Merge pull request #118 from bwhiteley/tryMap

Add a version of `map` that converts thrown errors to `Failure`s.

Result/Result.swift

@@ -205,4 +205,14 @@ public func >>- <T, U, Error> (result: Result<T, Error>, @noescape transform: T
 }
 
 
+// MARK: - ErrorTypeConvertible conformance
+
+/// Make NSError conform to ErrorTypeConvertible
+extension NSError: ErrorTypeConvertible {
+	public static func errorFromErrorType(error: ErrorType) -> NSError {
+		return error as NSError
+	}
+}
+
+
 import Foundation

Result/ResultType.swift

@@ -64,6 +64,29 @@ public extension ResultType {
 	}
 }
 
+/// Protocol used to constrain `tryMap` to `Result`s with compatible `Error`s.
+public protocol ErrorTypeConvertible: ErrorType {
+	typealias ConvertibleType = Self
+	static func errorFromErrorType(error: ErrorType) -> ConvertibleType
+}
+
+public extension ResultType where Error: ErrorTypeConvertible {
+
+	/// Returns the result of applying `transform` to `Success`es’ values, or wrapping thrown errors.
+	public func tryMap<U>(@noescape transform: Value throws -> U) -> Result<U, Error> {
+		return flatMap { value in
+			do {
+				return .Success(try transform(value))
+			}
+			catch {
+				let convertedError = Error.errorFromErrorType(error) as! Error
+				// Revisit this in a future version of Swift. https://twitter.com/jckarter/status/672931114944696321
+				return .Failure(convertedError)
+			}
+		}
+	}
+}
+
 // MARK: - Operators
 
 infix operator &&& {

ResultTests/ResultTests.swift

@@ -103,6 +103,16 @@ final class ResultTests: XCTestCase {
 		XCTAssertNil(result.error)
 	}
 
+	func testTryMapProducesSuccess() {
+		let result = success.tryMap(tryIsSuccess)
+		XCTAssert(result == success)
+	}
+
+	func testTryMapProducesFailure() {
+		let result = Result<String, NSError>.Success("fail").tryMap(tryIsSuccess)
+		XCTAssert(result == failure)
+	}
+
 	// MARK: Operators
 
 	func testConjunctionOperator() {
@@ -146,7 +156,7 @@ func attempt<T>(value: T, succeed: Bool, error: NSErrorPointer) -> T? {
 }
 
 func tryIsSuccess(text: String?) throws -> String {
-	guard let text = text else {
+	guard let text = text where text == "success" else {
 		throw error
 	}