Why is Equatable not defined for optional arrays

by

Update: Conditional conformance has been implemented in Swift 4.1. Arrays and optionals of Equatable elements are themselves
Equatable now, and your code

let a: [Int]? = [1]
let b: [Int]? = nil
a == b

compiles and works as expected in Xcode 9.3. The workarounds are not
needed anymore.

(Old answer:)
Optionals can be compared only if the underlying wrapped type is equatable:

public func ==<T : Equatable>(lhs: T?, rhs: T?) -> Bool

Now Arrays can be compared if the element type is equatable:

/// Returns true if these arrays contain the same elements.
public func ==<Element : Equatable>(lhs: [Element], rhs: [Element]) -> Bool

but even for equatable types T, Array<T> does not conform to the Equatable protocol.

At present, this is not possible in Swift, see for example
Why can’t I make Array conform to Equatable? for a discussion
in the Apple developer forum. This change with the implementation
of SE-0143 Conditional conformances
in Swift 4.

Your implementation looks correct, here is a possible different one
using switch/case with pattern matching:

func ==<T: Equatable>(lhs: [T]?, rhs: [T]?) -> Bool {

    switch (lhs, rhs) {
    case let (l?, r?) : // shortcut for (.Some(l), .Some(r))
        return l == r
    case (.None, .None):
        return true
    default:
        return false
    }
}

More Related Contents:

  1. Overriding methods in Swift extensions
  2. Swift: guard let vs if let
  3. .toInt() removed in Swift 2?
  4. Return instancetype in Swift
  5. Swift 2 – UnsafeMutablePointer to object
  6. Swift do-try-catch syntax
  7. The “++” and “–” operators have been deprecated Xcode 7.3
  8. Differences in NSDateComponents syntax?
  9. Swift 2 ( executeFetchRequest ) : error handling
  10. Get integer value from string in swift
  11. Swift 2 – Pattern matching in “if”
  12. Extend array types using where clause in Swift
  13. Is it possible to replicate Swifts automatic numeric value bridging to Foundation (NSNumber) for (U)Int8/16/32/64 types?
  14. Calling protocol default implementation from regular method
  15. Fix warning “C-style for Statement is deprecated” in Swift 3
  16. fatal error: swapping a location with itself is not supported with Swift 2.0
  17. Generic function taking a type name in Swift
  18. Selecting global or object print function
  19. Swift 2.0 : ‘enumerate’ is unavailable: call the ‘enumerate()’ method on the sequence
  20. Any reason not use use a singleton “variable” in Swift?
  21. try, try! & try? what’s the difference, and when to use each?
  22. print without newline in swift
  23. Convert Swift Array to Dictionary with indexes [duplicate]
  24. Iterate through a String Swift 2.0
  25. How do you enumerate OptionSetType in Swift?
  26. Convert Character to Int in Swift 2.0
  27. Whats the Swift animate WithDuration syntax?
  28. How to convert a binary to decimal in Swift?
  29. When are argument labels required in Swift?
  30. Keep window always on top?

Leave a Comment