SE-0235: Add Result<Success, Failure: Error> to Standard Library

stdlib/public/core/CMakeLists.txt

@@ -116,6 +116,7 @@ set(SWIFTLIB_ESSENTIAL
   ReflectionMirror.swift
   Repeat.swift
   REPL.swift
+  Result.swift
   Reverse.swift
   Runtime.swift.gyb
   RuntimeFunctionCounters.swift

stdlib/public/core/GroupInfo.json

@@ -220,5 +220,8 @@
     "Comparable.swift",
     "Codable.swift",
     "MigrationSupport.swift"
+  ],
+  "Result": [
+    "Result.swift"
   ]
 }

stdlib/public/core/Result.swift

@@ -0,0 +1,131 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2018 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/// A value that represents either a success or failure, capturing associated
+/// values in both cases.
+@_frozen
+public enum Result<Success, Failure: Error> {
+  /// A success, storing a `Success` value.
+  case success(Success)
+
+  /// A failure, storing a `Failure` value.
+  case failure(Failure)
+
+  /// Evaluates the given transform closure when this `Result` instance is
+  /// `.success`, passing the value as a parameter.
+  ///
+  /// Use the `map` method with a closure that returns a non-`Result` value.
+  ///
+  /// - Parameter transform: A closure that takes the successful value of the
+  ///   instance.
+  /// - Returns: A new `Result` instance with the result of the transform, if
+  ///   it was applied.
+  public func map<NewSuccess>(
+    _ transform: (Success) -> NewSuccess
+  ) -> Result<NewSuccess, Failure> {
+    switch self {
+    case let .success(success):
+      return .success(transform(success))
+    case let .failure(failure):
+      return .failure(failure)
+    }
+  }
+
+  /// Evaluates the given transform closure when this `Result` instance is
+  /// `.failure`, passing the error as a parameter.
+  ///
+  /// Use the `mapError` method with a closure that returns a non-`Result`
+  /// value.
+  ///
+  /// - Parameter transform: A closure that takes the failure value of the
+  ///   instance.
+  /// - Returns: A new `Result` instance with the result of the transform, if
+  ///   it was applied.
+  public func mapError<NewFailure>(
+    _ transform: (Failure) -> NewFailure
+  ) -> Result<Success, NewFailure> {
+    switch self {
+    case let .success(success):
+      return .success(success)
+    case let .failure(failure):
+      return .failure(transform(failure))
+    }
+  }
+
+  /// Evaluates the given transform closure when this `Result` instance is
+  /// `.success`, passing the value as a parameter and flattening the result.
+  ///
+  /// - Parameter transform: A closure that takes the successful value of the
+  ///   instance.
+  /// - Returns: A new `Result` instance, either from the transform or from
+  ///   the previous error value.
+  public func flatMap<NewSuccess>(
+    _ transform: (Success) -> Result<NewSuccess, Failure>
+  ) -> Result<NewSuccess, Failure> {
+    switch self {
+    case let .success(success):
+      return transform(success)
+    case let .failure(failure):
+      return .failure(failure)
+    }
+  }
+
+  /// Evaluates the given transform closure when this `Result` instance is
+  /// `.failure`, passing the error as a parameter and flattening the result.
+  ///
+  /// - Parameter transform: A closure that takes the error value of the
+  ///   instance.
+  /// - Returns: A new `Result` instance, either from the transform or from
+  ///   the previous success value.
+  public func flatMapError<NewFailure>(
+    _ transform: (Failure) -> Result<Success, NewFailure>
+  ) -> Result<Success, NewFailure> {
+    switch self {
+    case let .success(success):
+      return .success(success)
+    case let .failure(failure):
+      return transform(failure)
+    }
+  }
+
+  /// Attempts to get the `success` value as a throwing expression.
+  ///
+  /// - Returns: The success value, if the instance is a success.
+  /// - Throws:  The failure value, if the instance is a failure.
+  public func get() throws -> Success {
+    switch self {
+    case let .success(success):
+      return success
+    case let .failure(failure):
+      throw failure
+    }
+  }
+}
+
+extension Result where Failure == Swift.Error {
+  /// Create an instance by capturing the output of a throwing closure.
+  ///
+  /// - Parameter catching: A throwing closure to evaluate.
+  @_transparent
+  public init(catching body: () throws -> Success) {
+    do {
+      let value = try body()
+      self = .success(value)
+    } catch {
+      self = .failure(error)
+    }
+  }
+}
+
+extension Result : Equatable where Success : Equatable, Failure : Equatable { }
+
+extension Result : Hashable where Success : Hashable, Failure : Hashable { }

