I'd like to make the function below work both with Float and Double values:
func srgb2linear(_ S: Float) -> Float {
if S <= 0.04045 {
return S / 12.92
} else {
return pow((S + 0.055) / 1.055, 2.4)
}
}
The Swift 4 Documentation says that what I need is a FloatingPoint generic, to represent both Float and Double classes, like:
func srgb2linear<T: FloatingPoint>(_ S: T) -> T
However when I try to do this, it doesn't compile with the following errors:
Error: binary operator '<=' cannot be applied to operands of type 'T' and 'Double'
Error: binary operator '/' cannot be applied to operands of type 'T' and 'Double'
Error: binary operator '+' cannot be applied to operands of type 'T' and 'Double'
How is it possible that for a generic representing floating point numbers such operators are not implemented? And if not like this, how can I write this function in Swift?
One problem is that FloatingPoint is not a subprotocol of ExpressibleByFloatLiteral, so your floating-point literals cannot necessarily be converted to T. You can solve this either by changing FloatingPoint to BinaryFloatingPoint (which is a subprotocol of ExpressibleByFloatLiteral) or by adding ExpressibleByFloatLiteral as a separate requirement.
Then you will run into the problem that there is no pow function that is generic over FloatingPoint, and no member of FloatingPoint or BinaryFloatingPoint that performs exponentiation. You can solve this by creating a new protocol and conforming the existing floating-point types to it:
protocol Exponentiatable {
func toPower(_ power: Self) -> Self
}
extension Float: Exponentiatable {
func toPower(_ power: Float) -> Float { return pow(self, power) }
}
extension Double: Exponentiatable {
func toPower(_ power: Double) -> Double { return pow(self, power) }
}
extension CGFloat: Exponentiatable {
func toPower(_ power: CGFloat) -> CGFloat { return pow(self, power) }
}
Note that there is also a Float80 type, but the standard library doesn't provide a pow function for it.
Now we can write a working generic function:
func srgb2linear<T: FloatingPoint>(_ S: T) -> T
where T: ExpressibleByFloatLiteral, T: Exponentiatable
{
if S <= 0.04045 {
return S / 12.92
} else {
return ((S + 0.055) / 1.055).toPower(2.4)
}
}
You could define a second one with Double arguments:
func srgb2linear(_ S: Double) -> Double {
if S <= 0.04045 {
return S / 12.92
} else {
return pow((S + 0.055) / 1.055, 2.4)
}
}
or, if Float<->Double conversions are not a concern :
func srgb2linear(_ S: Double) -> Double {
return Double(srgb2linear(Float(S)))
}
I am trying to implement a basic protocol extension like so:
protocol Value {
func get() -> Float
mutating func set(to:Float)
}
extension Value {
static func min(of a:Value, and b:Value) -> Float {
if a < b { //Expression type 'Bool' is ambiguous without more context
return a.get()
}else{
return b.get()
}
}
static func < (a:Value, b:Value) -> Bool {
return a.get() < b.get()
}
}
At the if clause the compiler says:Expression type 'Bool' is ambiguous without more context. Why doesn't this work?
As touched on in this Q&A, there's a difference between operator overloads implemented as static members and operator overloads implemented as top-level functions. static members take an additional (implicit) self parameter, which the compiler needs to be able to infer.
So how is the value of self inferred? Well, it has to be done from either the operands or return type of the overload. For a protocol extension, this means one of those types needs to be Self. Bear in mind that you can't directly call an operator on a type (i.e you can't say (Self.<)(a, b)).
Consider the following example:
protocol Value {
func get() -> Float
}
extension Value {
static func < (a: Value, b: Value) -> Bool {
print("Being called on conforming type: \(self)")
return a.get() < b.get()
}
}
struct S : Value {
func get() -> Float { return 0 }
}
let value: Value = S()
print(value < value) // Ambiguous reference to member '<'
What's the value of self in the call to <? The compiler can't infer it (really I think it should error directly on the overload as it's un-callable). Bear in mind that self at static scope in a protocol extension must be a concrete conforming type; it can't just be Value.self (as static methods in protocol extensions are only available to call on concrete conforming types, not on the protocol type itself).
