[swift-evolution] Equality of optional arrays

[Jake Heiser]

Hi all!

Currently in Swift, checking the equality of two optional arrays is much
messier than it should be:

switch (optionalArray1, optionalArray2) {
    case (.Some(let a1), .Some(let a2)) where a1 == a2: return true
    case (.None, .None): return true
    default: return false
}

This is a result of the lack of a built-in overload of == for optional
arrays (and optional array slices for that matter). Overloads exist for
optional equatable objects and for non-optional arrays with equatable
elements, but not for optional arrays with equatable elements. The standard
library currently has these overloads (among others):

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

To correct the issue, I propose two functions be added to the standard
library:

public func ==<Element : Equatable>(lhs: ArraySlice<Element>?, rhs:
ArraySlice<Element>?) -> Bool
public func ==<Element : Equatable>(lhs: [Element]?, rhs: [Element]?) ->
Bool

This change would allow optional arrays to be checked for equality just as
other objects are:

optionalString1 == optionalString2 // Already possible
requiredArray1 == requiredArray2 // Already possible
optionalArray1 == optionalArray2 // Would be possible with these changes

Thanks for your consideration -
Jake Heiser
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