I would like to get the opposite of a boolean value using dot syntax.
E.g.
let digitsExceptForFive = [1, 2, 3, 4, 6, 7, 8, 9]
let fives = [1, 2, 3, 4, 5, 6, 7, 8, 9].filter(!digitsExceptForFive.contains)
// Cannot convert value of type '(Int) -> Bool' to expected argument type 'Bool'
The following code works, however.
let digitsExceptForFive = [1, 2, 3, 4, 6, 7, 8, 9]
let notFives = [1, 2, 3, 4, 5, 6, 7, 8, 9].filter(digitsExceptForFive.contains)
// [1, 2, 3, 4, 6, 7, 8, 9]
I need to get the opposite value functionally using the not operator. If it's possible I would like not to have to define a bool extension or custom operator just to do so.
How can you do this in Swift?
I would just extend Sequence protocol constraining Element to Equatable and implement a notContains method:
extension Sequence where Element: Equatable {
func notContains(_ element: Element) -> Bool { !contains(element) }
}
let digitsExceptForFive = [1, 2, 3, 4, 6, 7, 8, 9]
let fives = [1, 2, 3, 4, 5, 6, 7, 8, 9].filter(digitsExceptForFive.notContains) // [5]
You can also extend Bool and provide a negated property, this would allow to use negated KeyPath syntax:
extension Bool {
var negated: Bool { !self }
}
let string = "abc12345"
let nonDigits = string.filter(\.isWholeNumber.negated) // "abc"
if you really want to implement a custom prefix operator (I really prefer using the notConstains method and the Bool negated instance property) you can do something like:
prefix func !<T>(predicate: @escaping (T) -> Bool) -> (T) -> Bool { { !predicate($0) } }
Usage:
let string = "abc12345"
let nonDigits = string.filter(!\.isWholeNumber) // "abc"
let digitsExceptForFive = [1, 2, 3, 4, 6, 7, 8, 9]
let fives = [1, 2, 3, 4, 5, 6, 7, 8, 9].filter(!digitsExceptForFive.contains) // [5]