09 Mar 2010

Fractran is a Turing-complete esoteric programming language. A Fractran program is just an ordered list of positive, irreducible fractions. The program's output for an input n is the output of the program run on n multiplied by the first fraction in the list that results in an integer. If no such multiplication results in an integer, the output is the input n. Variables are encoded in the exponents of the prime factorization of the input and output.

Some time ago I thought up an idea for a short story involving Fractran. A mathematician accidentally creates a Fractran program that can trivially factor large composites. Think something like O(log n). It's just the right magical string of, say, 31 fractions.

The story would be a first-person narrative of the mathematician's thoughts during a short time after the discovery, considering many of the consequences of the program. For example, it would render much of cryptography, which plays an essential role in the modern world, useless. He would also wonder if mankind should deserve such a discovery, considering how accidental it was.

This whole idea vanished once I realized that this Fractran program is actually completely trivial. It even runs in O(1) time. It's so trivial as to be worthless. Remember that Fractran stores its data in the number's prime factorization? The Fractran program that can factor any number in constant time is the identity function. To decode the output, which matches the input, all you need to do is factor it!

Interestingly, it doesn't seem to actually be possible to implement the identity function in Fractran (But somehow it's Turing-complete? Hmmm... more investigation needed.), unless you can define your program in terms of its input. For example, the program 1/(n+1) is the identity function for input n.

18 Feb 2010

Here's something I only learned recently, since it came up when working on Wisp. In C function pointers are incompatible with normal pointers. For example, this is unportable C,

int func (int x);

int main ()
{
  void *p = (void *) &func;
}

This is because function pointers are larger than other pointers on some architectures. If the compiler in such a scenario allowed this perhaps it would overflow the pointer and corrupt some other data, like the stack, or, more likely, only part of the pointer would be copied. It wasn't until I added the -pedantic flag to gcc that it started warning me about situations like the above. The -W -Wall flags are silent here.

I bet this issue only comes up very rarely. How often do you have to store a function pointer in a void pointer? It subverts the type system and is generally a bad idea. I had to do it in Wisp as part of its value polymorphism, which is why it bit me. This is probably why gcc doesn't get very picky over it.

This also means function pointers have less support than normal pointers. For example, printing pointers with printf()'s %p won't work, since it expects a void pointer, so there's no printing them. You can't sort them with qsort(). You can even treat the function pointer as a blob of data to manipulate manually since there's no safe way to make a regular pointer to it. Really, almost any C library function that accepts pointers won't work with function pointers.

So if you want a tricky, unfair, interview question this could be one!

06 Feb 2010

I found this Common Lisp Quick Reference the other day from r/lisp, and I think it's fantastic. It's a comprehensive, libre booklet of the symbols defined by the Common Lisp ANSI standard. Very slick!

The main version is meant to be printed out and nested with a vertical fold, and it works quite well. If I ever get a chance to use Common Lisp at work (a man can dream), probably at a location without Internet access, this could come in handy. So I printed out one for myself,

In case the website ever disappears in the future, here's all the PDFs and LaTeX source for it. These could be out of date, so you should check the main website first.

clqr-a4-booklet-all.pdf
clqr-a4-booklet-four.pdf
clqr-a4-consec.pdf
clqr-letter-booklet-all.pdf
clqr-letter-booklet-four.pdf
clqr-letter-consec.pdf
clqr.tar.gz (LaTeX source)

And a local clone,

git clone http://git.nullprogram.com/clqr.git

04 Feb 2010

I've been chugging away on Wisp, announced in my last post, every day since I started it a few weeks ago, and it's becoming a pretty solid system. There's now an exception system, reference counting for dealing with garbage, and a reentrant parser. It's no replacement for any other lisps, but I've found it to be very fun to work on. I've put what exists so far of the Wisp User Guide up for viewing. The AsciiDoc source is in the Git repository, which is here,

git clone http://git.nullprogram.com/wisp.git

I wanted to show off some of the new features of Wisp, and since I was inspired by Full Disclojure, since it's so damn slick, I decided to make some screencasts of Wisp in action. All of the screencast software for GNU/Linux is pretty poor, but after a few hours of head-banging I managed to hobble something together for you. Enjoy!

