[swift-evolution] [Review] SE-0187: Introduce Sequence.filterMap(_:)

[Tino Heth]

> I prefer to think of it this way: it does a mapping, and keeps the results where the closure did not return nil. In the example given earlier, the closure returned Optional<Int?>.some(nil), which is not “nil”.
Exactly; therefore, flatMap can even „create“ nil values…
Filters in the physical world aren’t perfect either, and the spam problem has the same issues in the digital domain — but compared with that, eliminating nil is quite simple.
So people (not all, but imho many) will think that filterMap works like an idealized filter, where you pour dirty water in, and get a crystal clear and consumable result.

How would you rate a filter that adds dirt to its product, or a high-pass that adds a significant component below the cutoff frequency to a signal?

Filters aren’t supposed to do that, even if you feed them something strange like Optional<Optional<T>>… and when it adds something that wasn’t in the input, it might be a catalysator, but not a filter (the filter-method in Swift follows that rule as well: It can only map [T?] to [T?], not [T]).

So I have no doubt that flatMap is a better description of what is happening („you map, unwrap the results, and put them all in the same container“ — that’s true for collections as well as for optionals), but if many people have problems with it, something should be changed...
It wouldn’t be the first time where a problematic syntax is added because it pleases the user (whenever I write something like „array0 + array1“, the mathematician in me cries), but in this case, there is no need to choose something that veils what it is doing:
There are alternatives that don’t have this issue („collect“, „collectResults“...), but I actually would prefer just adding a warning to the problematic form of flatMap — simply because without that, nothing will stop people from misusing filterMap in the same way as they do with flatMap now.
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