test/stdlib/Result.swift

@@ -0,0 +1,171 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+
+import StdlibUnittest
+import Swift
+
+let ResultTests = TestSuite("Result")
+
+fileprivate enum Err: Error, Equatable {
+  case err
+  case derr
+}
+
+fileprivate let string = "string"
+
+fileprivate extension Result {
+  var success: Success? {
+    switch self {
+    case let .success(success):
+      return success
+    case .failure:
+      return nil
+    }
+  }
+
+  var failure: Failure? {
+    switch self {
+    case .success:
+      return nil
+    case let .failure(failure):
+      return failure
+    }
+  }
+}
+
+ResultTests.test("Construction") {
+  let result1: Result<String, Err> = .success(string)
+  let result2: Result<String, Err> = .failure(.err)
+  let string1: String? = {
+    switch result1 {
+      case let .success(string): 
+        return string
+      case .failure: 
+        expectUnreachable()
+        return nil
+    }
+  }()
+  let error: Err? = {
+    switch result2 {
+      case let .failure(failure): 
+        return failure
+      case .success: 
+        expectUnreachable()
+        return nil
+    }
+  }()
+
+  expectEqual(string1, string)
+  expectEqual(error, .err)
+}
+
+ResultTests.test("Throwing Initialization and Unwrapping") {
+  func notThrowing() throws -> String {
+    return string
+  }
+
+  func throwing() throws -> String {
+    throw Err.err
+  }
+
+  let result1 = Result { try throwing() }
+  let result2 = Result { try notThrowing() }
+
+  expectEqual(result1.failure as? Err, Err.err)
+  expectEqual(result2.success, string)
+
+  do {
+    _ = try result1.get()
+  } catch let error as Err {
+    expectEqual(error, Err.err)
+  } catch {
+    expectUnreachable()
+  }
+
+  do {
+    let unwrapped = try result2.get()
+    expectEqual(unwrapped, string)
+  } catch {
+    expectUnreachable()
+  }
+
+  // Test unwrapping strongly typed error.
+  let result3 = Result<String, Err>.failure(Err.err)
+  do {
+    _ = try result3.get()
+  } catch let error as Err {
+    expectEqual(error, Err.err)
+  } catch {
+    expectUnreachable()
+  }
+}
+
+ResultTests.test("Functional Transforms") {
+  func transformDouble(_ int: Int) -> Int {
+    return 2 * int
+  }
+
+  func transformTriple(_ int: Int) -> Int {
+    return 3 * int
+  }
+
+  func transformError(_ err: Err) -> Err {
+    if err == .err {
+      return .derr
+    } else {
+      return .err
+    }
+  }
+
+  func resultValueTransform(_ int: Int) -> Result<Int, Err> {
+    return .success(transformDouble(int))
+  }
+
+  func resultErrorTransform(_ err: Err) -> Result<Int, Err> {
+    return .failure(transformError(err))
+  }
+
+  let result1: Result<Int, Err> = .success(1)
+  let newResult1 = result1.map(transformDouble)
+
+  expectEqual(newResult1, .success(2))
+
+  let result2: Result<Int, Err> = .failure(.err)
+  let newResult2 = result2.mapError(transformError)
+
+  expectEqual(newResult2, .failure(.derr))
+
+  let result3: Result<Int, Err> = .success(1)
+  let newResult3 = result3.flatMap(resultValueTransform)
+
+  expectEqual(newResult3, .success(2))
+
+  let result4: Result<Int, Err> = .failure(.derr)
+  let newResult4 = result4.flatMapError(resultErrorTransform)
+
+  expectEqual(newResult4, .failure(.err))
+}
+
+ResultTests.test("Equatable") {
+  let result1: Result<Int, Err> = .success(1)
+  let result2: Result<Int, Err> = .failure(.err)
+
+  expectEqual(result1, .success(1))
+  expectNotEqual(result1, .success(2))
+  expectNotEqual(result1, .failure(.err))
+  expectNotEqual(result1, .failure(.derr))
+
+  expectNotEqual(result2, .success(1))
+  expectNotEqual(result2, .success(2))
+  expectEqual(result2, .failure(.err))
+  expectNotEqual(result2, .failure(.derr))
+}
+
+ResultTests.test("Hashable") {
+  let result1: Result<Int, Err> = .success(1)
+  let result2: Result<Int, Err> = .success(2)
+  let result3: Result<Int, Err> = .failure(.err)
+  checkHashable([result1, result2, result3], equalityOracle: { $0 == $1 })
+}
+
+runAllTests()