That video demonstrated the memoization function. It can be pulled in from the memoize library. You give it a symbol, which should have a function definition stored in it, and it will installed a wrapper around it. In the video I used the Fibonacci function from the examples library.

(require 'examples)
(fib 30) ; Slooooow ...
(memoize 'fib)
(fib 100) ; Fast!

This demonstrated the "detachment" feature of Wisp, which is similar to "futures" in Clojure. It forks off a new process, which executes the given function. The send function can be used in the detached process to send any lisp objects back to the parents, which can receive them with the receive function. The send function can be called any number of times to continually send data back. The receive function will block if there is no lisp object to receive yet.

(require 'examples)
(setq d (detach (lambda () (send (fib)))))
(receive d) ; Gets value from child process

This video shows off the point-free functions that have been defined: function composition and partial application (I accidentally say "partial evaluation" in the video). These are actually just simple macros that any lisp could do.

24 Jan 2010

Update 2010-2-04: A lot of the information below is out of date. There is an update here: Wisp Screencasts.

This is a project I've been wanting to do for some time, and I finally got around to doing it. I spent the last few days implementing my own lisp interpreter in C. Today, after sinking in about 48 hours of work, I believe I completed enough of it to consider it in a working state, with a code base stable enough that other interested people could contribute. It was really exciting to see everything come together today.

You can make a clone of the Wisp repository with Git. Go ahead; don't be shy,

git clone http://git.nullprogram.com/wisp.git

To build it, all you need is a C99 compiler, make, yacc (i.e. Bison), and lex (i.e. Flex).

It doesn't use the readline library or one of it's clones to make a nice interaction command line, so it's a good idea to run it with rlwrap. That's what I've been doing. If you plan on writing Wisp code in Emacs, putting this in your .emacs will give you all the syntax niceties,

(add-to-list 'auto-mode-alist '(".wisp\\'" . lisp-mode))

You should also be able to run it as an inferior lisp and send code to it like a normal lisp. I haven't done this yet myself.

I think the name is apt, because it really is a wisp of a lisp. As of this writing, it weighs in at 1500 lines of code and is still very feature-light. I haven't actually read any material about writing lisp interpreters, so I've been winging it based on my experience with it. It's already taught me a lot of subtle things about lisp that I hadn't been aware of before.

Right know it's very simple. It doesn't yet support any syntax beyond parenthesis (no ' quoting). No closures. No garbage collection, as I'm still working out how I'm going to do that. Dynamically scoped, since that's a lot easier to do. And, like Scheme, it's a lisp-1, meaning functions and variables share a common namespace. I hope to expand it to include some features from other lisps like Arc (particularly the anonymous function syntactical sugar), Common Lisp, and Scheme.

However, it does have already anonymous functions, which many popular languages still don't have. :-) It's far enough along to let you define crazy stuff like this,

(defun example (n)
  (if (<= n 0)
      (lambda (x) (* x 10))
    (lambda (x) (* x 2.0))))

Which you can call, interactively in this case, like so,

wisp> ((example  1) 20)
40.000000
wisp> ((example -1) 20)
200

Because there's no garbage collection yet, it leaks memory like a sieve. For garbage collection, I think I'll do a mark-and-sweep, marking objects based on their reachability from the symbol table. That still leaves some corner cases -- such as worrying about objects in limbo in the evaluator -- that I'm not sure about. I need to be careful not to free objects still in use. Still working that one out.

It has the multiplication, addition, subtraction, and division implemented, as well as the greater-than and less-than operators. Lisp macros are implemented. A number of special forms are defined, like let, if, set, defun, defmacro, car, cdr, not, progn, lambda, and while. All of the predicates are implemented. It has a C interface, which is how all the above got defined. I already have some functions and macros defined in terms of Wisp code, too. Most of the needed functions at this point are trivial to add, though a bit tedious, so I'm mostly trying to focus on the core parts of the interpreter right now.