We can fix both the above example, and your example by defining the overload as a top-level function instead:
protocol Value {
func get() -> Float
}
func < (a: Value, b: Value) -> Bool {
return a.get() < b.get()
}
struct S : Value {
func get() -> Float { return 0 }
}
let value: Value = S()
print(value < value) // false
This works because now we don't need to infer a value for self.
We could have also given the compiler a way to infer the value of self, by making one or both of the parameters take Self:
protocol Value {
func get() -> Float
}
extension Value {
static func < (a: Self, b: Self) -> Bool {
print("Being called on conforming type: \(self)")
return a.get() < b.get()
}
}
struct S : Value {
func get() -> Float { return 0 }
}
let s = S()
print(s < s)
// Being called on conforming type: S
// false
The compiler can now infer self from the static type of operands. However, as said above, this needs to be a concrete type, so you can't deal with heterogenous Value operands (you could work with one operand taking a Value; but not both as then there'd be no way to infer self).
Although note that if you're providing a default implementation of <, you should probably also provide a default implementation of ==. Unless you have a good reason not to, I would also advise you make these overloads take homogenous concrete operands (i.e parameters of type Self), such that they can provide a default implementation for Comparable.
Also rather than having get() and set(to:) requirements, I would advise a settable property requirement instead:
// Not deriving from Comparable could be useful if you need to use the protocol as
// an actual type; however note that you won't be able to access Comparable stuff,
// such as the auto >, <=, >= overloads from a protocol extension.
protocol Value {
var floatValue: Double { get set }
}
extension Value {
static func == (lhs: Self, rhs: Self) -> Bool {
return lhs.floatValue == rhs.floatValue
}
static func < (lhs: Self, rhs: Self) -> Bool {
return lhs.floatValue < rhs.floatValue
}
}
Finally, if Comparable conformance is essential for conformance to Value, you should make it derive from Comparable:
protocol Value : Comparable {
var floatValue: Double { get set }
}
You shouldn't need a min(of:and:) function in either case, as when the conforming type conforms to Comparable, it can use the top-level min(_:_:) function.
You can't write
if a < b {
because a and b have type Value which is NOT Comparable.
However you can compare the float value associated to a and b
if a.get() < b.get() {
If you want to be able to make types that can use operators such as >, <, ==, etc., they have to conform to the Comparable protocol:
protocol Value: Comparable {
func get() -> Float
mutating func set(to: Float)
}
This comes with more restrictions though. You will have to change all the Value types in the protocol extension to Self:
extension Value {
static func min(of a: Self, and b: Self) -> Float {
if a < b { //Expression type 'Bool' is ambiguous without more context
return a.get()
}else{
return b.get()
}
}
static func < (a: Self, b: Self) -> Bool {
return a.get() < b.get()
}
}
The Self types get replaced with the type that implements the protocol. So if I implemented Value on a type Container, the methods signatures would look like this:
class Container: Value {
static func min(of a: Container, and b: Container) -> Float
static func < (a: Container, b: Container) -> Bool
}
As a side note, if you want Value to conform to Comparable, you might want to also add the == operator to the Value extension:
static func <(lhs: Self, rhs: Self) -> Bool {
return lhs.get() < rhs.get()
}
I'd like to increase my knowledge of generics and I come to a problem I can't solve. I have two different types (Int and Double). Both types implement a function advanced(by:). I want to make a generic function to call advanced(by:) on a given type.
The benefit would be so that I could replace intAdvancedByTen(value:) and doubleAdvancedByTen(value:) with a single genericAdvancedByTen(value:).
Here's my playground:
let myDouble: Double = 1
let myInt: Int = 2
func intAdvanceByTen(value: Int) { // Replace this function
value.advanced(by: 10)
}
func doubleAdvanceByTen(value: Int) { // ...and this function
value.advanced(by: 10)
}
protocol CanAdvance {
func advance(_ by: Any)
}
// ...with this generic function
func genericAdvanceByTen(value: CanAdvance) {
value.advance(10)
}
genericAdvanceByTen(value: myInt) // Error: Argument "Int" does not conform to expected type "CanAdvance"
How to make the generic function know that the passed type implements the advanced(by:) method?
