chris blogs

Sloop: Sublanguage for Object Orientation with Prototypes

Tue Aug 29 16:22:13 CEST 2006

The first sublanguage I implemented is one for prototype-based object orientation, called Sloop.

Sloop was developed in a fully TDD-style and can be found in my darcs repository.

To show its look and feel, we can implement the canonical example of object orientation, an account.

Account = sloop {
  self.balance = 0

  def_deposit  { |v| self.balance += v }
  def_withdraw { |v| self.balance -= v }

  def_inspect { "#(an account with $#{balance})" }
}

In Sloop, everything is a first-class object, even methods (unlike Ruby, where only Method objects are first-class). Instead of def_inspect, I also could have written:

self.inspect = Sloop::Method.new { "#(an account with $#{balance})" }

However, the syntactic sugar with def_name is nicer to read and closer to ordinary Ruby.

Look how this code is used:

my_account = Account.clone
p my_account

puts "Depositing $10"
my_account.deposit 10

p my_account

On running, you see the expected output:

#(an account with $0)
Depositing $10
#(an account with $10)

The example however can’t fully show the flexibility of the Sloop object system. Have a look at the Circles and Ellipses example.

As you can see, a Circle really is an Ellipse, but they have different implementations! In fact, since we can change the “class” of objects at run-time, we even could cast Ellipses with the same radii transparently into a Circle (the question is “when?”).

The Sloop object system allows for a lot of other powerful things that don’t fit into this article (and after all, it’s still in development). For now, I refer you to the source and unit tests. (You may be curious about the condition mechanism, for example.)

NP: Spiritualized—I Think I’m in Love

Sublanguages: Embedded general-purpose languages

Tue Aug 29 15:15:18 CEST 2006

In the past, Ruby often has been popularized as a language suitable for building DSLs (Domain Specific Languages). Its flexible syntax (e.g. mostly optional parentheses or newline as statement terminator) and language facilities like blocks or powerful run-time introspection and metaprogramming make this possible.

Domain specific languages are, nomen est omen, specific to a certain domain that in some ways deserves programmability. Therefore, while certain languages considered as DSLs are in fact Turing-complete (think of XSLT, sendmail.cf or TeX), they aren’t very suitable for general-purpose programming. For Rubyists, this is no problem, given that embedded DSLs implemented in Ruby often can use the full power of Ruby.

However, Ruby is powerful enough not only to implement embedded DSLs, but also to implement embedded general-purpose languages. I’ll name these sublanguages.

Why sublanguages, if you have Ruby?, the sceptic may ask. The reason is easy: while libraries implement functions, sublanguages implement programming paradigms.

Out of the box, Ruby already supports some programming paradigms pretty well. These are:

• Class-based object oriented programming, since “everything” in Ruby is an object; this probably is Ruby’s main paradigm.

• Functional programming with powerful and side-effect free methods in the core (e.g. due to Enumerable) and code as first-class objects (Proc).

• Imperative programming due to modifyable variables, sequential execution and eager evaluation.

These three paradigms probably are the ones the creator of Ruby cared most about, which is why they are so well-supported by core Ruby. They also are the three most popular paradigms as of today.

However, due to Ruby’s flexibility, it is relatively easily possible to also implement other programming paradigms in Ruby. In a series of posts, I hope to present implementations of these:

• Prototype-based object oriented programming, featuring object-orientation not by defining classes, but by cloning and refining objects. (Inspired by Self and Io).

• Concurrent programming, with Erlang style processes and message passing.

• Declarative logic programming, as seen in Prolog.

Please note that almost everything is possible to implement using Ruby with some ugly hacks. I’ll try to avoid these very hard to make the code suitable to a general public, and not only for academic use. It’s still very hard to encode these paradigms with reasonable speed compared to a proper native implementation. Wherever possible, I’ll try to make useful optimizations.

Code can be found at my darcs repositories.

NP: Spiritualized—Cool Waves

Off to Munich and Essen

Sat Aug 26 22:02:25 CEST 2006

Vacation time has come again, and I’m off to Munich from Sunday till Tuesday, and in Essen from Wednesday till Sunday. Probably.

