In finance, US dollars are generally stored to four decimal places, because you need to deal with stuff like compounding interest or stock splits.

COBOL has a built in fixed point integer type, which makes defining a 4 digit decimal and doing math on it easy. (IBM designed it from the ground up to cater to people with a lot of money, who spend a lot of money, to work with lots of money, ie banks) Java has the BigDecimal type, which is a class in the class library, which means you need to import it. And because Java lacks operator overloading, doing calculations is tedious.

In the 90s, there was a huge push to replace COBOL with <something else>, and Java was the Rust of its day, so that's what everyone got behind. However, 4 digit COBOL decimals apparently round differently than 4 digit Java BigDecimals, so all the tests failed. And all the stuff like a\x+b had to be written like BigDecimal.add(BigDecimal.multiply(a,x),b) so development was taking forever.

Eventually they said "fuck it" and 20 years later we're still stuck with COBOL and everyone who remembers the original death march says "never again".

I have a feeling a lot of the problems came down to computer science people thinking money has two decimal digits but domain knowledge people knowing it has four. We programmers, as a group, make a lot of assumptions about other peoples' domains and we're wrong a lot*.

US currency can go to more than two decimal places...

http://blogs.reuters.com/ben-walsh/2013/11/18/do-stocks-real...

I guess it's time for someone to write an "Assumptions Programmers make about money" post.

>>...since for example US currency only ever goes to two decimal places

That is not correct. Stock settlement transactions often list four decimal places.

You can still get reasonable enough approximations with more than two decimals if you do something like `int64 myWorkingMoneyVal = currentMoney * 100000`, do your work, then divide the final result by 100000. You still risk some potential truncation if your work involves division, but the larger your multiplier that you're working with, the larger divisor at the end, which will help minimize how much of an error this ends up being. The 64 bit integer space is pretty darn big, so you typically don't risk an overflow, and you will typically get better performance than using a regular "decimal" type, since on-chip integer operations are usually very fast.

EDIT: Just a note, there's nothing special about the number 100000; pick the largest exponent of 10 that you can get away with a reasonable assurance that no overflow is possible. For a vast majority of money applications, I seriously doubt you're going to be hitting the limits of int64, so you could probably even get away with something like 1000000000.

I have developed a payment plan calculator for asset based finance and you would be amazed how many different rounding schemes and day counters (for fractional periods) exist and are actively used.

Counterexample: gas prices in the US are frequently displayed with 3 decimals (tenth of cents)

The basic representation is usually good enough at 2 decimals (so a plain int), but it is often needed to have a transient representation during calculations.

For instance if one needs to apply discounts, add taxes, split in equal parts, all of the above one after the other, there will be a more precise intermediary representation before rounding everything in a way that keeps the total amount consistent with the original amount.