[swift-evolution] TreeLiteralConvertible

[Milos Rankovic]

Hi John,

> On 14 Apr 2016, at 21:09, John McCall <rjmccall at apple.com> wrote:
> 
> I mean, you could just make your Tree type implement all the individual literal-convertible protocols.

It does sound like something like that should be doable, but it isn’t. The literal-convertible protocols only require one initialiser, but you need two: one that lifts leaf values to Self and the other that actually takes Self elements (it is this last one that provides for recursion). In other words, we’d need to overload our conformance:

extension Tree : ArrayLiteralConvertible { // error: does not conform!
	init(arrayLiteral elements: Tree...) {
		self = .Branches(elements)
	}
	init(arrayLiteral elements: Value...) {
		if elements.count == 1 { self = .Leaf(elements[0]) }
		else { self = .Branches(elements.map{ .Leaf($0) }) }
	}
}

But you can only conform in one or the other way, but not both! Therefore, for trees, we need something like:

protocol TreeLiteralConvertible {
	associatedtype LeafValue
	init(literal: Self.LeafValue...)
	init(literal: Self...)
}

milos
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