It's amazing how much the internal code looks like lisp written in a C dialect. I have CAR and CDR macros defined, which get used all over the place, and code frequently uses them to walk lists like lisp code would.

The core struct that everything works with the the object struct. It's defined as such,

typedef struct object
{
  type_t type;
  void *val;
} object_t;

The type_t field comes from this enumeration,

typedef enum types
  { INT, FLOAT, STRING, SYMBOL, CONS, CFUNC, SPECIAL } type_t;

The type indicates what type of data the void pointer points to, making it sort-of polymorphic. Note the CONS type, the cons cell, used to create lists,

typedef struct cons
{
  object_t *car;
  object_t *cdr;
} cons_t;

There's the familiar car and cdr pointers. There are a bunch of helper functions to manipulate and build these. For example, c_cons() creates a cons cell,

object_t *c_cons (object_t * car, object_t * cdr);

Look familiar? Yup, that's the lisp cons function. Since the nil symbol is available in C code as NIL you can chain these together in C to make a list,

object_t *lst = c_cons (c_int(10), c_cons (c_str ("hello"), NIL));

Which puts together the simple list,

(10 "hello")

Since that's so cumbersome to write out, there's a parser that can read nice lisp code and use all those same functions to make the lists. Hence, the lisp reader.

If you want to help, it's pretty easy to add more CFUNCs, C functions that are exposed to Wisp lisp code. Right now, I'd like to expose the whole C math.h library, provide a nice I/O interface, and expose a bunch of string functions. The TODO file in the repository contains more things to be done.

Wisp will probably be getting it's own "project page" here at some point in the future. When it does, I'll update this post to point to it.

Oh, and I decided to make this available under a 2-clause BSD license, so someone could easily plug it into another program as an extension language (once Wisp has matured first, of course). That would be cool.

15 Jan 2010

Common Lisp is possibly the most advanced programming language. Think of pretty much any programming language feature and Common Lisp probably has it. Since lisp is the programmable programming language, when someone invents a new language feature it can probably be added to Common Lisp without even touching the language core.

However, if you're interested in digging into Common Lisp to try it out, you may find yourself quickly running into walls just getting started. It's a lot different than other programming environments you may be used to. The Common Lisp tutorials generally skip this step, assuming the user has an environment, or leaving that setup for the "vendor" to handle. So, here's a guide to setting up a great Common Lisp environment with Emacs and SLIME. It should work with any Common Lisp implementation and any operating system that can run Emacs (i.e. most of them). Even a much less capable one like Windows.

First, you need to pick a Common Lisp implementation and install it. Ideally, it should end up in your PATH. Like C, the language is defined solely by its standardized specification, rather than some canonical implementation. Steel Bank Common Lisp (SBCL) is currently the highest performing implementation, it's Free Software, and it runs on a wide variety of platforms, so take a look at that one if you're not sure.

Next, install Emacs. We're using Emacs not just because it's the best text editor ever created. :-D It's because that's what SLIME is written for, and Emacs is a lisp-aware editor. Really, Emacs is a lisp interpreter that happens to be geared towards text-editing. It's accused of breaking the rules of unix by being a single, monolithic program, but it's really a whole bunch of small lisp programs. You can even have a lisp REPL in Emacs (ielm), similar to what we will have once we're done here. It's plays very well with Common Lisp.

If you're unfamiliar with Emacs, you should stop here and familiarize yourself with it a bit. Really, you could spend a decade learning Emacs and still have more to learn. The tutorial should be good enough for now. Fire up Emacs and run the tutorial by pressing control+h then t. In Emacs notation, that's C-h t. C-h is the help/documentation prefix, which can be used to look up variables/symbols (v), functions (f), key bindings (k), info manuals (i), the current mode (m), and apropos (searching) (a). In the info manuals, you should be able to find the full Emacs manual, Elisp reference, and Elisp tutorial, since they are generally installed alongside Emacs these days. Nearly anything you might need to know can be found inside the included documentation.

