Practical correctness.
WHAT TO NEVER USE FOR MONEY
Floating point numbers.
WHY
They cannot represent common decimal values exactly, and errors accumulate.
WHAT TO USE INSTEAD
Integers in the smallest unit, or a decimal type.
WHAT THE SMALLEST UNIT IS
Kobo for naira, cents for dollars, and currencies with no subunit at all.
WHY THAT VARIATION MATTERS
Assuming two decimal places breaks for currencies that do not have them.
WHAT TO ALWAYS STORE WITH AN AMOUNT
Its currency.
WHY
An amount without a currency is meaningless and dangerous.
WHAT TO NEVER DO
Add amounts in different currencies Compare them without conversion
WHAT ROUNDING REQUIRES
A defined rule, applied consistently.
WHERE IT MATTERS MOST
Splitting amounts, where remainders must go somewhere.
WHAT TO DO WITH REMAINDERS
Allocate them deliberately, and record the rule.
WHAT TO BE CAREFUL WITH
Percentages applied to amounts Tax calculations, where the rule is prescribed Fees calculated on already-rounded values
WHAT TO TEST
Amounts producing awkward divisions Very large and very small amounts Every currency you handle
