Generic sum() and mean() extensions for Swift numeric collections

sumAndMean.swift

/// To use reduce() to sum a sequence, we need
///
/// - a zero (identity) element
/// - an addition operator

protocol Summable {

    /// Identity element for this type and the + operation.
    static var Zero: Self { get }

    /// + operator must be defined for this type
    func +(lhs: Self, rhs: Self) -> Self
}

extension SequenceType where Generator.Element: Summable {

    /// Return sum of all values in the sequence
    func sum() -> Generator.Element {
        return self.reduce(Generator.Element.Zero, combine: +)
    }
}

// Define Zero for Int, Double, and String so we can sum() them.
//
// Extend this to any other types for which you want to use sum().

extension Int: Summable {
    /// The Int value 0
    static var Zero: Int { return 0 }
}

extension Double: Summable {
    /// The Double value 0.0
    static var Zero: Double { return 0.0 }
}

extension String: Summable {
    /// Empty string
    static var Zero: String { return "" }
}


/// A generic mean() function requires that we be able
/// to convert values to Double.

protocol DoubleConvertible {

    /// Return value as Double type
    var doubleValue: Double { get }
}

// Define doubleValue() for Int and Double.
//
// Extend this to any other types for which you want
// to use mean().

extension Int: DoubleConvertible {

    /// Return Int as Double
    var doubleValue: Double { return Double(self) }
}

extension Double: DoubleConvertible {

    /// Return Double value
    var doubleValue: Double { return self }
}


extension CollectionType where
    Generator.Element: DoubleConvertible,
    Generator.Element: Summable,
    Index.Distance: DoubleConvertible {

    /// Return arithmetic mean of values in collection
    func mean() -> Double {
        assert(!self.isEmpty, "cannot calculate mean of empty collection")

        return self.sum().doubleValue / self.count.doubleValue
    }
}


// Examples

[1, 2, 3, 4, 5].sum()                         // 15

[1.1, 2.2, 3.3, 4.4, 5.5].sum()               // 16.5

["a", "b", "c", "d", "e"].sum()               // "abcde"

[1, 2, 3, 5].mean()                           // 2.75
[0.4, 1.2, 3.2].mean()                        // 1.6

Awesome work man!