Next, install SLIME. I'll be a bit more specific for this one. Make a .emacs.d directory in your home directory (whatever your HOME environmental variable is set to). This is a common place to put user-installed Emacs extensions. You will be putting your slime directory in here. There are two basic ways to obtain SLIME, as indicated right on their main page. You can do a CVS checkout of the SLIME repository, which allows you to follow it and run the latest version. Or you can grab a snapshot of the repository, which is provided, and dump it in there. Since I like you so much, I'll give you a third option. Here's a Git repository, maintained by someone very kind, that follows SLIME's CVS repository,

git clone git://git.boinkor.net/slime.git

Ultimately, you should have a directory ~/.emacs.d/slime/ that contains a bunch of SLIME source files directly inside.

Now, we tell Emacs where SLIME is and how to use it. Make a .emacs file in your home directory, if you haven't already, and put this in it,

(add-to-list 'load-path "~/.emacs.d/slime/")
(require 'slime)
(slime-setup '(slime-repl))

Once it's saved, either restart Emacs, or simply evaluate those lines by putting the cursor after each them in turn and typing C-x C-e. If you did everything right so far, you shouldn't have any errors. (If you did, go back up and see what you did wrong.) If your Common Lisp installation didn't end up in your PATH as "lisp" (not uncommon) for some reason, you may need to tell Emacs where it is. For example, I can point directly to my SBCL installation with this line,

(setq inferior-lisp-program "/usr/bin/sbcl")

If everything is set up right, fire up SLIME with "M-x slime". It should compile the back-end, called swank, and run a Common Lisp REPL as an inferior process to Emacs. You should end up with a nice prompt like this,

CL-USER>

At this line, you can start evaluating lisp expressions as you please. But this isn't where the true power of SLIME comes in yet. I'll give you an example: make a new file with a .lisp extension and open it. Throw some lisp in there,

(defun adder (x)
  (lambda (y) (+ x y)))

Type C-x C-k and it will send the current buffer over to be compiled and loaded. This code here uses a closure, so you know you aren't accidentally using Emacs lisp, as it doesn't have closures. At the REPL you can call it,

CL-USER> (funcall (adder 5) 6)

Which will print the return value, 11. That's all there is to it. You write code in the buffer, then with a simple keystroke send it to the Common Lisp system to be evaluated and loaded. Because the SLIME key bindings eclipse the Emacs lisp key bindings, you can type this same line in the lisp source buffer place the cursor at the end, and type C-x C-e, which will send it out to Common Lisp to be evaluated. Look at the mode help (C-h m) to see all the key bindings made available.

This is a great programming environment that makes Common Lisp all the more fun to use. You run a single, continuous instance if your program growing it gradually. (This is exactly how I built my Emacs web server with elisp.) You can test your code as soon as soon as it's written.

The setup can get even more advanced. The Common Lisp REPL need not be running on the same computer. It can be running on another computer, as long as SLIME is able to connect to it over the network. Several developers could even share a single Common Lisp process running on a common machine. Lots of possibilities.

If you don't have a Common Lisp book yet, there's Practical Common Lisp, which you can read at no cost online or download for reading offline. It's based on an Emacs and SLIME setup, so you'll be right on track.

21 Dec 2009

For a long time I couldn't figure out how to make decent thumbnails with ImageMagick. Specifically, I wanted to create uniform sized thumbnails from arbitrary images. Over the weekend I came across the ImageMagick Examples page, which shows exactly how to do this. Here's the command for a 150x150 thumbnail,

convert orig.jpg -thumbnail 150x150^ -gravity center \
        -extent 150x150 thumb.jpg

It cuts out the largest square possible from the center of the image and resizes that to 150x150. This capability has actually only been available for 2 years now! It wasn't there last time I needed it.

