> On Nov 15, 2017, at 3:05 PM, Tino Heth via swift-evolution
> <[email protected]> wrote:
>
> Odd… exactly that is the reason why I think filterMap is the worst choice:
>
> Both are established terms of art, but one has a meaning that doesn’t fit to
> the operation.
> Applying filter can remove elements, but it can never change types (I feel
> kind of silly to repeat this over and over, but so far, nobody took the time
> to falsify this).
The concern about filter changing types is only relevant if you think of the
filter applying to the result of the map, instead of being a part of the
filterMap operation itself (an operation that is distinct from map).
Let’s imagine that we had this instead:
enum SelectiveMapResult<T> {
case use(T)
case ignore
}
extension Sequence {
func selectiveMap<T>(_ selectiveTransform:
(Element)->SelectiveMapResult<T>) -> [T]
}
let actualNumbers =
["1", "2", "apple", "banana", "5"].selectiveMap({
(x)->SelectiveMapResult<Int> in
if let value = Int(x) { return .use(value) }
else { return .ignore }
})
actualNumbers == [1, 2, 5]
The “selective” part of this operation doesn’t feel like it’s changing the type
of the result, because SelectiveMapResult is easily understood to not be part
of the mapping transformation; it just exists to tell us whether we should use
the result of that particular transformation. Likewise, I don’t feel like the
optional in filterMap is part of the mapping operation; it’s just serving the
same role as SelectiveMapResult. (It should be obvious that SelectiveMapResult
is just Optional with another name here.)
The name filterMap focuses on removing the ignored values, as does compactMap.
The name selectiveMap focuses on retaining the non-ignored values. I’m not sure
whether focusing on the positive or negative aspects is clearer here. I don’t
particularly like the name compactMap, simply because I don’t have a lot of
experience with languages that use “compact” to mean “drop the nil values”, and
without that experience it doesn’t seem intuitive. I think filterMap is better.
But if we introduced Sequence.compact() alongside .compactMap(), I’d probably
get used to it.
-BJ
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