From 96512e9029754bc8e3334a372db0ed1fc341d7a7 Mon Sep 17 00:00:00 2001
From: SooHwanCho <51935215+JCSooHwanCho@users.noreply.github.com>
Date: Thu, 24 Mar 2022 14:32:28 +0900
Subject: [PATCH] opeators: add a MapError opeator

---
 README.md                                     | 19 ++++
 .../Operators/AsyncSequence+MapError.swift    | 97 +++++++++++++++++++
 .../AsyncSequence+MapErrorTests.swift         | 35 +++++++
 3 files changed, 151 insertions(+)
 create mode 100644 Sources/Operators/AsyncSequence+MapError.swift
 create mode 100644 Tests/Operators/AsyncSequence+MapErrorTests.swift

diff --git a/README.md b/README.md
index 2405e96..7d43aca 100644
--- a/README.md
+++ b/README.md
@@ -48,6 +48,7 @@ AsyncSequences
 * [Share](#Share)
 * [WithLatestFrom](#WithLatestFrom)
 * [EraseToAnyAsyncSequence](#EraseToAnyAsyncSequence)
+* [MapError](#MapError)
 
 More operators and extensions are to come. Pull requests are of course welcome.
 
@@ -545,3 +546,21 @@ seq1.send(3)
 ### EraseToAnyAsyncSequence
 
 `eraseToAnyAsyncSequence()` type-erases the async sequence into an AnyAsyncSequence.
+
+### MapError
+
+`mapError(_:)` transforms error from upstream async sequence.
+
+```
+struct CustomError: Error { }
+
+let failSequence = AsyncSequences.Fail<Int, Swift.Error>(error: NSError(domain: "", code: 1))
+
+do {
+    for try await element in failSequence.mapError({ _ in CustomError() }) {
+        print(element) // will never execute
+    }
+} catch {
+    print(error) // CustomError()
+}
+```
diff --git a/Sources/Operators/AsyncSequence+MapError.swift b/Sources/Operators/AsyncSequence+MapError.swift
new file mode 100644
index 0000000..58ac89c
--- /dev/null
+++ b/Sources/Operators/AsyncSequence+MapError.swift
@@ -0,0 +1,97 @@
+//
+//  AsyncSequence+MapError.swift
+//
+//  Created by JCSooHwanCho on 2022/03/24.
+//
+
+public extension AsyncSequence {
+    /// transform error from upstream async sequence
+    ///
+    /// ```
+    /// struct CustomError: Error { }
+    ///
+    /// let failSequence = AsyncSequences.Fail<Int, Swift.Error>(error: NSError(domain: "", code: 1))
+    ///
+    /// do {
+    ///   for try await element in failSequence.mapError({ _ in CustomError() }) {
+    ///       print(element)
+    ///   }
+    /// } catch {
+    ///    print(error) // CustomError()
+    /// }
+    ///
+    /// ```
+    ///
+    /// - Parameter transformError: A transform to apply to error that upstream emits.
+    /// - Returns: The async sequence error transformed
+  @inlinable
+  __consuming func mapError(
+    _ transformError: @Sendable @escaping (Error) async -> Error
+  ) -> AsyncMapErrorSequence<Self> {
+    return AsyncMapErrorSequence(self, transformError: transformError)
+  }
+}
+
+public struct AsyncMapErrorSequence<Base: AsyncSequence> {
+  @usableFromInline
+  let base: Base
+
+  @usableFromInline
+  let transformError: (Error) async -> Error
+
+  @usableFromInline
+  init(
+    _ base: Base,
+    transformError: @escaping (Error) async -> Error
+  ) {
+    self.base = base
+    self.transformError = transformError
+  }
+}
+
+extension AsyncMapErrorSequence: AsyncSequence {
+
+  public typealias Element = Base.Element
+
+  public typealias AsyncIterator = Iterator
+
+  public struct Iterator: AsyncIteratorProtocol {
+    @usableFromInline
+    var baseIterator: Base.AsyncIterator
+
+    @usableFromInline
+    let transformError: (Error) async -> Error
+
+    @usableFromInline
+    init(
+      _ baseIterator: Base.AsyncIterator,
+      transformError: @escaping (Error) async -> Error
+    ) {
+      self.baseIterator = baseIterator
+      self.transformError = transformError
+    }
+
+    @inlinable
+    public mutating func next() async rethrows -> Element? {
+        do {
+            return try await baseIterator.next()
+        } catch {
+            throw await transformError(error)
+        }
+    }
+  }
+
+  @inlinable
+  public __consuming func makeAsyncIterator() -> Iterator {
+      return Iterator(base.makeAsyncIterator(), transformError: transformError)
+  }
+}
+
+extension AsyncMapErrorSequence: @unchecked Sendable
+  where Base: Sendable,
+        Base.Element: Sendable { }
+
+extension AsyncMapErrorSequence.Iterator: @unchecked Sendable
+  where Base.AsyncIterator: Sendable,
+        Base.Element: Sendable { }
+
diff --git a/Tests/Operators/AsyncSequence+MapErrorTests.swift b/Tests/Operators/AsyncSequence+MapErrorTests.swift
new file mode 100644
index 0000000..4dddb9a
--- /dev/null
+++ b/Tests/Operators/AsyncSequence+MapErrorTests.swift
@@ -0,0 +1,35 @@
+//
+//  AsyncSequence+MapErrorTests.swift
+//  
+//
+//  Created by JCSooHwanCho on 2022/03/24.
+//
+
+import AsyncExtensions
+import XCTest
+
+private struct CustomError: Error, Equatable { }
+
+final class AsyncSequence_MapErrorTests: XCTestCase {
+    func testMapError_with_CustomError() async throws {
+        let failSequence = AsyncSequences.Fail<Int>(error: NSError(domain: "", code: 1))
+
+        do {
+            for try await _ in failSequence.mapError({ _ in CustomError() }) {
+                XCTFail()
+            }
+        } catch {
+            XCTAssertEqual(error as! CustomError, CustomError())
+        }
+    }
+
+    func testMappError_doesnot_affect_on_normal_finis() async throws {
+        let intSequence = (1...5).asyncElements
+
+        let reduced = try await intSequence
+            .mapError { _ in CustomError() }
+            .reduce(into: 0, +=)
+
+        XCTAssertEqual(reduced, 15)
+    }
+}