@pure function attribute

[xwu (Xiaodi Wu)]

You and I have had this exact conversation before:

Add `filter` and `ifSome(then:)` to Optional

It's true that map is borrowed from functional programming, but Swift has no concept of 'pure' functions and has never made such a requirement for map. The Swift Programming Language itself has (or at least, one of its editions once had) an example of map(print), so I think it's fair to say that such usage is 'idiomatic' Swift.