Swift Language Generics Advanced Type Constraints
Example
It's possible to specify several type constraints for generics using the where clause:
func doSomething<T where T: Comparable, T: Hashable>(first: T, second: T) {
// Access hashable function
guard first.hashValue == second.hashValue else {
return
}
// Access comparable function
if first == second {
print("\(first) and \(second) are equal.")
}
}
It's also valid to write the where clause after the argument list:
func doSomething<T>(first: T, second: T) where T: Comparable, T: Hashable {
// Access hashable function
guard first.hashValue == second.hashValue else {
return
}
// Access comparable function
if first == second {
print("\(first) and \(second) are equal.")
}
}
Extensions can be restricted to types that satisfy conditions. The function is only available to instances which satisfy the type conditions:
// "Element" is the generics type defined by "Array". For this example, we
// want to add a function that requires that "Element" can be compared, that
// is: it needs to adhere to the Equatable protocol.
public extension Array where Element: Equatable {
/// Removes the given object from the array.
mutating func remove(_ element: Element) {
// We could also use "self.index(of: element)" here, as "index(of:_)"
// is also defined in an extension with "where Element: Equatable".
// For the sake of this example, explicitly make use of the Equatable.
if let index = self.index(where: { $0 == element }) {
self.remove(at: index)
} else {
fatalError("Removal error, no such element:\"\(element)\" in array.\n")
}
}
}
