According to the compiler, the closure argument for flatMap is not optional or
defaulted, so it’s always required. I think it should default to { $0 }, but
whatever.
Jon
> On Feb 13, 2017, at 8:26 PM, Saagar Jha via swift-users
> <swift-users@swift.org> wrote:
>
> flatMap was designed to work this way. How I rationalize it is that flatMap
> “extracts” the value from an array’s elements and expands it. For an Array,
> this is just taking out the individual Elements, but for an Optional, which a
> “wrapper” around one value, it just takes this value out. Optionals with no
> value (the .none case, or nil as it’s more commonly known) have nothing to
> contribute and thus are filtered out.
>
> Saagar Jha
>
> P.S. You can call flatMap without a closure: deep.flatMap().flatMap()
>
>> On Feb 13, 2017, at 4:31 PM, Maxim Veksler via swift-users
>> <swift-users@swift.org <mailto:swift-users@swift.org>> wrote:
>>
>> Hi everyone,
>>
>> I've discovered today that Swift will actually choose 2 very differently
>> behaving types of flatMap implementation based on the input signature.
>>
>> For a Sequence of options it will call a flatMap that filters out nil's. For
>> a Sequence of Sequence's it will call a flattening function, without
>> filtering.
>>
>> Leading to code that (IMHO) reads very not inconsistency, and unexpected.
>> Sometime even looking a bit funny such as collection.flatMap.flatMap:
>>
>> 5> let deep = [["1989", nil], [nil, "Red"], [nil, nil]]
>> deep: [[String?]] = 3 values {
>> [0] = 2 values {
>> [0] = "1989"
>> [1] = nil
>> }
>> [1] = 2 values {
>> [0] = nil
>> [1] = "Red"
>> }
>> [2] = 2 values {
>> [0] = nil
>> [1] = nil
>> }
>> }
>> 6> deep.flatMap { $0 }
>> $R1: [String?] = 6 values {
>> [0] = "1989"
>> [1] = nil
>> [2] = nil
>> [3] = "Red"
>> [4] = nil
>> [5] = nil
>> }
>> 7> deep.flatMap { $0 }.flatMap { $0 }
>> $R2: [String] = 2 values {
>> [0] = "1989"
>> [1] = "Red"
>> }
>> I wonder why it was implemented this way?
>>
>>
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