[wux]

The term "currency type" is used here to mean a type that is commonly used.

For example, although there are a variety of range types in the Swift standard library ('Range', 'ClosedRange', 'PartialRangeFrom', 'PartialRangeUpTo', etc.), 'Range' is considered the currency type. Similarly, among string types, 'String' is considered the currency type, as opposed to 'Substring', 'StaticString', etc.

Like currency (money), the idea is that APIs in different libraries across different domains of programming will generally take values of the currency type as input and produce values of the currency type as output unless there's a good reason to use a different type.

For Swift System, the stated goal is to provide low-level currency types; if that goal is accomplished, other users of Swift can rely on these types instead of supporting multiple disparate third-party wrappers of system calls that may provide similar functionality just so that they can interoperate with other libraries.

[lilyball]

I’ve never before heard of the phrase “currency types”. I even searched the web for “system currency types” and only got money-related results.

[wux]

It may just be jargon specific to the Swift community:

https://www.google.com/search?q=swift+evolution+%22currency+...