On Friday 28 August 2015 08:55:53 John Ralls wrote: > I’ve pushed a feature branch, single-price, to my Github repo > (https://github.com/jralls/gnucash) which covers most of what we’ve > discussed here. I’m still wrestling with the math of how to sensibly > handle rounding itself, so what’s there still uses the hard-coded > 10^6 denom. The branch is from maint because I’d like to put these > changes in 2.6.8. > > The actual changes are explained in the commit notes. In my limited > testing it appears to work and to provide stability when doing > multiple transactions with the same exchange rate. Please test some > more; I’m sure I didn’t think of every possible variation. > Hi John,
Thanks for your work on this. From my first tests and reading the code I have the following observations: - Let me start with a nit-pick: while reading through the commits I got confused by the return values of check_account and check_edit. They return TRUE if the check fails (that is when anything is not ok to continue with a transfer). From a distance that seems backwards. I usually expect a check function to return TRUE if all checks pass correctly. - Next I created a vendor bill, posted it and then paid it in a foreign currency. This also pops up the transfer dialog. I entered a price and continued. This added the price to the db (as expected). Next I remove the payment transaction (from the bank account in the foreign currency) and issue a new payment via the payment dialog, again in the same foreign currency. This time however, I enter a to amount directly (ensuring it would result in a completely different price). Check the price db and note the existing price hasn't changed. - The currencies I was playing with are € (from currency) and HKD (to currency). Before your changes my price db listed a HKD security EUR currency. On your branch the code now adds a EUR security in HKD currency. That change is fine in itself. I prefer your normalized way to store (currency) exchange rates in the db. The issue with this however is that F::Q won't return an exchange rate for the new price, while it did (and still does) for the old one. > As for the math, here’s the conundrum: I proposed earlier to base the > rounding on what would make a 1 scu change in the “to” commodity. The > problems with that idea are that it depends entirely on the amount in > the “from” commodity and that prices are often quoted in fractions of > a scu. For example, the Wall Street Journal website quotes the Yen at > 120.98 to the USD. The Yen’s scu is 1, and the change in the rate to > make a 1¥ change in the value is different if the USD amount is $10 > from what it would be if the amount was $1000. Carry that to its > illogical conclusion and we need infinite precision, and that’s > ignoring the fact that we need infinite precision to exactly > represent a lot of rational fractions, but since all the real money > systems use decimal math nowadays that’s not really germane. > > So I have a new proposal: If the commodities are both currencies, > store exchange rates in the direction where the rate > 1, set the > denominator to 1000, and round-half-up. The price retrieval code > already checks in both directions. If only one of the commodities is > a currency then it’s a price and we store it in the currency with the > denominator = the currency’s scu * 10000. > See above: the check in both directions is not reliable/borked. > That leaves commodity-commodity prices. The most common example in > modern life is stock-for-stock exchanges resulting from mergers or > spin-offs. These tend to be one-offs, so no rounding required. Barter > exchange, where one exchanges one commodity for another (e.g. two > bushels of corn for a cow), is similarly fractional rather than > decimal, so again not rounding is appropriate. The third case is the > problem: Bitcoin and similar pseudo-currencies. For maint I think > we’re going to have to leave those prices unrounded as well, but > perhaps for master we should consider creating a separate commodity > category so that users can create commodities that GnuCash treats as > currencies. > That looks like a very sensible proposal to me. Regards, Geert _______________________________________________ gnucash-devel mailing list [email protected] https://lists.gnucash.org/mailman/listinfo/gnucash-devel