Try this:
protocol CanAdvance {
// This method is intentionally designed to have the same signature as the
// methods built into Int and Double
func advanced(by: Self) -> Self
// We need this primarily for the definition of the constant 10. The built
// in `advanced` function requires the distance to be of the same type.
//
// The conversion syntax in Swift is via init:
// let aDouble = Double(anInt)
// Not the C-like cast:
// let aDouble = anInt as! Double // invalid
//
// Hence we must have a way to convert 10 to the appropriate Int or Double.
// Conveniently, both types can convert from an Int32 so we put this
// requirement in the protocol
init(_ value: Int32)
}
extension Int : CanAdvance { }
extension Double : CanAdvance { }
func genericAdvanceByTen<T: CanAdvance>(value: T) -> T {
let distance = T(10)
return value.advanced(by: distance)
}
genericAdvanceByTen(value: 2) // 12
genericAdvanceByTen(value: 3.14) // 13.14
No need to define your own protocol – advanced(by:) is defined by the standard libary's Strideable protocol:
public protocol Strideable : Comparable {
/// A type that can represent the distance between two values of `Self`.
associatedtype Stride : SignedNumber
// ...
/// Returns a `Self` `x` such that `self.distance(to: x)` approximates `n`.
///
/// If `Stride` conforms to `Integer`, then `self.distance(to: x) == n`.
///
/// - Complexity: O(1).
public func advanced(by n: Self.Stride) -> Self
}
Therefore you simply want to constrain your function to take a Strideable input.
Given that the Stride associated type (what advanced(by:) expects as an argument) is constrained to SignedNumber, it must conform to ExpressibleByIntegerLiteral – which allows us to pass an integer literal directly to it.
For example:
func genericAdvancedByTen<T : Strideable>(value: T) -> T {
// Utilise the fact that T.Stride is ExpressibleByIntegerLiteral.
return value.advanced(by: 10)
}
print(genericAdvancedByTen(value: 2)) // 12
print(genericAdvancedByTen(value: 3.14)) // 13.14
I want to use first order citizen Type in Swift to decide which function to call.
func sf(v: [Float]) -> Float{
}
func df(v: [Double]) -> Double {
}
func f<T: RealType>(seq ls: [T]) -> T {
if T.self == Float.self {
return sf(ls) // 1. where sf: Float -> Void
} else if T.self == Double.self {
return df(ls) // 2. where df : Double -> Void
}
}
The type inference system couldn't notice that under one branch T == Float and Double in the other ?
Is here a missing feature, complex feature or bug ?
Edit:
typealias RealType = protocol<FloatingPointType, Hashable, Equatable, Comparable, FloatLiteralConvertible, SignedNumberType>
for my prototype but will become a protocol
You are trying to combine static resolution given by generic with runtime decisions, and this is not possible.
You can simply overload f for both Float and Double to obtain what you need:
func f(seq ls: [Float]) -> Float {
return sf(ls) // 1. where sf: Float -> Void
}
func f(seq ls: [Double]) -> Double {
return df(ls) // 2. where df : Double -> Void
}
However, if you want RealType to be a generic placeholder that you can use over other types than Float or, Double, then you can do something like this:
protocol RealType {
static func processArray(v: [Self]) -> Self
}
extension Float: RealType {
static func processArray(v: [Float]) -> Float {
return sf(v)
}
}
extension Double: RealType {
static func processArray(v: [Double]) -> Double {
return df(v)
}
}
func sf(v: [Float]) -> Float{
return 0
}
func df(v: [Double]) -> Double {
return 0
}
func f<T: RealType>(seq ls: [T]) -> T {
return T.processArray(ls)
}
This will give you both type safety, which is one of Swift's main advantages, and scalability as whenever you need to add support for f over another type, you need to only declare that type as conforming to RealType, and implement the processArray method.
