1) A hint about potential prior art: F# (and/or C#?) have a first-class unit system (physical units I think), which sounds a bit similar to monetary calculations. However, physical unit are easier to model than monetary unit I think.
2) Currently, I am building something tangentially related in Rust: A backtester (test trading strategies on historical and/or simulated data) with focus on accuracy. So one thing I included already is that Assets (like etfs) are valued in a currency.
If I may be so bold: How would you approach these questions / where could I read more about that? 1) When simulating, can I always assume that the exchange will work? (For assets, that is not always the case, sometimes one has to wait a bit until there is a buyer, or there might not be a buyer at all) 2) Is there public domain data in exchange rates? 3) If I have to choose, which exchange rate should I pick? The lower one, higher? What would make the most sense in the context of trading etfs, stocks, options, crypto etc.? 4) How to approach rounding, is there a best practise? 5) I assume it is best to immediately substract taxes in every transaction, even if they are formally defined annually, right? 6) Would you model inflation? I currently plan to ignore it and present it at the very end: "Your portfolio has a final value of X.X ¥. Adjusted for inflation, that would be Y.Y ¥ today (2024-10-08)."