How can I extend Array so equality check of arrays of optionals are possible?

Question

I want to unit-test some code that creates values with types of the form [T?] (arrays of optionals). But == is not defined for arrays of optionals:

Binary Operator '==' cannot be applied to two '[String?]' operands

So I extend Array accordingly (following this answer):

extension Array {
    static func ==<T: Equatable>(lhs: [T?], rhs: [T?]) -> Bool {
        if lhs.count != rhs.count {
            return false
        }
        else {
            return zip(lhs,rhs).reduce(true) { $0 && ($1.0 == $1.1) }
        }
    }
}

But now I get:

Ambiguous reference to member '=='

In combination, the messages don't seem to make sense; how can I go from zero fitting methods/functions to more than one?

How can I extend Array for checking equality of arrays of optionals?

Answer 1

If you want to keep it as an extension to Array:

extension Array where Element : Equatable {
    static func ==(lhs: [Element?], rhs: [Element?]) -> Bool {
        if lhs.count != rhs.count {
            return false
        }
        else {
            for i in 0..<lhs.count {
                if lhs[i] != rhs[i] { return false }
            }
            return true
        }
    }
}

let a: [String?] = ["John", nil]
let b: [String?] = ["John", nil]
let c: [String?] = ["Jack", nil]

print(a == b) // true
print(a == c) // false

The for loop here is more efficient for 2 reasons: (a) you don't have to construct a temporary array of tuples like with zip and (b) it returns false as soon as a non-match is found.