Of course, anyone can make a Map function for any particular type combination, but Tim Bray has a point. The generic, functional map function cannot be written, and never will.

There is an issue of elegance. If generics were to be added, lists and maps would need to have similar syntax, because they feel and behave like generics. However, they cannot. Their syntax is… not generic enough.

Oddly enough, elegance through consistency was never reached when designing built-in functions like make(): those can be applied on any type, but user-defined functions don't have this freedom. Essentially, there is a Java-like divide between built-in types (which, for instance, you cannot create methods on) and user types. Their rules of elegance don't apply to the built-ins, which is quite odd.

I doubt that this impedes on programming efficiency compared to C. However, some of the duplicated code out there cannot be factored into a library, and map is an example of that.