• Proposal: SE-0174
• Author: Ben Cohen
• Review Manager: Doug Gregor
• Status: Implemented (Swift 4.2)
• Decision Notes: Rationale
• Bug: SR-3444

This proposal implements the filter operation on RangeReplaceableCollection to return the same type as the filtered collection.

The recently accepted SE-165 introduced a version of filter on Dictionary that returned a Dictionary. This had both performance and usability benefits: in most cases, a Dictionary is what the user wanted from the filter, and creating one directly from the filter operation is much more efficient than first creating an array then creating a Dictionary from it.

However, this does result in some inconsistencies. Users may be surprised that this one specific collection returns Self, while other collections that would benefit from the same change still return [Element]. And some collections, such as String, might also benefit from usability and performance win similar to Dictionary. Additionally, these wins will be lost in generic code – if you pass a Dictionary into an algorithm that takes a Sequence, then when you filter it, you will still get an Array.

An implementation of filter on RangeReplaceableCollection will be provided, using init() and append(_:), so all range-replaceable collections will have a filter method returning of Self. Per SE-163, this will include String.

Note, many sequences (for example, strides or ranges), cannot represent a filtered self as Self and will continue to return an array. If this is a performance problem, lazy remains a good solution.

Add a default implementation of filter to RangeReplaceableCollection returning Self:

extension RangeReplaceableCollection {
    func filter(_ isIncluded: (Iterator.Element) throws -> Bool) rethrows -> Self {
        var result = Self()
        for element in self {
            if try isIncluded(element) {
                result.append(element)
            }
        }
        return result
    }
}

Specific concrete collections may choose to implement a faster version, but this is an implementation detail.

This change is subtly source breaking. In most cases users will not notice. They may be be relying on an array being returned (albeit often in order to then transform it back into the original type), but this version will still be available (via the extension on Sequence) and will be called if forced through type context. The only code that will break is if this operation spans multiple lines:

// filtered used to be [Character], now String
let filtered = "abcd".filter { $0 == "a" }
useArray(filtered) // won't compile

Because of this, the new implementation of RangeReplaceableCollection.filter will only be available in Swift 4.

This change will affect the ABI of RangeReplaceableCollection and needs to be made before declaring ABI stability.

N/A

Status-quo. There are benefits to the consistency of always returning [Element]. The version on Sequence can be reached via type context ("abc".filter(predicate) as [Element]).

It could be worthwhile to make a similar change to map, but this is beyond the capabilities of the current generics system because map does not preserve the element type (more specifically, you cannot express a type that is Self except with a different Element in order to provide the implementation on RangeReplaceableCollection).

This proposal originally included a new associated type Filtered on Sequence. However, this was unimplementable due to requiring a recursive type constraint (Filtered: Sequence). While these were supported in later Swift versions, the additional associated type was not implemented and that portion of the proposal has expired.