Sequence extensions for KeyPath

Sequence+KeyPath.swift

extension Sequence {

    /// Map over properties of sequence elements
    /// ```
    /// let counts = ["Hello", "World!"].map(\.count)
    /// // returns [5, 6]
    /// ```
    ///
    /// - Returns: An array of element properties at the keypath
    @inlinable
    public func map<U>(_ keyPath: KeyPath<Element, U>) -> [U] {
        return map { $0[keyPath: keyPath] }
    }

    /// Map and flatten over sequence properties of sequence elements
    /// ```
    /// let subviews = [view1, view2].flatMap(\.subviews)
    /// // returns an array combining the subviews of view1 and view2
    /// ```
    ///
    /// - Returns: An array of element values at the keypath
    @inlinable
    public func flatMap<S: Sequence>(_ keyPath: KeyPath<Element, S>) -> [S.Element] {
        return flatMap { $0[keyPath: keyPath] }
    }

    /// Map and compact over optional properties of sequence elements
    /// ```
    /// let initials = ["Key", "Value", "Coding"].compactMap(\.first)
    /// // returns ["K", "V", "C"]
    /// ```
    ///
    /// - Returns: An array of element values at the keypath
    @inlinable
    public func compactMap<U>(_ keyPath: KeyPath<Element, U?>) -> [U] {
        return compactMap { $0[keyPath: keyPath] }
    }

    /// Map and compact over optional properties of optional sequence elements
    /// ```
    /// let words = ["Three", nil, "Little", "", "Birds"].compactMap(\.first)
    /// // returns ["T", "L", "B"]
    /// ```
    ///
    /// - Returns: An array of element values at the keypath
    @inlinable
    public func compactMap<T, U>(_ keyPath: KeyPath<T, U?>) -> [U] where Element == T? {
        return compactMap { $0?[keyPath: keyPath] }
    }
}