Swift lets you create an Array extension that sums Integer's with:
extension Array {
func sum() -> Int {
return self.map { $0 as Int }.reduce(0) { $0 + $1 }
}
}
Which can now be used to sum Int[] like:
[1,2,3].sum() //6
But how can we make a generic version that supports summing other Number types like Double[] as well?
[1.1,2.1,3.1].sum() //fails
This question is NOT how to sum numbers, but how to create a generic Array Extension to do it.
Getting Closer
This is the closest I've been able to get if it helps anyone get closer to the solution:
You can create a protocol that can fulfills what we need to do, i.e:
protocol Addable {
func +(lhs: Self, rhs: Self) -> Self
init()
}
Then extend each of the types we want to support that conforms to the above protocol:
extension Int : Addable {
}
extension Double : Addable {
}
And then add an extension with that constraint:
extension Array {
func sum<T : Addable>(min:T) -> T
{
return self.map { $0 as T }.reduce(min) { $0 + $1 }
}
}
Which can now be used against numbers that we've extended to support the protocol, i.e:
[1,2,3].sum(0) //6
[1.1,2.1,3.1].sum(0.0) //6.3
Unfortunately I haven't been able to get it working without having to supply an argument, i.e:
func sum<T : Addable>(x:T...) -> T?
{
return self.map { $0 as T }.reduce(T()) { $0 + $1 }
}
The modified method still works with 1 argument:
[1,2,3].sum(0) //6
But is unable to resolve the method when calling it with no arguments, i.e:
[1,2,3].sum() //Could not find member 'sum'
Adding Integer to the method signature also doesn't help method resolution:
func sum<T where T : Integer, T: Addable>() -> T?
{
return self.map { $0 as T }.reduce(T()) { $0 + $1 }
}
But hopefully this will help others come closer to the solution.
Some Progress
From #GabrielePetronella answer, it looks like we can call the above method if we explicitly specify the type on the call-site like:
let i:Int = [1,2,3].sum()
let d:Double = [1.1,2.2,3.3].sum()
As of Swift 2 it's possible to do this using protocol extensions. (See The Swift Programming Language: Protocols for more information).
First of all, the Addable protocol:
protocol Addable: IntegerLiteralConvertible {
func + (lhs: Self, rhs: Self) -> Self
}
extension Int : Addable {}
extension Double: Addable {}
// ...
Next, extend SequenceType to add sequences of Addable elements:
extension SequenceType where Generator.Element: Addable {
var sum: Generator.Element {
return reduce(0, combine: +)
}
}
Usage:
let ints = [0, 1, 2, 3]
print(ints.sum) // Prints: "6"
let doubles = [0.0, 1.0, 2.0, 3.0]
print(doubles.sum) // Prints: "6.0"
I think I found a reasonable way of doing it, borrowing some ideas from scalaz and starting from your proposed implementation.
Basically what we want is to have typeclasses that represents monoids.
In other words, we need:
an associative function
an identity value (i.e. a zero)
Here's a proposed solution, which works around the swift type system limitations
First of all, our friendly Addable typeclass
protocol Addable {
class func add(lhs: Self, _ rhs: Self) -> Self
class func zero() -> Self
}
Now let's make Int implement it.
extension Int: Addable {
static func add(lhs: Int, _ rhs: Int) -> Int {
return lhs + rhs
}
static func zero() -> Int {
return 0
}
}
So far so good. Now we have all the pieces we need to build a generic `sum function:
extension Array {
func sum<T : Addable>() -> T {
return self.map { $0 as T }.reduce(T.zero()) { T.add($0, $1) }
}
}
Let's test it
let result: Int = [1,2,3].sum() // 6, yay!
Due to limitations of the type system, you need to explicitly cast the result type, since the compiler is not able to figure by itself that Addable resolves to Int.
So you cannot just do:
let result = [1,2,3].sum()
I think it's a bearable drawback of this approach.
Of course, this is completely generic and it can be used on any class, for any kind of monoid.
The reason why I'm not using the default + operator, but I'm instead defining an add function, is that this allows any type to implement the Addable typeclass. If you use +, then a type which has no + operator defined, then you need to implement such operator in the global scope, which I kind of dislike.
