I ran into trouble with custom collections the other day and wondered how to reliably find out what the bare minimum for a custom CollectionType is.

The compiler will only help a bit:

struct Foo: CollectionType {}

It’s conforming to neither CollectionType nor SequenceType. Now I know SequenceType requires the generate() method – but don’t let this fool you. CollectionType will provide that for you if you meet its requirements.

You’ll need:

• startIndex,
• endIndex, and
• subscript.

The indexes have to be of the same kind; the parameter for subscript, too. Here’s a very simple custom collection that raises an exception when you reach a point past the end, just like Array does:

struct Foo: CollectionType {

    var startIndex: Int { return 0 }
    var endIndex: Int { return 10 }

    subscript (index: Int) -> String {

        precondition(index < endIndex)

        return ".\(index)."
    }
}

print(Foo().map { "x\($0)x" })
// ["x.0.x", "x.1.x", "x.2.x", "x.3.x", "x.4.x", "x.5.x", 
//  "x.6.x", "x.7.x", "x.8.x", "x.9.x"]

You can leave the precondition out for stuff like mapping and return an optional String? instead. Works well, too, but changes you collection’s type, of course:

struct Bar: CollectionType {

    var startIndex: Int { return 0 }
    var endIndex: Int { return 10 }

    subscript (index: Int) -> String? {

        guard index < endIndex else { return nil }

        return ".\(index)."
    }
}

print(Bar().flatMap { $0 }.map { "_\($0)_" })
// ["_.0._", "_.1._", "_.2._", "_.3._", "_.4._", "_.5._", 
//  "_.6._", "_.7._", "_.8._", "_.9._"]