I recognize that this thread hasn't been touched in over a year, but I was wondering if this ever actually ended up going anywhere?
It received very positive feedback and I recently needed this functionality and guess I sort of expected it to be included in swift 4.2, because I was surprised that I couldn't map over a KeyPath.
Would the next step be to make this into an official proposal?
I think what’s keeping this from getting implemented is that key paths should really behave more like functions (ie. KeyPath<T, U> should be a subtype of T -> U or something like that), and then we would get mapping for free. So the last time this came up, the conclusion was that adding a separate overload of map and flatMap for keypaths would be pure sugar and we should come up with a more general solution.
Ah, found it, quoting @Joe_Groff here:
Instead of introducing a subtype relationship, we could (and should) extend the key path literal syntax to be valid in function type context, which I think covers the most common use case for this conversion of passing a literal property reference as an argument to map and friends.
It seems the thread you've linked to is the real pitch we should be discussing since its purpose is for more than just map/flatMap