Pointfree style in Python

Additional discussion: Hacker News

I’ve been getting back into Python lately… I just wrote a small module that provides support for pointfree style programming with a combination of automatic partial function/method application and operator overloading for function composition:

from pointfree import *
from operator import add

fn = pfmap(len) \
  >> pfmap(lambda n: n**2) \
  >> pfreduce(add, initial=0)

fn(["foo", "barr", "bazzz"]) # == 50

See the overview for a lengthier introduction. You can install the pointfree module from the Python Package Index:

$ pip install pointfree


Martin Kleppmann: Accounting for Computer Scientists

I just found a great blog post by Martin Kleppmann titled Accounting for Computer Scientists, a succinct introduction to double-entry accounting (although he doesn’t refer to it as such) in terms of basic graph theory:

Eventually I figured it out: basic accounting is just graph theory. The traditional ways of representing financial information hide that structure astonishingly well, but once I had figured out that it was just a graph, it suddenly all made sense.

He goes on to illustrate how a profit-and-loss statement and a balance sheet can be visualized on a simple DAG.  Good stuff…

Updating your music collection with PowerShell

I’ve been banging my head against the wall on account of different music players which can’t play the subset of songs I have encoded as either Vorbis or AAC. So I’m slowly converting my entire music library over to MP3, which works everywhere, even if it’s less efficient.

But before I go digging through my old CDs I need to identify which albums I have to re-encode. My music is organized in folders by artist and then album, e.g.:

~\Music\Library\Pixies\Surfer Rosa\Where Is My Mind.mp3

So this means I effectively have to list the names of folders containing non-MP3 music files. Fortunately Windows PowerShell makes this a one-liner (though admittedly it’s a pretty long line; the backtick is PowerShell’s line continuation syntax):

ls -r Music\Library `
| ?{ $_.PSIsContainer -And ( $_.GetFiles() `
| ?{ $_.Name -Match "\.(m4[ap]|ogg|wma)$" } ) } `
| %{ New-Object PSObject -Property `
@{ Artist = (gi $_.PSParentPath).Name; Album = $_.Name } }

This will give you a nice list of non-MP3 albums, like:

Album              Artist
-----              ------
That's Your Fire   Aloha
Noble Beast        Andrew Bird
Believe It Mammals Bats & Mice
Charm School       Bishop Allen

And thanks to PowerShell’s object-oriented pipes, this is done without having to worry about any quoting or whitespace issues.

The more I learn PowerShell, the less I find myself relying on Cygwin for simple Windows administration tasks. I think Microsoft did a nice job balancing all the different requirements they had to meet with PowerShell. At the very least, it’s good to finally see a strong command shell built into Windows.

PXE booting OpenBSD on an ALIX via Ubuntu Live CD

Update: I’ve expanded the contents of this post into a full guide to running an OpenBSD router on an ALIX board.

This is a quick guide to booting the OpenBSD installer on a PC Engines ALIX board with tinyBIOS (such as the ALIX 2d3) via PXE, using just the following:

  • PC with two network interfaces. One of these needs to be Ethernet, and the other must connect to the Internet. For example, any standard PC laptop with both WiFi and Ethernet adapters will work if there’s a WiFi Internet connection available.
  • Null modem cable
  • Ethernet crossover cable
  • USB-serial adapter (unless your PC has a built-in RS-232 port)
  • Ubuntu Linux 10.10 desktop live CD

Thanks to the versatility of the Ubuntu live CD (specifically the use of AUFS to provide a writable root directory in RAM), you can set up the necessary PXE boot server without making any permanent changes to your PC.

Ubuntu packages

Boot the Ubuntu live CD and quit the installer. Ensure that Ubuntu has a working Internet connection, then enable the “universe” package repository by uncommenting the corresponding lines in /etc/apt/sources.list. Now open a terminal and run the following commands to install prerequisite packages:

$ sudo -s
# apt-get update
# apt-get install dhcp3-server tftpd xinetd cu

Network configuration & NAT

Run this command to configure a static address on the Ethernet interface:

# ifconfig eth0 up netmask

I’ve found you may also need to configure the static address in the “Network Connections” dialog (under Preferences in the System menu) to prevent Network Manager from getting in the way. This is sort of hackish, but we only need it to work for the duration of the install.

Now enable routing and configure a simple NAT using iptables so that the ALIX board can access the internet through your PC’s wireless connection:

# echo 1 > /proc/sys/net/ipv4/ip_forward
# iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE
# iptables -A FORWARD -i eth0 -j ACCEPT
# iptables -A FORWARD -i wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT

Connect your PC’s Ethernet port to the first port on your ALIX board using the crossover cable. On the ALIX 2d3, the first port is the one adjacent to the USB ports.

DHCP server

Replace /etc/dhcp3/dhcpd.conf with the following contents:


shared-network LOCAL-NET {
    option domain-name-servers;

    subnet netmask {
        option routers;
    filename "pxeboot";
    default-lease-time 600;
    max-lease-time 7200;

Also, edit the file /etc/default/dhcp3-server so that the last line reads:


Now you can start the DHCP server.

# /etc/init.d/dhcp3-server start

TFTP server

Create an xinetd file /etc/xinetd.d/tftp as:

service tftp
    socket_type = dgram
    protocol = udp
    wait = yes
    user = root
    server = /usr/sbin/in.tftpd
    server_args = -s /tftpboot

Next create the aforementioned directory /tftpboot. Download the files bsd.rd and pxeboot from the /4.8/i386/ directory on your favorite OpenBSD mirror and copy them into this directory.

Restart xinetd to load the new configuration.

# /etc/init.d/xinetd restart

Serial console

Connect your laptop’s serial port (or plugged-in USB-serial adapter) to the ALIX board’s serial port with your null modem cable, then use the cu command to connect to the serial console. For example, if you’re using a USB adapter and your ALIX’s BIOS has the default serial port settings:

# cu -e -o -s 38400 -l /dev/ttyUSB0

PXE boot

With your serial console ready, plug in the ALIX board’s power adapter, and you should see the board begin to boot. While the memory check is being performed, press the ‘s’ key to enter the tinyBIOS settings, and verify that PXE boot is enabled (if it isn’t, press ‘e’ to toggle it).

After exiting the BIOS settings menu, the board will reboot. It should find your PXE server and bring you to an OpenBSD boot menu. Enter the following at this menu:

boot> stty com0 38400
boot> set tty com0
boot> bsd.rd

The installer will boot from the bsd.rd image that you downloaded. Now perform the installation as normal, but remember to configure the serial port as your system console in the installer.

Advanced Kindle store search

I just found a great web site providing a better Kindle content search than what’s baked into Amazon: eReaderIQ.com. You can search by price, publication date, reading level, and whether the book you want is in the public domain, among other things.

For example, this query lists only free Kindle books in the public domain. Very handy.