Jon Pretty

Sometimes the simplest problems are harder than they first seem. A client once specified a requirement to me: we'll give you the VAT-inclusive (20%) price; you need to generate invoices showing the net and VAT amounts. Sounds easy, right? net = round(tot*5/6) vat = round(tot/6)

Some time passed. The client came back to me: "You've been calculating the invoices wrong! Our accountant has had to work an extra few days because about one in five of your invoices has an error of £0.01, and our accounts don't balance!"

Did you spot my error? So, what's the moral of this story? I'm not sure, because it's quite subtle, but I could claim that it's a case of "make impossible states unrepresentable". The challenge was in recognising that we were representing impossible states. Suggestions welcome.

I do not get it... Have you used `double` for monetary calculation? But that would not be subtle...

What's your suggestion to deal with moneteray calculation in Scala? Do you have a library you'd recommend?