## Calculating the internal rate of return with hledger

Published 2012-12-07 in sections English, Haskell.

For more than ten years now, I am a regular and happy user of GnuCash, the free accountancy program. Especially the integrated online banking support for HBCI that allows me not only to fetch my transactions, but also to transfer money using a cryptographic makes it a great tool. Nevertheless, it has shortcomings, especially when it comes to reports. And for some reason I never felt the urge to hack on that C/Guile code (although I recently hacked a bit on the UI).

Joey Hess’ blog post about Simon Michael’s hledger made me look at that command line based accountancy tool. It certainly won’t replace Gnucash for me, but luckily ledger, of which hledger is a re-implementation, can read GnuCash’s file format and convert it to a ledger file that hledger can read as well. So now I can do analysis on my financial data in Haskell – what would you want more?

One report that I was missing was the internal rate of investment of some account. This is the annual rate of a hypothetical fixed interest-bearing savings account that would, given the same deposits and payouts, result in the same final value. I now implemented this as the hledger-irr command and – as you would expect – you can find it on hackage. See the package description for documentation and example output.

It is very fresh, hardly tested code, so don’t base important decisions on it yet. Also, it needs work to work correctly with multiple currencies or things with varying prices, and I am not yet sure what to do there.

Update: Simon asked me to elaborate the difference to hledger-interest by Peter Simons (on which my code was initial based, but eventually I rewrote everything but the option parsing code). The difference is that hledger-interest answers the question „I have this investment with this yearly interest rate (which may even change), how large are the interest payments going to be.“ I imagine a usecase is that you have borrowed some money to a friend at a certain (surely amicable) rate and need to figure out what he owes you, even when you gave him the money mid year and he payed you some back shortly after easter. hledger-irr serves a different purpose: Here you already know your investment payments, gains and losses and you want to have a number describing “how good” your investment was. One usecase might be that, at the end of a live of paying into a pension fund, you ask yourself: “Given all the feeds and commissions, was this a good idea or should I just have invested the money myself?” Then the internal rate of investment tells you how lucrative an alternative fixed-rate investment would have had to been to match your pension fund investment. (Or course this is not the only aspect that you should look at when comparing investments, as it totally ignores risk and possible insurance elements of the investment.)