Mutation.swift

import Foundation

/// A type that privides mutation interface for a wrapped value.
public struct Mutation<Value> {
  /// The wrapped value.
  public var value: Value

  /// Creates an instance that wraps the given value.
  ///
  /// - Parameter value: The value to store.
  public init(_ value: Value) {
    self.value = value
  }

  /// Returns a mutated copy of the original wrapped value using the given
  /// closure.
  ///
  /// - Parameter body: A closure that takes the wrapped value of the instance.
  /// - Returns: The mutated copy of the wrapped value.
  public func mutated(_ body: (inout Value) throws -> Void) rethrows -> Value {
    var v = self.value
    try body(&v)
    return v
  }

  /// Invokes the given closure with a pointer for the wrapped value.
  ///
  /// - Parameter body: A closure that takes the wrapped value of the instance.
  public mutating func mutate(_ body: (inout Value) throws -> Void) rethrows {
    try body(&self.value)
  }
}

extension Mutation where Value: NSObject {
  /// Creates an instance that stores a value initialized using `init()`.
  public init() {
    self.value = Value()
  }

  /// Returns a mutated value which initialized using `init()`.
  ///
  /// - Parameter body: A closure that takes a value initialized using `init()`.
  /// - Returns: The mutated value.
  public static func mutated(_ body: (inout Value) throws -> Void) rethrows -> Value {
    return try Mutation().mutated(body)
  }
}