Pitch: Reducing a sequence onto its own elements to simplify code and reduce errors

Evolution Pitches
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FWIW, there is a way to solve the issue without introducing an optional in many cases. We could introduce a Monoid protocol:

protocol Monoid {
    associatedtype Value

    /// must obey identity laws with respect to `combine`
    static var identity: Value

    /// must be associative
    static func combine(_ lhs: Value, _ rhs: Value) -> Value
}

With conformances such as:

extension Int {
    enum Add: Monoid {
        static var identity = 0
        static func combine(_ lhs: Int, _ rhs: Int) { return lhs + rhs }
    }
    enum Multiply: Monoid {
        static var identity = 1
        static func combine(_ lhs: Int, _ rhs: Int) { return lhs * rhs }
    }
}

and this overload of fold:

public func fold<M: Monoid>(_ Monoid.Type) -> Element where M.Value == Element

which would then be called as follows:

let sum: Int = sequenceOfInt.fold(Add.self)
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