I've found there are situations where primitives are fine and wrapping doesn't add much value, e.g., (using F#) let calcCorrelation (symbol1:string) (symbol2

I've found there are situations where primitives are fine and wrapping doesn't add much value, e.g., (using F#) let calcCorrelation (symbol1:string) (symbol2:string) = ... Here, calling the function with misordered arguments won't change the result, so the wrapping\unwrapping friction doesn't have much ROI. but here, where 'beta' is the slope of a least-squares fitted line: let calcBeta (stockSym:string) (indexSym:string) = ... that's a mistake waiting to happen, because if you accidentally flip the arguments when you call the function you get a different number. Better to have: type StockSymbol = StockSymbol of string type IndexSymbol = IndexSymbol of string let calcBeta (IndexSymbol index) (StockSymbol stock) = ... then call the function like this: let beta = calcBeta (IndexSymbol "SPX") (StockSymbol "AMZN") Of course you could still make a mistake: let beta = calcBeta (IndexSymbol "AMZN") (StockSymbol "SPX") but that would stick out like a sore thumb.