I'm not sure how a type system can protect against accounting errors, and I'm even less sure how the type system would know what accounting rules to apply without human input.
Does catching a $0.0001 accounting error fall into the "surprising amount" or "not all" bucket? Because that's the example at hand.
I'm not surprised at all by what you can do with a type system. But type system fanboys (yes, I have to use the word) always have the same arguments. "You can do lots of stuff with types. Oh, except maybe for your actual example." It's not convincing, it's just noise that shows up whenever somebody is using the "wrong" language.
If you could flesh out your example, I could try to answer whether the $0.0001 accounting error might be catchable. Thanks!
A type system like Haskell's can guide your programming in general and leave more of your mental resources to solve the problem at hand. As an anecdote: I have found, that with some QuickCheck tests/rules and some hard thinking about the right types, I can extend my programs even when I can't concentrate 100%. (A situation that usually just gives me Segmentation Faults in C.)
if (customer_should_get_charged_00001()) {
account += 0.00001;
}
It wasn't my example to start, but I'll roll with it. The bug, of course, is that we're adding and not subtracting.
The more obvious example where somebody has 0.01 dollars and it gets put in a cents variable, turning into 0.0001 dollars, is also easily solvable with the C++ type system.
For a serious accounting package, I would wrap up all money calculations, in such a way that you can't get away from double ledger accounting. So no money would ever be lost or created and all the flows are traceable.
Again, I don't see how that's anywhere near difficult in C++. This discussion doesn't seem to be going anywhere, especially since there was never a real bug to start with. I'm just annoyed that every time someone even mentions a (fictional!) bug, the problem is "using C++".
Oh, sorry. I just replied to your comment doubting the use of type systems. I did not want to make a statement about C++. I guess you could make C++ dance, if you tried and were smart enough.
