[swift-evolution] [Accepted and Focused Re-review] SE-0187: Introduce Sequence.filterMap(_:)

[Wallacy]

I vote for “mapNonNil”, “mapDroppingNil” or “unwrappingMap”(or some
variations using unwrap).

It’s not pretty, but does the job!



Em qua, 15 de nov de 2017 às 22:36, Greg Parker via swift-evolution <
swift-evolution at swift.org> escreveu:

> On Nov 15, 2017, at 2:31 PM, BJ Homer via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> On Nov 15, 2017, at 3:05 PM, Tino Heth via swift-evolution <
> swift-evolution at swift.org> 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.)
>
>
> "selectiveMap" feels better in part due to grammar. "map" is obviously the
> verb and "selective" is obviously a modification of "map". "selectiveMap"
> is therefore performing some sort of special map operation.
>
> "filterMap" feels bad for the same reason that "selectMap" would feel
> worse than "selectiveMap". "filter" and "map" are both verbs in this
> context. Grammatically the analogue to "selectiveMap" would be
> "filteredMap" or "filteringMap".
>
> But even then "filteredMap" or "filteringMap" is insufficient to describe
> the operation. You additionally need to know that the "filter" here is not
> ordinary "filter", but instead the special case "filter { $0 != nil }".
>
>
> 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.
>
>
> Swift doesn't use "filter" to mean "drop the nil values" either.
>
>
> "compactMap" is okay if "compact" is added. Is "compact" a common enough
> operation in practice to pull its own weight?
>
> "mapSome" is great if you know about Optional.Some but terrible if you
> don't. ("Okay, it maps some elements, but which ones?")
>
> "mapNonNil" is obvious and ugly and perhaps its obviousness makes it a
> winner.
>
>
> --
> Greg Parker     gparker at apple.com     Runtime Wrangler
>
>
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