[swift-evolution] Reconsidering the (Element -> T?) variant of SequenceType.flatMap

Kevin Ballard kevin at sb.org
Fri Dec 4 17:34:57 CST 2015

I use that flatMap() variant on sequences very often. I don't think I've
actually _ever_ used the variant that returns [T]. I understand the
monadic argument you're making, but I believe the practical choice here
is to keep this flatMap() variant as-is. Giving it a different name
would I think reduce clarity, since it's conceptually performing the
same operation (e.g. the closure returns some value that is flattened;
whether the values an array or an optional it doesn't matter), and
adding .filterNils() would not only reduce clarity but would, as you
said, have a performance impact.

Personally, I'd be in favor of making Optional conform to SequenceType.
I've filed a radar on it before, and I seem to recall a message
(probably to this list) yesterday suggesting the exact same thing. It
may even want to go ahead and adopt CollectionType too, completely
replacing CollectionOfOne (though the ability to subscript an optional
may be confusing, so it may be better to just adopt SequenceType and
keep CollectionOfOne around for CollectionType use-cases).

-Kevin Ballard

On Fri, Dec 4, 2015, at 03:26 PM, Andy Matuschak wrote:
> Hello, all! This SequenceType-implemented flatMap recently caused some
> confusion on my team:
>> func flatMap<T>(@noescape transform: (Self.Generator.Element) throws
>> -> T?) rethrows -> [T]
> I’m a big fan of this operator in various functional libraries, but I
> admit I was a bit surprised to see the “flatMap” terminology appear in
> the Swift stdlib in the first place—its naming is certainly a notch
> obscure!
> From the reactions of teammates in code reviews involving these
> methods, there was a significant difference in comprehensibility
> between the Element -> [T] variant and the Element -> T? variant. The
> former was easily explained by “it’s a map, followed by a flatten,”
> whereas the same explanation failed in the latter case.
> I expect that the inspiration came from Scala, where the equivalent
> definition has a transformer essentially of type  Element ->
> GeneratorType<T>; separately, their optionals are implicitly
> convertible to (their equivalent of) GeneratorType. So, in the end, in
> Scala, you can effectively flatMap with an Element -> T? transformer.
> But Optional doesn’t implement GeneratorType, and I’d (weakly) argue
> that it shouldn’t. And if we think about flatMap in the context of a
> monadic bind (I do, anyway!), it’s especially surprising that the
> transformer is operating in a different monadic context (Optional)
> than the receiver (SequenceType). Unless we made Optional adopt
> SequenceType, in which case we could consider the bind to be happening
> in that context.
> In conclusion, I argue that this overload is confusing both to folks
> unfamiliar with FP (because it doesn’t feel like Optionals can be
> flattened) and to folks familiar with FP (because it implies binding
> across monadic contexts).
> ~
> In terms of what to do instead: I do think that this is a useful
> method, and I’d like to keep this functionality easily accessible!
> Two ideas:
> 1. We expose a separate operator like:
>> extension SequenceType where Generator.Element: OptionalType { func
>> filterNils() -> [Generator.Element.Wrapped] }
>> // To deal with limitations on protocol extension type restriction:
>> protocol OptionalType { typealias Wrapped func
>> flatMap<Result>(@noescape f: Wrapped -> Result?) -> Result? }
>> extension Optional: OptionalType {}
> Clients would do myArray.map(optionalReturningTransform).filterNils().
> There would be some performance impact from the intermediate array.
> 2. We give the foo variant a more specific name, e.g.
>    mappedArrayFilteringNils etc. Naming this is tricky (which probably
>    implies it should be decomposed?).
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