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I'm sure there is something I don't know here, but how is working with integers "brittle"? The only issue I see is rounding down by default, not sure if that is even an issue or not. At any rate, it seems a lot less brittle than floats or bigdecimal style number classes.

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The brittleness comes from the fact that the number of implied decimal digits per currency isn't always well-defined across all stakeholders and systems.

If you're only working in a single currency, there's usually no issue.

As a general rule, you always include the currency code (EUR, SEK, USD etc.) and if possible also the amount of decimals, when using minor units.

Currency codes can be found in ISO 4217.

Yes, definitely always include the number of digits, but at your system boundary you still have to pray that whoever you're working with isn't silently dropping that number and re-deriving it from their own, almost-4217-compliant currency database.

Redundancy can be great, but it's not a panacea, since it's not guaranteed to be used in an optimal way.

How you expose you present your data to third parties is not how you represent it internally.

Ideally it’s not, but I’ve seen it leak so often that I’d rather be safe than sorry.

ISO 4217 also defines the number of significant digits after the decimal separator if Wikipedia is to be believed.

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