Explicit conversion operator for Swift.

Example.swift

import Foundation
import ExplicitlyConvertible

let i8: Int8 = 8
let i64: Int64 = 64

i64 + ^i8                                 // => 72

struct Person {
    var name: String
    var birthDate: Date
}
let people: [Person] = []

let names = people.map(^\.name)
type(of: names)                           // => Array<String>

ExplicitlyConvertible.swift

prefix operator ^

/// Conforming types can be unconditionally, losslessly converted to 
/// another type through the explicit conversion operator, `^`.
public protocol ExplicitlyConvertible {
    associatedtype ExplicitConversionSupertype
    
    /// Converts `self` to an equivalent supertype, such as a wider 
    /// integer type.
    static prefix func ^(operand: Self) -> ExplicitConversionSupertype
}

// Operators below this point simply let you indirectly convert through 
// several explicit supertypes at once. Five is an arbitrary choice; it 
// just happens to be more than high enough to get you from the 
// smallest integers to the largest.

extension ExplicitlyConvertible where ExplicitConversionSupertype: ExplicitlyConvertible {
    public static prefix func ^(operand: Self) -> ExplicitConversionSupertype.ExplicitConversionSupertype {
        return ^(^operand)
    }
}

extension ExplicitlyConvertible where ExplicitConversionSupertype: ExplicitlyConvertible, ExplicitConversionSupertype.ExplicitConversionSupertype: ExplicitlyConvertible {
    public static prefix func ^(operand: Self) -> ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype {
        return ^(^(^operand))
    }
}

extension ExplicitlyConvertible where ExplicitConversionSupertype: ExplicitlyConvertible, ExplicitConversionSupertype.ExplicitConversionSupertype: ExplicitlyConvertible, ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype: ExplicitlyConvertible {
    public static prefix func ^(operand: Self) -> ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype {
        return ^(^(^(^operand)))
    }
}

extension ExplicitlyConvertible where ExplicitConversionSupertype: ExplicitlyConvertible, ExplicitConversionSupertype.ExplicitConversionSupertype: ExplicitlyConvertible, ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype: ExplicitlyConvertible, ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype: ExplicitlyConvertible {
    public static prefix func ^(operand: Self) -> ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype.ExplicitConversionSupertype {
        return ^(^(^(^(^operand))))
    }
}

Integer.swift

extension Int8: ExplicitlyConvertible {
    public typealias ExplicitConversionSupertype = Int16
}

extension Int16: ExplicitlyConvertible {
    public typealias ExplicitConversionSupertype = Int32
}

extension Int32: ExplicitlyConvertible {
    public typealias ExplicitConversionSupertype = Int
}

extension Int: ExplicitlyConvertible {
    public typealias ExplicitConversionSupertype = Int64
}

extension UInt8: ExplicitlyConvertible {
    public typealias ExplicitConversionSupertype = UInt16
}

extension UInt16: ExplicitlyConvertible {
    public typealias ExplicitConversionSupertype = UInt32
}

extension UInt32: ExplicitlyConvertible {
    public typealias ExplicitConversionSupertype = UInt
}

extension UInt: ExplicitlyConvertible {
    public typealias ExplicitConversionSupertype = UInt64
}

extension ExplicitlyConvertible where Self: BinaryInteger, ExplicitConversionSupertype: BinaryInteger {
    /// Converts the integer to a wider integer type.
    public static prefix func ^(operand: Self) -> ExplicitConversionSupertype {
        return ExplicitConversionSupertype(operand)
    }
}

KeyPath.swift

extension KeyPath: ExplicitlyConvertible {
    public typealias ExplicitConversionSupertype = (Root) -> Value
    
    /// Converts the KeyPath to a getter function.
    public static prefix func ^(operand: KeyPath<Root, Value>) -> (Root) -> Value {
        return { $0[keyPath: operand] }
    }
}