Apologies in advance for writing a long blog post about the dull and
specialised subject of
double-entry accounting
from the Unix tools perspective, that ends up talking about Monads to boot.
I can't believe I'm about to write such a thing, but I'm finding it an oddly
interesting subject.
double-entry accounting
I've known of, though probably not really understood double entry
accounting for a while, thanks to
GnuCash. I think
GnuCash did something rather special in making such a subject approachable
to the layman, and I've been happy to recommend GnuCash to friends. I was
stoked to find a chapter in my sister Anna's
new book
that happily and plausibly suggests readers use GnuCash.
But for my personal use, GnuCash is clunky. I mean, I'm a programmer,
but I can't bring any of my abilities to bear on it, without a deep dive
into the code. The file format is opaque (and a real pain to keep checked
into git with all the log files); the user interface is often confusing,
but there's no benefit to its learning curve, it never seems to get better
than manually entering data into its ledger, or perhaps importing data from
a bank. I've never found the reports very useful.
I've got perhaps a year of data in GnuCash, but it's fragmented and
incomplete and not something I've been able to motivate myself to keep up
with. So I have less financial data than I'd like. I'm hoping ledger will
change that.
ledger
I've known about ledger for a while, at least since
This Linux Weekly News article.
It's a quintessential Unix tool, that simply processes text files.
The genius of it is the simplicity of the file format, that gets the essence
and full power of double entry bookeeping down to something that approaches a
meme. Once you get the file format stuck in your head, you're done for.
2004/05/27 Book Store
Expenses:Books $20.00
Liabilities:Visa
starting to use hledger
Now as a Haskell guy, I was immediately drawn to
the Haskell clone,
hledger. It's nice that there
are two (mostly) compatable implementations of ledger too. So from here on,
I'll be talking about hledger.
I sat down and built a hledger setup for myself the other day.
I started by converting the GnuCash books I
have been keeping up-to-date,
for a small side business (a rental property). It quickly turned into
something like programming, in the best way, as I used features like:
- Include directives, so I can keep my business data in its own file,
while pulling it into my main one.
- Simple refactorings, like putting "Y 2012" at the top, so I don't have
to write the year in each transaction.
- Account aliases, so I can just type "rent", rather than
"income:rental" and "repairs:contractor" rather than
"expenses:home repair:contractor"
- All the power of my favorite editor.
a modern unix program
While I've been diving into hledger, I've been drawing all kinds of parallels
between it and other modern Unix-friendly programs I use lately. I think
we've gone over a bit of a watershed recently. Unix tools used to be either
very simple and crude (though often quite useful), or really baroque and
complex with far too many options (often due to 10-30 years of evolution).
Or they were a graphical user interface, like GnuCash, and completely
divorced from Unix traditions.
The new unix programs have some commonalities...
- They're a single command, with subcommands. This keeps
the complexity of doing any one thing down, and provides many
opportunities for traditional unix tools philosophy, without
locking the entire program into being a single-purpose unix tool.
hledger's subcommands range from querying and reports, to pipable
print, to a interactive interface.
- They have a simple but powerful idea at their core, that can be
expressed with a phrase like "double entry accounting is a simple
file format" (ledger), or "files, trees, commits" (git).
Building a tool on a central idea is something I strive
to do myself. So the way ledger manages it is particularly interesting to
me.
- They are not afraid to target users who have a rather special kind
of literacy (probably the literacy you need to have gotten to here
in this post). And reward them with a lot of power.
Ledger avoids a lot of the often confusing terminology around accounting,
and assumes a mind primed with the Unix tools philosophy.
- If there's a GUI, it's probably web based. There's little trust in
traditional toolkits having staying power, and the web is where the
momentum is. The GUI is not the main focus, but does offer special
features of its own.
hledger's web UI completely parallels what I've doing with the git-annex
webapp, right down to being implemented using Yesod -- which really needs
to be improved to use some methods I've developed to make it easier to make
webapps that integrate with the desktop and are more secure, if there are
going to be a lot more programs like this using it.
importing data
After manually converting my GnuCash data, I imported all my PayPal history
into hledger. And happily it calculates the same balance Paypal does. It
also tells me I've paid PayPal $180 in transaction fees over the years,
which is something PayPal certianly doesn't tell you on their website.
