Posted by Tom Moertel
Mon, 05 May 2008 13:58:00 GMT
There’s a great way to explain delimited continuations in the notes of Oleg’s Continuation Fest talk on using delimited continuations for CGI programming. Just so it doesn’t get overlooked, here it is:
I’m obsessed in pointing out that every programmer already knows
and understands the delimited continuations; they might not know that
word though. Everyone knows that when a process executes a system
call like read, it gets suspended. When the disk
delivers the data, the process is resumed. That suspension of a
process is its continuation. It is delimited: it is not the
check-point of the whole OS, it is the check-point of a process only,
from the invocation of main() up to the point
main() returns. Normally these suspensions are resumed
only once, but can be zero times (exit) or twice
(fork).
I especially like the final part about exit and
fork, which drives home the notion that something more
subtle than returning from a typical function call is going on. If
anybody is confused over what suspended means, that last part
ought to clear things up.
The next time I need to explain delimited continuations, I know how
I’m going to do it.
Posted in functional programming
Tags continuations, fp, oleg
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Posted by Tom Moertel
Fri, 11 Apr 2008 15:58:00 GMT
Via Chris:
$ history | awk '{print $2}' | sort | uniq -c | sort -rn | head
196 git
110 l
102 cd
70 make
34 darcs
30 pushd
23 ssh
23 m
23 ls
20 rm
The l and m commands are aliases:
Posted in interesting stuff
Tags life, memes, programming
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Posted by Tom Moertel
Thu, 20 Mar 2008 02:34:00 GMT
At work recently I was writing some tests with Python’s out-of-the-box
unit-testing framework
unittest. I’m new
to Python and accustomed to Perl and Haskell’s testing frameworks,
which are lightweight and let you write tests without much
hoop-jumping. In particular,
QuickCheck and
LectroTest make it easy
to test at the property level instead of the test-case level.
With unittest, I was having to write a lot of code
to get the same level of abstraction.
By “property level,” here’s what I mean. Say I’m testing this thing,
let’s call it a subscriber pool. It has two fundamental properties:
- Subscribe. For all initial states of the pool, if you call subscribe(user), then, assuming there have been no other operations on the pool, user must be in the pool.
- Unsubscribe. For all initial states of the pool, if you call unsubscribe(user), then, assuming there have been no other operations on the pool, user must not be in the pool.
That’s it. If my implementation satisfies both properties, it’s
correct. (This is a simplified version of my real testing problem,
which required additional property checks.)
To test whether my implementation satisfies each property, I must
write individual test cases that together “cover” the property. For
example, to test whether the Subscribe property holds, I might write
four test cases:
class SubscribeProperty(unittest.TestCase):
def setUp(self):
initialize_pool()
def tearDown(self):
destroy_pool()
def testEmpty(self):
load_pool_with_members([])
subscribe("1")
self.assert_("1" in pool_members())
def testOtherGuyAlreadyInPool(self):
load_pool_with_members(["2"])
subscribe("1")
self.assert_("1" in pool_members())
def testSubscriberAlreadyInPool(self):
load_pool_with_members(["1"])
subscribe("1")
self.assert_("1" in pool_members())
def testSubscriberAndOtherGuyAlreadyInPool(self):
load_pool_with_members(["1", "2"])
subscribe("1")
self.assert_("1" in pool_members())
Every one of the test cases has the same form. The repetition
makes me want to refactor the whole thing.
Okay, let’s do it:
Read more...
Posted in testing
Tags nose, properties, python, testing, unittest
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Posted by Tom Moertel
Tue, 18 Dec 2007 03:33:00 GMT
XML is fine for representing document-like things, but when it’s
twisted to represent build recipes, configuration files, and little
programming languages, it opens the gates to XML Hell. Once the
gates are opened, the demons of cargo-cult thinking are loosed upon
the world, where they are free to trick innocent programmers into
working with grotesquely twisted XML documents – something no human
mind was designed to comprehend. Ensnared, these programmers are
slowly drawn into the depths of XML Hell, from which their
lamentations echo across the
universe.
When the demons of cargo-cult thinking come for you, don’t be
ensnared! Instead, be prepared – with PXSL – the Parsimonious XML
Shorthand Language
(pronounced “pixel”).
What’s PXSL? It’s a luxurious, thermonuclear smoking jacket that you
can slip on using a convenient preprocessor. Use it whenever you see
grotesque XML on the horizon. Within PXSL’s plush (and stylish)
protection, you can create all the nasty, twisted XML that may be
demanded of you, but you need not descend into XML Hell to do it.
Instead, you can work from the comfort of a well-stocked lounge, where
clarity and conciseness are always on tap.