As usual, unlikely to have network in Munich (well, maybe some open WLAN is strong enough by now), and unlikely to have lots of time anyway. Mail and IRC usage will be reduced, tumblelogging probably interrupted.

If you got something important, or want to meet me, you’ll figure how you can contact me.

Happy holiday. Maybe I’ll get some writing for the blog done.

NP: Spiritualized—Electricity

Side-effects in real life

Wed Aug 16 15:21:31 CEST 2006

Two years ago today, I’d flip my LSB. Since I’m doing a lot of purely functional programming these days, it’s not as easy like that this year:

import Control.Monad.State

data Guy = Guy { name :: String, age :: Integer }

introduce guy =
    concat ["Hey, I'm ", name guy,
            ", and I'm ", show $ age guy, " years old."]

getOlder :: State Guy ()
getOlder = modify (\guy -> guy { age = (age guy) + 1 })

main =
  let guy = (Guy "chris" 18) in do
    print $ introduce guy
    print $ introduce $ execState getOlder $ guy

Facts: Nineteen is a centered triangular number, centered hexagonal number, and an octahedral number. Every positive integer is the sum of at most nineteen fourth powers. Nineteen is a Heegner number and a strictly non-palindromic number…

NP: Ton Steine Scherben—Heute Nacht

MonetDB XQuery

Thu Aug 10 17:52:05 CEST 2006

XQuery is an XML technique that unfortunately doesn’t get as much attention as it deserves.

Part of the reason is that there are not many XQuery implementations, many use Java, some have ugly licenses and others are incomplete. One that’s written in C with libxml2, Mozilla-licensed and pretty complete is MonetDB XQuery, based upon the MonetDB RDBMS.

A special thing about MonetDB is that it is not inherently SQL-based, but supports several front-ends, among them SQL, but also XQuery. Queries in both languages get translated into MIL (MonetDB Interpreter Language), respective MAL (MonetDB Assembler Language) for version 5, which doesn’t yet support XQuery, however.

MonetDB and the XQuery front-end compile (almost) out of the box, but contain huge .c-files (up to 5 megabytes) due to code-generation techniques. Be sure to have enough free RAM when you want to compile it.

XQuery is a nice language (unlike it’s totally evil bastard brother, XQueryX), reminding of languages like OCaml, Links or XDuce. It’s probably what XSLT always should have been.

I also think XQuery would make a great templating language (I used to template websites with XSLT for years, which can be pretty painful at times.), especially given virtual XML views. (I’m sure one can implement those with MonetDB, but I didn’t try yet.) XQuery templates would allow well-formed XML output by definition and generally ease development.

Unfortunately, most XQuery engines, MonetDB inclusive, are pretty much database-like. MonetDB provides a few helpers to automatically add XML documents you use to its database, but they will be “shredded” into relations nevertheless. This is probably a speed problem if you want to apply one-shot XQuery programs on a large set of documents. If you can store the data in MonetDB anyway, however, I could imagine neat tricks given a light HTTP to XQuery wrapper. MonetDB doesn’t support XUpdate at the moment, so XQuery-based web applications would be read-only for now.

XQuery certainly is a thing that deserves more investigation.

NP: Bob Dylan—Like A Rolling Stone

Halbjahreszeugnis 12/2

Wed Aug 02 15:56:40 CEST 2006

Pünktlich nach Schützen kommen jetzt dann die Sommerferien, und heute gabs natürlich Zeugnisse:

Dieses Jahr also marginal schlechter als letztes (11.471 gegenüber 11.867), aber das ist man im zweiten Halbjahr eigentlich immer.

Sport war scheiße, wird eben geklammert.

Außerdem hab ich noch einige ungebloggte Quotes gefunden:

Kann man auch nur 30€ zahlen und nicht in die Schule gehen?

Magdad Fenster auf!

Kannsch au bei uns komma.

[Relilehrerin:] Ich glaube nur, was ich sehe.

Die Katholische Kirche beledigt den menschlichen Verstand.

Wenn ich ein Buch lese, merke ich gar nicht, wie mich mein Freund fickt.