Anyway, here's how it would work if you need for instance to make both Int and String 'multipliable', given that * is defined for Int but not for `String.
protocol Multipliable {
func *(lhs: Self, rhs: Self) -> Self
class func m_zero() -> Self
}
func *(lhs: String, rhs: String) -> String {
return rhs + lhs
}
extension String: Multipliable {
static func m_zero() -> String {
return ""
}
}
extension Int: Multipliable {
static func m_zero() -> Int {
return 1
}
}
extension Array {
func mult<T: Multipliable>() -> T {
return self.map { $0 as T }.reduce(T.m_zero()) { $0 * $1 }
}
}
let y: String = ["hello", " ", "world"].mult()
Now array of String can use the method mult to perform a reverse concatenation (just a silly example), and the implementation uses the * operator, newly defined for String, whereas Int keeps using its usual * operator and we only need to define a zero for the monoid.
For code cleanness, I much prefer having the whole typeclass implementation to live in the extension scope, but I guess it's a matter of taste.
In Swift 2, you can solve it like this:
Define the monoid for addition as protocol
protocol Addable {
init()
func +(lhs: Self, rhs: Self) -> Self
static var zero: Self { get }
}
extension Addable {
static var zero: Self { return Self() }
}
In addition to other solutions, this explicitly defines the zero element using the standard initializer.
Then declare Int and Double as Addable:
extension Int: Addable {}
extension Double: Addable {}
Now you can define a sum() method for all Arrays storing Addable elements:
extension Array where Element: Addable {
func sum() -> Element {
return self.reduce(Element.zero, combine: +)
}
}
Here's a silly implementation:
extension Array {
func sum(arr:Array<Int>) -> Int {
return arr.reduce(0, {(e1:Int, e2:Int) -> Int in return e1 + e2})
}
func sum(arr:Array<Double>) -> Double {
return arr.reduce(0, {(e1:Double, e2:Double) -> Double in return e1 + e2})
}
}
It's silly because you have to say arr.sum(arr). In other words, it isn't encapsulated; it's a "free" function sum that just happens to be hiding inside Array. Thus I failed to solve the problem you're really trying to solve.
3> [1,2,3].reduce(0, +)
$R2: Int = 6
4> [1.1,2.1,3.1].reduce(0, +)
$R3: Double = 6.3000000000000007
Map, Filter, Reduce and more
From my understanding of the swift grammar, a type identifier cannot be used with generic parameters, only a generic argument. Hence, the extension declaration can only be used with a concrete type.
It's doable based on prior answers in Swift 1.x with minimal effort:
import Foundation
protocol Addable {
func +(lhs: Self, rhs: Self) -> Self
init(_: Int)
init()
}
extension Int : Addable {}
extension Int8 : Addable {}
extension Int16 : Addable {}
extension Int32 : Addable {}
extension Int64 : Addable {}
extension UInt : Addable {}
extension UInt8 : Addable {}
extension UInt16 : Addable {}
extension UInt32 : Addable {}
extension UInt64 : Addable {}
extension Double : Addable {}
extension Float : Addable {}
extension Float80 : Addable {}
// NSNumber is a messy, fat class for ObjC to box non-NSObject values
// Bit is weird
extension Array {
func sum<T : Addable>(min: T = T(0)) -> T {
return map { $0 as! T }.reduce(min) { $0 + $1 }
}
}
And here: https://gist.github.com/46c1d4d1e9425f730b08
Swift 2, as used elsewhere, plans major improvements, including exception handling, promises and better generic metaprogramming.
Help for anyone else struggling to apply the extension to all Numeric values without it looking messy:
extension Numeric where Self: Comparable {
/// Limits a numerical value.
///
/// - Parameter range: The range the value is limited to be in.
/// - Returns: The numerical value clipped to the range.
func limit(to range: ClosedRange<Self>) -> Self {
if self < range.lowerBound {
return range.lowerBound
} else if self > range.upperBound {
return range.upperBound
} else {
return self
}
}
}