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Gonna shoutout F#'s unit-of-measure types

> It's "small" but rather important not to mix units!

With unit of measure, it's possible to easily/simply create types like CustomerId, which is actually a Guid/Int under the covers. Means you can't pass a CustomerId to something that expects an OrderId. These types are erased at compile time, so there's zero effect on runtime performance. It's lovely.

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See also: Haskell newtypes and Kotlin value classes (née inline classes).

Does Kotlin have the equivalent of GeneralizedNewTypeDeriving or whatever is required to make math operators work without boilerplate?

One unexpected aspect of unit-of-measurement types is that they don't generalize well at all to anything more complex than basic arithmetic. For example, it's very hard to create a type-safe library that allows you to compute the dot product of (1m 1Hz) and (1Hz 1m) and get 2m/s, but doesn't allow you to accidentally compute the dot product of (1m 1Hz) with itself.

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