For example, here’s a snippet from an XSLT stylesheet, in the
original XML:
<xsl:template match="/">
<xsl:for-each select="//*/@src|//*/@href">
<xsl:value-of select="."/>
<xsl:text> </xsl:text>
</xsl:for-each>
</xsl:template>
And here’s the same snippet, written in PXSL:
template /
for-each //*/@src|//*/@href
value-of .
text << >>
Isn’t that refreshing?
Why PXSL?
There are lots of XML shorthands available. (The PXSL FAQ lists about ten of them.) So why choose
PXSL? Here’s why:
Also, PXSL is battle tested. It was first released in 2003 and has
been saving people from XML Hell since. People who try it seem to like it:
- I think PXSL could do wonders for soothing my irrational hatred for all things XML. —kowey
- Impressive… I converted some of my files from XML to PXSL and the readability was much improved. —chris
- Quite aside from the fact that XSLT is finally somewhat readable, the fact that you’ve added a serious macro system means that some serious scripting of XML can occur. I’m very impressed. —invisible
The next time you’re headed for XML Hell, why not give the venerable PXSL a try? You might just find that you like it, too.
This public service announcement was brought to you in celebration of
the 1.0 release of the pxsl-tools package. The PXSL-to-XML compiler
pxslcc is written in Haskell and uses the
cross-platform Haskell Cabal
build/package system to let you use PXSL just about anywhere.
Posted in programming
Tags haskell, pxsl, xml, xslt
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Posted by Tom Moertel
Mon, 10 Dec 2007 21:52:00 GMT
About three years ago, I switched to
Darcs
as my primary source-code management system. It was simple,
intuitive, and powerful, and it made managing my projects more fun and
less frustrating than any centralized VCS ever had. That it was
written in Haskell, one of my favorite programming languages, made
it even better. I was hooked.
Since then, the distributed SCM landscape has changed. Darcs hasn’t
improved much, but its competitors have made long strides, especially
Git and
Mercurial. Both
are crazy fast, vigorously developed, and widely used on large, highly
active real-world projects, such as the Linux kernel and Mozilla 2.
In comparison, Darcs has
stagnated.
When I started working for a new company recently, I had to consider
whether to advocate Darcs or something else. In the end, I decided
that Darcs would be a hard sell. Nobody else at the company uses
Haskell, and having to explain how to avoid the occasional corner
case
seemed liked a losing proposition.
After researching and playing around with Git and Mercurial, I settled
on Git. I like Git’s underlying hashed-blobs model better than
Mercurial’s revlogs, and Git seems to have slightly more development
momentum. Still, it was a close call. Either choice would have been
completely reasonable.
Missing Darcs
When I started using Git on real projects, the one thing I really
missed was the ability to easily amend earlier patches, something
Darcs made trivial. Let me
explain. The typical development workflow goes something like this:
- Checkout copy of upstream code base.
- Implement feature X.
- Commit.
- Implement independent feature Y.
- Commit.
- Implement independent feature Z.
- Commit.
- Push new features back upstream.
Now, what really happens is that when I’m implementing Y or Z,
I’ll realize that I made a mistake in X. The trick is then
fixing X so that my fix is part of the changeset/patch for X that
ultimately gets pushed upstream in the last step. That way, the
upstream folks will see only a single, clean patch for feature X – not
a mishmash of patches that together represent X.
In Darcs, amending the original patch is easy because its patch theory
lets me tweak the patch for X independently of the other patches.
Darcs will simply ask me which patch I want to amend, and I’ll select
the orignal patch for X:
$ emacs # fix X
$ darcs amend-record # amend original patch for X
Mon Dec 10 14:43:13 EST 2007 Tom Moertel <tom@moertel.com>
* Implemented Z
Shall I amend this patch? [yNvpq], or ? for help: n
Mon Dec 10 14:42:12 EST 2007 Tom Moertel <tom@moertel.com>
* Implemented Y
Shall I amend this patch? [yNvpq], or ? for help: n
Mon Dec 10 14:41:46 EST 2007 Tom Moertel <tom@moertel.com>
* Implemented X
Shall I amend this patch? [yNvpq], or ? for help: y
hunk ./x 1
-X1
+X2
Shall I add this change? (1/?) [ynWsfqadjkc], or ? for help: y
Finished amending patch:
Mon Dec 10 14:43:25 EST 2007 Tom Moertel <tom@moertel.com>
* Implemented X
That’s it. The exact same process will work regardless of when I
realize I need to fix X: before I start Y, while I’m implementing Y,
after I’ve committed Y, while I’m working on Z, or after I’ve committed
Z.
