Switch Generic Numeric Function with Protocol - swift

Question

This is working great for Doubles and Floats. But getting other numerals like Ints involved is proving really difficult.
public protocol TemperatureConvertable: FloatLiteralConvertible, CustomStringConvertible {
func +(lhs: Self, rhs: Self) -> Self
func -(lhs: Self, rhs: Self) -> Self
func *(lhs: Self, rhs: Self) -> Self
func /(lhs: Self, rhs: Self) -> Self
}
extension Double: TemperatureConvertable {}
extension Float: TemperatureConvertable {}
public func fahrenheitToCelsius<T: TemperatureConvertable>(fahrenheit: T) -> T {
// (°F − 32) ÷ 1.8 =°C
return (fahrenheit - 32.0) / 1.8
}
public func celsiusToFahrenheit<T: TemperatureConvertable>(celsius: T) -> T {
// (°C × 1.8) + 32 =°F
return (celsius * 1.8) + 32.0
}
From what I can find there isn't a NumberLiteralConvertible or similar. And just creating a Numeric protocol like others suggest makes the arithmetic fail because the types don't match. Ideally I'd like something like this thats more generic but this I get Binary operator '-' cannot be applied to operands of type 'T' and 'Double' on the line of actual math for fahrenheit - 32.0 because the lhs and rhs aren't both Self.
public protocol TemperatureConvertable: CustomStringConvertible {
func +(lhs: Self, rhs: Self) -> Self
func -(lhs: Self, rhs: Self) -> Self
func *(lhs: Self, rhs: Self) -> Self
func /(lhs: Self, rhs: Self) -> Self
}
extension Int: TemperatureConvertable {}
extension Double: TemperatureConvertable {}
extension Float: TemperatureConvertable {}
public func fahrenheitToCelsius<T: TemperatureConvertable>(fahrenheit: T) -> T {
// (°F − 32) ÷ 1.8 =°C
return (fahrenheit - 32.0) / 1.8
}
public func celsiusToFahrenheit<T: TemperatureConvertable>(celsius: T) -> T {
// (°C × 1.8) + 32 =°F
return (celsius * 1.8) + 32.0
}

Answer 1

You need a way to create a TemperatureConvertable from a Double and viceversa.
So your functions will be able to internally use Double(s) to make the operations. And finally convert the result to T for the output.
Step 1
public protocol TemperatureConvertible: CustomStringConvertible {
// this part is not needed
// func +(lhs: Self, rhs: Self) -> Self
// func -(lhs: Self, rhs: Self) -> Self
// func *(lhs: Self, rhs: Self) -> Self
// func /(lhs: Self, rhs: Self) -> Self
init(_ other: Double)
var double: Double { get }
}
Now lets update your extensions to Double, Float and Int.
You don't need to add the initializer we declared above since these types already have it.
extension Int: TemperatureConvertible {
public var double: Double { return Double(self) }
}
extension Double: TemperatureConvertible {
public var double: Double { return self }
}
extension Float: TemperatureConvertible {
public var double: Double { return Double(self) }
}
Step 2
Now you can rewrite your functions like this
public func fahrenheitToCelsius<T: TemperatureConvertible>(fahrenheit: T) -> T {
// (°F − 32) ÷ 1.8 =°C
let celsius = (fahrenheit.double - 32.0) / 1.8
return T(celsius)
}
public func celsiusToFahrenheit<T: TemperatureConvertible>(celsius: T) -> T {
// (°C × 1.8) + 32 =°F
let fahrenheit = (celsius.double * 1.8) + 32.0
return T(fahrenheit)
}
Test
let celsius: Int = 20
let fahrenheit = celsiusToFahrenheit(celsius)
// ^ it's an Int
Tip
For readability and consistency with the Cocoa framework I suggest you to rename your functions like this
public func fahrenheitFromCelsius<T: TemperatureConvertible>(_ celsius: T) -> T
public func celsiusFromFahrenheit<T: TemperatureConvertible>(_ fahrenheit: T) -> T