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I'm working on an iOS app with Realm as the database and I'm confronted to a problem I can't fix without your help.

I have a function that queries objects from my Realm instance like so:

static func trainings(containing exercise: Exercise) -> Results<Training>? {
    return Realm.workoutDatabase?.objects(TrainingEntry.self)
                                 .filter({ $0.exercise == exercise })
                                 .compactMap({ $0.training })
}

This code produces the following error:

Cannot convert return expression of type 'LazyMapCollection<LazyFilterCollection<LazyMapCollection<LazyFilterCollection<Results<TrainingEntry>>, Training?>>, Training>?' to return type 'Results?'`

There error is obviously that the return type should not be Results<Training>? but LazyMapCollection<LazyFilterCollection<LazyMapCollection<LazyFilterCollection<Results<TrainingEntry>>, Training?>>, Training>?.

Outch! That type is soooooo long.

I tried to set [Training]? as the return type for the function and it worked.
But by doing so, I'm afraid that this will implicitly cast the returned result of the expression into an array, thus loosing the laziness of the collection?

As far I as know, I need a type-eraser to get a much shorter type, but I'm definitely not an expert on that particular subject.

I know that the Swift Standard Library provides a few type-erasure structs, such as AnyGenerator, AnySequence but I'm afraid I don't know enough about type-erasure to use them in my case.

So my question is, how to use type-erasure to get a cleaner return type for my function?

Edit:

I tried to cast the expression to AnyRandomAccessCollection but I get the following error:

Type 'LazyFilterCollection<LazyMapCollection<LazyFilterCollection<Results<TrainingEntry>>, Training?>>' does not conform to protocol 'RandomAccessCollection'

I also tried to cast to AnyCollection and AnyBidirectionalCollection, both of them work, but then I'm loosing the ability to subscript with an Int, which is something I want to keep.

Vin Gazoil
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  • could it be related to this: https://stackoverflow.com/questions/42912998/realm-predicate-returning-lazyfiltercollection-how-to-convert-to-resultst ? – Fyodor Volchyok Jul 10 '18 at 22:51
  • @FyodorVolchyok Yes and no, I just tried to cast the expression to AnyBidirectionalCollection. That does works but it seems that I can't subscript with a simple Int now, I get the following error : `Cannot subscript a value of type 'AnyBidirectionalCollection' with an index of type 'Int'` – Vin Gazoil Jul 10 '18 at 23:12

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