Learning to love Git
With Git, however, I can amend a commit only if I haven’t committed anything else before making my fix. In Git’s mind, Y depends on X, and Z
depends on Y, even if they really are independent of one another.
So if I commit the original patch for X and then immediately realize I
need to make a fix, before I start working on Y or Z, it’s easy:
$ emacs # implement X
$ git commit -m 'Implemented X'
# discover problem in X
$ emacs # fix X
$ git commit --amend # amend original patch
More typically, it’s only while I’m working on Y that I’ll
realize I need to fix X. Then it’s more complicated
to amend the original commit:
$ emacs # implement X
$ git commit -m 'Implemented X'
$ emacs # start working on Y
# discover problem in X
$ git stash # stash away half-completed work on Y
$ emacs # fix X
$ git commit --amend # amend original patch for X
$ git stash apply # restore work on Y
$ emacs # continue working on Y
While not as convenient as Darcs’s workflow, it’s perfectly workable.
Now let’s consider another fairly typical case: I commit X and Y and
then start working on Z before I notice the problem in X. I used to
think that Git couldn’t handle this case, but it can, thanks to
git rebase --interactive:
$ emacs # implement X
$ git commit -m 'Implemented X'
$ emacs # implement Y
$ git commit -m 'Implemented Y'
$ emacs # start working on Z
# discover problem in X
$ git stash # stash away half-completed work on Z
$ emacs # fix X
$ git commit -m 'Fixed X'
$ git rebase --interactive HEAD~3 # see comments below
$ git stash apply # restore work on Z
$ emacs # continue working on Z
The
git rebase --interactive command is
powerful. What the
command does, as called in the snippet above, is invoke my editor of
choice on a text file describing the last 3 commits (that’s the
HEAD~3 part):
# Rebasing 3ad99a7..b9a8405 onto 3ad99a7
#
# Commands:
# pick = use commit
# edit = use commit, but stop for amending
# squash = use commit, but meld into previous commit
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
pick 0885540 Implemented X
pick 320b115 Implemented Y
pick b9a8405 Fixed X
I can then edit the file to reorder, merge (squash), and/or remove
the commits. In this example, I want to merge the fix for X into
the original commit that implemented X. So I edit the file like so:
pick 0885540 Implemented X
squash b9a8405 Fixed X
pick 320b115 Implemented Y
Then I save the file, at which point Git takes over and makes the
requested changes, merging the fix for X into the
original commit for X. Now the log shows the original implementation
and fix as one commit:
$ git log
commit f387d650976246c0854d028b040cca40e542be56
Author: Tom Moertel <tom@moertel.com>
Date: Mon Dec 10 15:11:26 2007 -0500
Implemented Y
commit 82a1c849ffd1bd688d5bc9d99be0e63548a89c4c
Author: Tom Moertel <tom@moertel.com>
Date: Mon Dec 10 15:13:03 2007 -0500
Implemented X
Fixed X
commit 3ad99a7ef537b7ae99e435e0d2b4b0d03de92c65
Author: Tom Moertel <tom@moertel.com>
Date: Mon Dec 10 15:11:14 2007 -0500
Initial checkin
Once I figured out how to use git rebase --interactive, I stopped
missing Darcs and started loving Git.
Posted in programming
Tags darcs, dvcs, git, haskell, scm
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Posted by Tom Moertel
Mon, 19 Nov 2007 18:07:00 GMT
Issue 9 of The Monad.Reader is hot off the presses! The issue focuses on three Google-Summer-of-Code projects for Haskell: Cabal configurations, Darcs’s Patch Theory, and the typechecker-framework TaiChi. Good stuff.
I know what I’ll be reading for lunch today.
Posted in haskell
Tags haskell, tmr
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Posted by Tom Moertel
Thu, 15 Nov 2007 07:30:00 GMT
I recently started using Puppet
to automate my server-build processes. The basic idea behind Puppet
is that you create “manifests” that declare
a directed graph of “resources” that represents the desired state of
your machines. Puppet-managed machines on your network then query a
master server to obtain the latest copy of the graph, which they then
reconcile with their current states to make whatever changes are
necessary to bring themselves up to date.
For the most part, everything works well. I have encountered a couple
of snags when writing manifests, however, so I’m going to explain them
here as reminder until I get the time to fix them in the Puppet code and send
patches upstream.
First, don’t use hyphens in class names. While hyphens are legal
in class names, they are not allowed in qualified variables, thus
variables defined within hyphen-named classes are inaccessible
from the outside world.
