When define prop as this, we can mutate property with function composition #CodePiece #tryswiftnyc

CodePiece.swift

func prop<Root, Value>(
    _ kp: WritableKeyPath<Root, Value>
    )
    -> (@escaping (Value) -> Value)
    -> (Root) -> Root {
        return { transformPart in
            return { root in
                var root = root
                root[keyPath: kp] = transformPart(root[keyPath: kp])
                return root
            }
        }
}

user.age
let newUser = user
    |> prop(\.age)(incr)
    >>> (prop(\.name)) { $0.uppercased() }
newUser.age
newUser.name

[user, user, user]
    |> map(prop(\.age)(incr)
        >>> (prop(\.name)) { $0.uppercased() })