(However, my current import from PayPal's CSV files is a hackish, and only
handles USD currency, so I miss some currency conversion fees.)
I also imported my Amazon Payments history, which includes all the
Kickstarter transactions. I almost got this to balance, but hledger and
Amazon disagree about how many hundreths of a cent remain in my account.
Still, pretty good, and I know how much I paid Amazon in fees for my
kickstarter, and how much was kicked back to Kickstarter as well.
(Look for a cost breakdown in some future blog post.)
At this point,
hledger stats says I have 3700 transactions on file,
which is not bad for what was just a few hours work yesterday.
One problem is hledger's gotten a little slow with this many transactions. It
takes 5 seconds to get a balance. The
ledger program, written in C, is
reportedly much faster. hledger recently had a
O(n^2) slowdown fixed,
which makes me think it's probably only starting to be optimised. With
Haskell code, you can get lucky and get near C (language, not speed of
light) performace without doing much, or less lucky and get not much better
than python performance until you dive into optimising. So there's hope.
harnessing haskell
If there's one place hledger misses out on being a state of the art modern
Unix program, it's in the rules files that are used to drive CSV
imports. I found these really hard to use; the manual notes that "rules
file parse errors are not the greatest"; and it's just really inflexible.
I think the state of the art would be to use a Domain Specific Language here.
For both my Amazon and PayPal imports I had CVS data something like:
date, description, amount, fees, gross
I want to take the feeds into account, and make a split transaction, like this:
date description
assets:accounts:PayPal $9.90
expenses:transaction fees:PayPal $0.10
income:misc:PayPal -$10.00
This does not seem possible with the rules file. I also wanted to combine
multiple CVS lines, to do with currency conversions, into a single
transaction, and couldn't.
The problem is that the rules file is an ad-hoc format, not a fully
programmable one. If instead, hledger's rules files were compiled into
standalone haskell programs that performed the import, arbitrarily
complicated conversions like the above could be done.
So, I'm thinking about something like this for a DSL.. I doubt I'll get
much further than this, since I have a hacked together multi-pass importer
that meets my basic needs. Still, this would be nice, and being able to
think about adding thing kind of thing to a program cleanly is one of the
reasons I reach for the Haskell version when possible these days.
First, here's part of one of my two paypal import rules files
(the other one extracts the transaction fees):
amount-field 7
date-field 0
description-field %(3) - %(4) - %(11)
base-account assets:accounts:PayPal
Bank Account
assets:accounts:checking
.*
expenses:misc:PayPal
That fills out the basic fields, and makes things with "Bank Account"
in their description be categorised as bank transfers.
Here's how it'd look as Haskell, carefully written to avoid the
$
operator that's more than a little confusing in this context. :)
main :: IO ()
main = convert paypalConveter
paypalConverter :: [CSVLine] -> [Maybe Transaction]
paypalConverter = map convert
where
convert = do
setAmount =<< field 7
setDate =<< field 0
setDescription =<< combine " - " [field 3, field 4, field 11]
defaultAccounts
"assets:accounts:PayPal" ==> "expenses:misc:PayPal"
inDescription "Bank Account" ?
"assets:accounts:PayPal" ==> "assets:accounts:checking"
That seems like something non-haskell people could get their heads around,
especially if they didn't need to write the boilerplate function
definitions and types at the top. But I may be biased. :)
Internally, this seems to be using a combination Reader and Writer monad
that can get at fields from a CSV line and build up a Transaction.
But I really just made up a simple DSL as I went along and thew in
enough syntax to make it seem practical to implement. :)
Of course, a Haskell programmer can skip the monads entirely, or use others
they prefer. And could do arbitrarily complicated stuff during imports,
including building split transactions, and combining together multiple
related CVS lines into a single transaction.
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