Check if a type implements a protocol - swift

Question

I am writing a library that creates extensions for default Swift types.
I would like to have a check on my Array extensions whether a certain type implements a certain protocol. See this method for example:
extension Array {
/// Compares the items using the given comparer and only returns non-equal values
/// :returns: the first items that are unique according to the comparer
func distinct(comparer: (T, T) -> Bool) -> [T] {
var result: [T] = []
outerLoop: for item in self {
for resultItem in result {
if comparer(item, resultItem) {
continue outerLoop
}
}
result.append(item)
}
return result
}
}
Now I'd like to rewrite this method to check if T is Equatable as such:
/// Compares the items using the given comparer and only returns non-equal values
/// :returns: the first items that are unique according to the comparer
func distinct(comparer: ((T, T) -> Bool)?) -> [T] {
var result: [T] = []
outerLoop: for item in self {
for resultItem in result {
if isEquatable ? comparer!(item, resultItem) : item == resultItem {
continue outerLoop
}
}
result.append(item)
}
return result
}
where isEquatable is a Bool value that tells me if T is Equatable. How can I find this out?

Answer 1

There isn’t a good way to do this in Swift at the moment.* This is why functions like sorted are either free-functions, or in the case of the member, take a predicate. The main problem with the test-and-cast approach you’re looking for is that Equatable and similar protocols have an associated type or rely on Self, and so can only be used inside a generic function as a constraint.
I’m guessing your goal is that the caller can skip supplying the comparator function, and so it will fall back to Equatable if available? And crash if it isn’t? The problem here is that the function is determining something at run time (the argument is Equatable) when this really ought to be determinable at compile time. This is not great - it’s much better to determine these things fully at compile time.
So you can write a free function that requires Equatable:
func distinct<C: CollectionType where C.Generator.Element: Equatable>
(source: C) -> [C.Generator.Element] {
var seen: [C.Generator.Element] = []
return filter(source) {
if contains(seen, $0) {
return false
}
else {
seen.append($0)
return true
}
}
}
let uniques = distinct([1,2,3,1,1,2]) // [1,2,3]
and then if you tried to call it with something that wasn’t comparable, you’d get a compile-time error:
let incomparable = [1,2,3] as [Any]
distinct(incomparable) // compiler barfs - Any isn’t Equatable
With the runtime approach, you’d only find this out when you ran the program.
The good news is, there are upsides too. The problem with searching an array for each element is the function will be very slow for large arrays, because for every element, the list of already-seen elements must be searched linearly. If you overload distinct with another version that requires the elements be Hashable (which Equatable things often are), you can use a set to track them:
func distinct<C: CollectionType where C.Generator.Element: Hashable>
(source: C) -> [C.Generator.Element] {
var seen: Set<C.Generator.Element> = []
return filter(source) {
if seen.contains($0) {
return false
}
else {
seen.insert($0)
return true
}
}
}
At compile time, the compiler will choose the best possible version of the function and use that. If your thing is hashable, that version gets picked, if it’s only equatable, it’ll use the slower one (this is because Hashable inherits from Equatable, and the compiler picks the more specialized function). Doing this at compile time instead of run time means you pay no penalty for the check, it’s all determined up front.
*there are ugly ways, but since the goal is appealing syntax, what’s the point… Perhaps the next version will allow constraints on methods, which would be nice.