Second, and this one is both tricky and important, Puppet handles
prerequisites for definitions by silently passing those prerequisites on
to all of the resources within the definitions. Definitions, in
effect, don’t really have their own prerequisites, they just pass them on to
their children. But – and here’s the problem – if those child
resources declare their own prerequisites, those prerequisites will
overwrite the passed-on prerequisites, effectively causing them to
be ignored.
This problem bit me hard when trying to create a definition for
installing Ruby Gems from a local cache of gems:
define local_gem($gem) {
$path = "/var/local/local-gems/$gem"
file { $path:
ensure => present,
source => "puppet://puppet/files/gems/$gem",
require => File["local-gems-dir"],
owner => root,
group => root,
mode => 0664,
}
package { $title:
ensure => installed,
provider => "gem",
require => [ Package["rubygems"], File[$path] ],
source => $path,
}
}
The intent was to be able to declare a local gem like so:
local_gem { "sqlite3-ruby":
gem => "sqlite3-ruby-1.2.1.gem",
require => Package["sqlite-devel"]
}
Thus the “sqlite3-ruby” local gem has the single prerequisite of the
“sqlite-devel” package – or at least that’s what I expected. What
happened on deployment was that the prerequisite was ignored because
when it was passed on to the inner file and package resources, those
resources had their own require parameters, and those parameters
overwrote the passed-on prerequisite.
The work-around is somewhat hacky. I augmented the definition with a do-nothing resource
that has no require parameter of its own. This
resource does nothing but capture the passed-on prerequisites. Then I made
all of the other resources in the definition include the do-nothing
resource as one of their prerequisites. Thus they are made to inherit the
passed-on prerequisites.
My final definition looks like this:
define local_gem($gem) {
# dummy exec to propagate requires from local_gem
exec { $name: command => "/bin/true" }
$path = "/var/local/local-gems/$gem"
file { $path:
ensure => present,
source => "puppet://puppet/files/gems/$gem",
require => [ Exec[$name], File["local-gems-dir"] ],
owner => root,
group => root,
mode => 0664,
}
package { $title:
ensure => installed,
provider => "gem",
require => [ Exec[$name], Package["rubygems"], File[$path] ],
source => $path,
}
}
Notice how the file and package resource both require the dummy exec resource.
That’s the trick that allows them to require the prerequisites passed on from
the local_gem definition.
It’s not pretty, but it works. See this email on the puppet-users mailing list for more on the problem.
Posted in sysadmin
Tags gems, manifests, puppet, rails
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Posted by Tom Moertel
Fri, 02 Nov 2007 19:32:00 GMT
I recently got a Palm Centro smartphone, and so far I love it. Like
most modern cell phones, it has a built-in camera and takes decent
snapshots and even records short movies. It’s great for
spur-of-the-moment shots when I don’t have my real camera. The
trick – and there’s always a trick when it comes to cell phones – is getting
the photos off the camera and onto my computer.
To get at my pictures, Sprint would prefer that I sign up for their
ludicrously expensive “PictureMail” service. Leave it to weasely
telecom execs to come up with another way to squeeze money from
teenagers: charge them $5 each month for the “privilege” of sharing
their pictures with friends. This fee, of course, is in addition to the
fee for “unlimited” mobile Internet use. I guess picture bits are
somehow more expensive to move over the air than other kinds of bits.
In any case, my next goal after
getting my Centro to hotsync with my Linux
workstation was to figure out how
to download my photos and movies.
After a bit of hacking, I figured out that the Centro stores images in
a typical digital-camera-image (DCIM) hierarchy. For
example, I have a 4-GB microSD card installed in my Centro, and I
store my photos in the “Palm” album on it. This album ends up stored
in the /DCIM/Palm directory on the card.
Using the pilot-xfer program
from the pilot-link project, I was able
to find the directory and its contents. The trick was to use the
sparsely documented –D flag to work with the Centro’s virtual
filesystem. Here, for example, is how I list the contents of the Palm album:
$ pilot-xfer -p usb: -D /DCIM/Palm -l
Listening for incoming connection on usb:... connected!
Directory of /DCIM/Palm...
652 Fri Nov 2 08:17:06 2007 Album.db
292053 Fri Nov 2 09:04:20 2007 Photo_110207_001.jpg
78493 Fri Nov 2 08:17:06 2007 Video_110207_001.3g2
20 Wed Oct 31 12:09:20 2007 Thumbnail.db
Thank you for using pilot-link.
Here, you can see that I have one photo and one movie in the album.
(Movies are stored in .3g2 files that contain MPEG4 video.)
To download the files, I again turned to pilot-xfer, this time using the
–f (fetch) flag to fetch a list of files.
