MapApply.swift

extension Sequence {
    /// Returns an array created by applying an instance
    /// method to each element of a source sequence.
    /// This method address the problem caused by mapping
    /// instance methods:
    ///
    ///     ["a", "b", "c"].map(String.uppercased)
    ///
    /// This statement returns `[(Function), (Function), (Function)]`,
    /// which is essentially `[{ String.uppercased("a"), ... }]`
    /// An instance method is `Type.method(instance)()`. Using
    /// `mapApply` forces each partial result to execute and returns those
    /// results, producing ["A", "B", "C"] as expected.
    ///
    ///     ["a", "b", "c"].mapApply(String.uppercased) // ["A", "B", "C"]
    ///
    /// Thanks, Malcolm Jarvis, Gordon Fontenot
    ///
    /// - Parameter transform: The partially applicable instance method
    /// - Parameter element: The sequence element to be transformed by the
    ///   applied method
    /// - Returns: An array containing the results of applying the method
    ///   to each value of the source sequence
    public func mapApply<T>(_ transform: (_ element: Element) throws -> () -> T) rethrows -> [T] {
        return try map(transform).map({ $0() })
    }
    
    /// Returns an array created by applying a keypath to
    /// each element of a source sequence.
    ///
    ///     let keyPath = \String.count
    ///     ["hello", "the", "sailor"].map(keyPath) // 5, 3, 6
    ///
    /// Thanks, Malcolm Jarvis, Gordon Fontenot
    ///
    /// - Parameter keyPath: A keyPath on the `Element`
    /// - Returns: An array containing the results of applying
    ///   the keyPath to each element of the source sequence.
    public func map<T>(_ keyPath: KeyPath<Element, T>) -> [T] {
        return map({ $0[keyPath: keyPath] })
    }
}