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

Links:

• Module documentation
• Github project page
• Python Package Index page

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…

I’ve been using OpenBSD as a NAT router for a few years now. Here’s a guide showing how I got it up and running, in case anyone is interested in doing the same.

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.

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.

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

# ifconfig eth0 up 192.168.2.1 netmask 255.255.255.0

# 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

authoritative;

shared-network LOCAL-NET {
    option domain-name-servers 8.8.8.8;

    subnet 192.168.2.0 netmask 255.255.255.0 {
        option routers 192.168.2.1;
    filename "pxeboot";
    range 192.168.2.100 192.168.2.200;
    default-lease-time 600;
    max-lease-time 7200;
    }
}

INTERFACES="eth0"

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

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

# /etc/init.d/xinetd restart

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

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

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.