Here, for example, I’ll fetch the image from the listing above:
$ pilot-xfer -p usb: -D /DCIM/Palm -f Photo_110207_001.jpg
Listening for incoming connection on usb:... connected!
Fetching '/DCIM/Palm' ... (292053 bytes) 285 KiB total.
Thank you for using pilot-link.
So that’s the process. It’s kind of clunky, so I wrote a small Python
program to automate it. (I’m learning Python. If you’re a Pythonista, please
consider critiquing my code. I would be especially thankful if you
could point out any helpful idioms that I may have overlooked.)
Here’s how to use the program:
$ get-pilot-photos.py --help
Usage: get-pilot-photos.py [options]
Options:
-h, --help show this help message and exit
-s SRCDIR, --srcdir=SRCDIR
VFS dir on Palm device from which to fetch images
-d DESTDIR, --destdir=DESTDIR
Where to save the images on your computer
Both the —srcdir and —dstdir options are optional. If you
omit the first, the program will download photos and movies from the
/DCIM/Palm album. If you omit the second, the program will save the
downloads to a new, timestamped directory within your home directory.
That’s it. The code is below.
Read more...
Posted in hacks
Tags centro, download, hotsync, images, movies, palm, python
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Posted by Tom Moertel
Wed, 31 Oct 2007 04:35:00 GMT
I just got a Palm Centro smartphone, and I love it. Getting it to sync
with my Linux workstation, however, was tricky, so I’m posting this
recipe in hopes that it might save you some time.
The “visor” kernel driver is supposed to make compatible Palm
handhelds look like serial devices when attached via a USB cable. For
me, it didn’t work. Instead, I had to blacklist the driver and
then use libusb to talk to the Centro. Here’s the recipe:
First, blacklist the “visor” kernel driver:
# echo blacklist visor >> /etc/modprobe.conf
# modprobe -qr visor
Second, make sure libusb is installed:
# yum install libusb
Third, edit the system’s udev rules to make sure your user account can
access the device files used to talk to the Centro. On my Fedora 7
setup, I found the right rule in /etc/udev/rules.d/50-udev.rules:
ACTION=="add", SUBSYSTEM=="usb_endpoint", \
ATTR{bEndpointAddress}=="?*", ATTRS{devnum}=="?*", ATTRS{busnum}=="?*", \
NAME="bus/usb/$attr{busnum}/$attr{devnum}_ep/$attr{bEndpointAddress}", \
MODE="0644", SYMLINK+="%k"
I edited the last line of the rule, changing the mode to 0664 and adding
a GROUP key to assign the Centro devices to my exclusive user group:
MODE="0664", SYMLINK+="%k", GROUP="thor"
This change lets my account talk to the Centro without having to
take on root privileges. (For bonus points you could set up a more-specific rule to match just your Centro. The rule above, as is, will actually match other devices, too.)
Fourth, tell udev to reload the rules:
# udevcontrol reload_rules
Finally, set up a Palm-device connection via gnome-pilot. Be sure to
select USB for the Type and “usb:” from the Device drop-down list.
That’s it. If you’re lucky like me, you should now be ready to
hotsync your Palm Centro!
Update: Even better, this handy HOWTO shows you to
sync via Bluetooth, which is more convenient than hooking up a USB
cable. I’m now using this method.
Update 2: If you want to use USB to hotsync your Centro, there is a
method that’s more convenient than setting up udev rules. Just create a perms file for
pam_console_apply that tells it to give the console user permission to
access your Centro. To do so, create a file
/etc/security/console.perms.d/60-libpisock.perms and put the following in it:
<libpisock>=/dev/usbdev* /dev/bus/usb/[0-9]*/[0-9]*
<console> 0644 <libpisock> 0644 root
That’s it. (You’ll still need to use libusb.)
Posted in linux
Tags centro, fedora, hotsync, linux, palm, usb
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Posted by Tom Moertel
Fri, 26 Oct 2007 17:23:00 GMT
Alex Smith, a 20-year-old EE student in the UK, proved that the 2, 3 Turing machine is universal. In doing so, he was able to claim the $25,000 prize that Stephen Wolfram offered for the first proof (or disproof) of the 2, 3 machine’s universality.
This story has been getting a lot of attention lately, but one part of
the story has not: that the Perl programming language is featured in the proof. In his documentation of the proof, Universality of Wolfram’s 2, 3 Turing
Machine,
Smith wrote, “I have written several Perl programs, to demonstrate the
constructions given in the proof and to interpret the systems given in
various conjectures.” Smith’s proof includes no fewer than 7 Perl
programs.
Go Perl!
Posted in perl
Tags math, perl, proof
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