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

[John McCall]

> On Nov 15, 2017, at 9:16 PM, Greg Parker <gparker at apple.com> wrote:
> 
> 
>> On Nov 15, 2017, at 5:53 PM, John McCall <rjmccall at apple.com <mailto:rjmccall at apple.com>> wrote:
>> 
>>> On Nov 15, 2017, at 7:36 PM, Greg Parker via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> On Nov 15, 2017, at 2:31 PM, BJ Homer via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>>> On Nov 15, 2017, at 3:05 PM, Tino Heth via swift-evolution <swift-evolution at swift.org <mailto: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.
>> 
>> mapSome and especially mapNonNil both sound to me like they only map the non-nil values in the source sequence.
> 
> I thought it did map the non-nil values in the source sequence. Did I misunderstand what the proposed filterMap does? It occurs to me that the proposal does not have a standalone description of the filterMap operation.

func filterMap<T>(_ f: (Element) -> T?) -> [T] {
  return self.map(f).filter { $0 != nil }.map( $0! } // not an efficient implementation
}

A mapping function is applied to all elements of the source sequence.  Unlike map, however, the mapping function can return nil, causing that value to be dropped from the result sequence.  Since the result sequence can therefore not contain any (directly) nil values, a level of optionality is removed from the result element type.  A common use case — perhaps the most common — is to simultaneously map and filter a sequence without introducing any unnecessary intermediate sequences, which is why it is often called filterMap in functional languages.

John.
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