> On Apr 14, 2016, at 1:34 PM, Milos Rankovic <[email protected]>
> wrote:
> Hi John,
>
>> On 14 Apr 2016, at 21:09, John McCall <[email protected]
>> <mailto:[email protected]>> 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...)
> }
No, you just need Tree to conform to both ArrayLiteralConvertible and
IntegerLiteralConvertible, and it implements the latter by building a Value out
of it.
This would be easily done with conditional conformance; as it is, you'll simply
have to make your Tree less generic, e.g. by always requiring Value to be
IntegerLiteralConvertible. Of course, this would not be a problem for a JSON
tree, which would not be generic at all.
John.
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