[swift-evolution] [Review] SE-0174: Change `filter` to return an associated type

[Nevin Brackett-Rozinsky]

Another possibility is to make “map” generic on the return type, something
like:

extension Collection {
    func map<T: RangeReplaceableCollection> (transform: (Iterator.Element)
throws -> T.Iterator.Element) rethrows -> T {
        var result = T()
        for e in self { try result.append(transform(e)) }
        return result
    }
}

That way the user can choose what type they want. And since there is also a
more-specific implementation returning an Array, that is what you’ll get if
context does not constrain the type, so existing code will still work the
same.

We could do the same for “filter”, in which case the current proposal would
just change what the default type is. So…what I’m talking about here would
be purely additive and can happen later.

In any case, I do like the idea being proposed in SE–0174. If I have a
collection and I filter it down, it makes sense to still be the same kind
of collection. So, +1 from me.

Nevin
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