best, most lightweight way to implement a JavaScript-based programming language I have found so far.

Basically, I get away from JS as quickly and painlessly as possible, and start writing the language in itself.

So I define a very small set of primitives on the joint foundation of Kernel-like first-class lexical environments and fexprs and delimited continuations. Fexprs are a great tool for language-oriented programming, and delimited continuations allow me to escape from the browser's (and Node's) async hell and implement any concurrency and effect system I like.

To fexprs I also add macros. When a macro is used as the operator of a form, the form's code gets  changed to the macro's output when the macro is first called, a technique I learned from here. I like macros because they make syntactic abstraction cost-free - with fexprs alone there is always an interpretative overhead. Still, Wat macros, like fexprs, do not work with quoted identifiers, but with first-class values, so many hygiene problems are avoided.

To delimited control I also add classic first-order control (sequential, conditional, loop, throw, catch, finally). This runs on the ordinary JS stack. Only when a continuation is captured does the stack get reified on the heap.

And last but not least, I use a JSON-based syntax for writing the language in itself. At first this was just intended as a quick way to not have to specify a parser for Wat, but I'm starting to like it. It allows Wat to truly be embedded in JavaScript.

Wat does not have a type tagging or object system. It uses the raw JavaScript values.

The whole implementation is roughly 350 lines of JavaScript. After these 350 lines, I can already write Wat in Wat, which is just great.

Some progress on the Wat VM

Wat is back! If you'll recall, Wat is my ultra-minimal (~500 lines of JS) interpreter for a Kernel-based language with delimited continuations as well as first-order control, and hygienic macros as well as fexprs.

I'm pretty excited by some recent and ongoing changes, which make Wat even smaller and turn it into more of a VM than a full language. Wat will provide (just) the following features:

• delimited continuations and delimited dynamic binding (higher-order control); these will be used to build cooperative multithreading with thread-local bindings
• try/catch/finally (first-order control) integrated with the JS stack, but suspendable by continuation capture
• fexprs as well as in-source self-modifying-code memoizing macros (which are hygienic, as they're built on Kernel)
• a native JS interface

And that's about it. This should give an extremely minimal yet powerful infrastructure for building JavaScript-based languages.

And I gave up on quasiquotation and Scheme-style hygienic macros again. I just cannot get them to work in a satisfying manner.

Exempli gratia, here's some initial Wat VM "microcode" for bootstrapping a vaporlanguage.

A quasiquote I can understand

I've written two Lisps (1, 2) with quasiquotation, and in both, quasiquotation was the most difficult thing to implement, and gave me the most headaches. That shouldn't be, right? After all, it only creates new forms.

I think now I've found a formulation for quasiquote that has a really simple implementation, and yields more or less the same results as existing quasiquote implementations.

Some definitions:

• `foo stands for (quasiquote foo), `(foo) stands for (quasiquote (foo)).
• ,foo stands for (unquote foo) and is only allowed within a quasiquote.
• ,@foo stands for (unquote-splicing foo) and is only allowed within a quasiquote.
• Quasiquote, unquote, and unquote-splicing only ever take a single operand.
• `foo = 'foo, i.e. a quasiquoted symbol yields simply the symbol.
• `"foo" = "foo", `12 = 12, and likewise for other literals.

The main difficulty I previously had with quasiquote came from unquote-splicing, which inserts a list of multiple elements into the constructed list (whereas nested quoted or unquoted forms only insert a single element). The main idea in this new formulation is to make inserting multiple elements the default, and define nested quoted or unquoted elements as simply inserting a list containing a single element.

Every `(...) expression therefore stands for an APPEND of the list's further processed elements.

For example, given

(define foo 1)

(define bar 2)

(define quux '(3 4))

the quasiquote expression

`(foo ,bar ,@quux)

stands for

(append (list 'foo) (list bar) quux)

which produces the following when evaluated:

'(foo 2 3 4)

So, processing a quasiquoted list works by wrapping each element, except for unquote-splicing forms, in a call to LIST, and APPENDing the results. Quoted elements (foo) get processed recursively. Unquoted elements (bar) are passed to the call to LIST unprocessed. Unquote-splicing forms (quux) are inserted directly into the APPEND form.

I haven't implemented this yet, but I think defining a quasiquoted list `(...) as an APPEND really simplifies things.

Taf's translation to O'Caml for type-checking

Taf is my new vapor-Lisp with row polymorphism, delimited continuations, and hygienic macros.

(Warning: incoherent rambling ahead!) Taf has a class-based object system with no inheritance. A class defines which slots an instance of this class has, and which methods are applicable to it. Every class also implicitly defines a class type. In addition to class types, there are also interface types or simply interfaces. An interface type defines a suite of methods applicable to objects of this type. Everything is an object of a single class, but may have many compatible class types and interface types. Every object is a member of a special top type. There is no implicit subtyping: objects need to be upcast to top.

All Taf objects are encoded as O'Caml objects. There is one O'Caml class for each Taf class. All classes inherit from a top class. Interfaces (method suites) are also defined as O'Caml classes. Any object can be statically upcast to top or any of the interfaces it implements. This is structural: an object can be upcast to an interface if it has all its methods. Objects have full RTTI, so they can also be dynamically downcast, resulting in an exception if the object is not of the given type. (A more convenient TYPECASE is provided as a wrapper.) Internally, downcasting is implemented via Obj.magic on the O'Caml side, and via a dynamic type-check in the VM. So Taf supports for example heterogenous containers containing arbitrary instances of top or of any other type. Any object can be put into the container, and taken out again, and cast back to its original class type. Likewise, it's possible to write methods that operate on objects of arbitrary types, such as Java's EQUALS. Types are parametric.

Another aspect is the semantics of the global environment. O'Caml's is basically that of a LET* for values, and LETREC only for groups of functions. But Lisp requires LETREC* for all values. So every binding must be encoded as a reference cell containing a Maybe on the O'Caml side, to model bindings which may be undefined.

The runtime, and also the code that produces O'Caml code will run in the browser. Eventually, the type-checker will be implemented in the language itself, so O'Caml will no longer be needed.

Update: here's a sneak preview of the Taf Language Manual.

Current project

In my quest for a good Lisp, I could no longer ignore static types.

See Taf - A plan for a statically-typed Lisp.

There shouldn't be any difficult roadblocks, so I expect a release sometime in or before summer.