Compare https://bugs.swift.org/browse/SR-1570 : Swift can infer the return
type from single-statement closures, but not from multi-statement closures.
Therefore in
let result = strArr.flatMap { x in
return x
}
the closure type is inferred as (String)-> String, and therefore the flatMap
call matches exactly the
func flatMap<SegmentOfResult>(_ transform: (Self.Element) throws ->
SegmentOfResult) rethrows -> [SegmentOfResult.Element]
where SegmentOfResult : Sequence
Sequence method, with Self.Element == String and SegmentOfResult == String, and
flatMap returns [SegmentOfResult.Element] == [Character].
In
let result = strArr.flatMap { x in
print(x)
return x
}
the closure return type is not inferred from the closure itself, because it is
a multi-statement closure. Here the compiler (apparently) chooses the
public func flatMap(_ transform: (Element) throws -> String?) rethrows ->
[String]
Array method, and promotes `String` to `String?`. You would get the same result
with an explicit return type in the single-statement closure:
let result = strArr.flatMap { x -> String? in
return x
}
But why do you use flatMap at all? If the intention is to map [String] to
[String] then map() would be the right method:
let strArr = ["Hi", "hello"]
let result: [String] = strArr.map { x in
return x
}
print(result) // ["Hi", "hello"]
Regards, Martin
> On 21. Oct 2017, at 02:50, Santiago Gil via swift-users
> <swift-users@swift.org> wrote:
>
> Hi Swift users,
>
> I just ran into a bug after a Swift 4 migration where we expected [String]
> and got [Character] from a flatMap since flatMap will flatten the String
> since it's now a collection. While I understand why flatMap behaves this way
> now that string are collections, in testing this I found some weird
> behaviour...
>
> var strArr = ["Hi", "hello"]
>
> let result = strArr.flatMap { x in
>
> return x
>
> }
>
>
> The type of results ends up being [Character] in the above case. However,
> adding a print statement changes things.
>
>
> var strArr = ["Hi", "hello"]
>
> let result = strArr.flatMap { x in
>
> print(x)
>
> return x
>
> }
>
> In this case, result is of type [String]
>
> This seems like a bug, or is this expected Swift behaviour?
>
> Thanks,
>
> Santiago
>
> _______________________________________________
> swift-users mailing list
> swift-users@swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
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