I can think of one way to improve it: instead of selecting the center, it selects the area with the highest information density. This could be measured by edge detection, corner detection, or some other statistical method. It would be selected by changing the gravity option to, say, "entropy".

I'm listing this here mostly for my own future reference. :-)

13 Dec 2009

Sources:

git clone http://git.nullprogram.com/GameOfLife.git

Compiled (2009-12-13): GoL.jar (6.7kb)

Since I recently got back into Java recently, I threw together this little Game of Life implementation in Java. It looks a lot like my maze generator/solver on the inside, reflecting the way I think about these things. Gavin wrote a competing version of the game in Processing which we were partially discussing the other night, so I made my own.

The individual cells are actually objects themselves, so you could inherit the abstract Cell class and drop in your own rules. I bet you could even write a Cell that does the iterated prisoner's dilemma cellular automata. The Cell objects are wired together into a sort of mesh network. Instead of growing it wraps around on the sides.

It takes up to four arguments right now, with three types of cells, basic, implementing the basic Conway's Game of Life, growth, which is a cell that matures over time, and random which mixes both types together (seen in the screenshot). The arguments work as follows,

java -jar GoL.jar [<cell type> [<width> <height> [<cell pixel width>]]]

I may look into extending this to do some things beyond simple cellular automata.

06 Dec 2009

Developing C in Emacs is a real joy, and it's mostly thanks to the compile command. Once you have your Makefile -- or SConstruct or whatever build system you like -- setup and you want to compile your latest changes, just run M-x compile, which will run your build system in a buffer. You can then step through the errors and warnings with C-x `, and Emacs will take you to them. It's a very nice way to write code.

I use the compile command so much that I bound it to C-x C-k (C-k tends to be part of compile key bindings),

(global-set-key "\C-x\C-k" 'compile)

Until recently, I didn't have as nice of a setup for Java. Since they generally force offensive IDEs onto me at work this wasn't something I needed yet anyway, but I get to choose my environment on a new project this time. If you're using Makefiles for some reason when building your Java project, it still works out fairly well because they're usually called recursively. It gets more complicated with Ant, where there is only one top-level build file. Emacs' compile command only runs the build command in the buffer's current directory.

I know three solutions to this problem. One is to provide the build file's absolute path when compile asks for the command with the -buildfile (-f) option. You only need to type it once per Emacs session, so that's not too bad.

ant -emacs -buildfile /path/to/build.xml

It's not well documented, but there is a -find option that can be given to Ant that will cause it to search for the build file itself. This is even nicer than the previous solution. Just remember to place it last, unless you give it the build filename too. For example, if you wanted to run the clean target,

ant -emacs clean -find

To keep the actual call as simple as possible, I wrote a wrapper for compile, and put a hook in java-mode to change the local binding. The wrapper, ant-compile, searches for the build file the same way -find would do.

(defun ant-compile ()
  "Traveling up the path, find build.xml file and run compile."
  (interactive)
  (with-temp-buffer
    (while (and (not (file-exists-p "build.xml"))
                (not (equal "/" default-directory)))
      (cd ".."))
    (call-interactively 'compile)))

So I can transparently keep using my muscle memory compile binding, I set up the key binding in a hook,

(add-hook 'java-mode-hook
          (lambda () (local-set-key "\C-x\C-k" 'ant-compile)))

Voila! Java works looks a little bit more like C.

05 Dec 2009

Yesterday was a slightly chilly day, in the mid 50s (Fahrenheit), so when my officemate said we were getting snow I thought she was kidding. Well, today we woke up to this winter wonderland,

My wife leapt for her camera and filmed that as soon as she realized what was going on. We had much more than that by late afternoon. According to the Baltimore Sun it has snowed on December 5th for six of the last eight years, and we don't get much snow around here. My only wish would be for this to have happened on a weekday. I work for a university, and educational organizations tend to fold and give snow days really easily. This would probably have given me a nice paid day off!