Add method JSONError.withPrefix(_:)

Fixes #26.

README.md

@@ -262,6 +262,12 @@ Unless you explicitly state otherwise, any contribution intentionally submitted
 
 ## Version History
 
+#### Development
+
+* Add method `JSONError.withPrefix(_:)` that returns a new error by prepending a prefix onto the path. This can be used in custom parsing code to produce good errors if the existing convenience functions don't do what you want. ([#26][])
+
+[#26]: https://github.com/postmates/PMJSON/issues/26 "GitHub: JSONError should have structured way to add prefixes"
+
 #### v3.1.1 (2018-05-17)
 
 * Squelch Swift 4.1 warnings.

Sources/JSONError.swift

@@ -93,16 +93,34 @@ public enum JSONError: Error, CustomStringConvertible {
                 prefix += key.stringValue
             }
         }
-        return prefix.isEmpty ? self : withPrefix(prefix)
+        return prefix.isEmpty ? self : withPrefix(.key(prefix))
     }
 
-    fileprivate func withPrefix(_ prefix: String) -> JSONError {
-        func prefixPath(_ path: String?, with prefix: String) -> String {
-            guard let path = path, !path.isEmpty else { return prefix }
+    /// An element that can be prefixed onto the error path.
+    public enum Prefix {
+        /// A key, as from an object.
+        case key(String)
+        /// An index used to subscript an array.
+        case index(Int)
+
+        fileprivate var asString: String {
+            switch self {
+            case .key(let key): return key
+            case .index(let x): return "[\(x)]"
+            }
+        }
+    }
+
+    /// Returns a new `JSONError` by prefixing the given `Prefix` onto the path.
+    /// - Parameter prefix: The prefix to prepend to the error path.
+    /// - Returns: A new `JSONError`.
+    public func withPrefix(_ prefix: Prefix) -> JSONError {
+        func prefixPath(_ path: String?, with prefix: Prefix) -> String {
+            guard let path = path, !path.isEmpty else { return prefix.asString }
             if path.unicodeScalars.first == "[" {
-                return prefix + path
+                return prefix.asString + path
             } else {
-                return "\(prefix).\(path)"
+                return "\(prefix.asString).\(path)"
             }
         }
         switch self {
@@ -1782,15 +1800,15 @@ internal func scoped<T>(_ key: String, _ f: () throws -> T) rethrows -> T {
     do {
         return try f()
     } catch let error as JSONError {
-        throw error.withPrefix(key)
+        throw error.withPrefix(.key(key))
     }
 }
 
 internal func scoped<T>(_ index: Int, _ f: () throws -> T) rethrows -> T {
     do {
         return try f()
     } catch let error as JSONError {
-        throw error.withPrefix("[\(index)]")
+        throw error.withPrefix(.index(index))
     }
 }
 

Tests/PMJSONTests/JSONAccessorTests.swift

@@ -319,4 +319,37 @@ public final class JSONAccessorTests: XCTestCase {
         XCTAssertEqual(JSON.double(42.1), JSON.decimal(42.1))
         XCTAssertEqual(JSON.double(1e100), JSON.decimal(Decimal(string: "1e100")!)) // Decimal(_: Double) can produce incorrect values
     }
+
+    func testErrorPaths() {
+        XCTAssertThrowsError(try JSON.int64(42).getString()) { (error) in
+            switch error {
+            case let error as JSONError: XCTAssertNil(error.path)
+            default: XCTFail("Expected JSONError, got \(error)")
+            }
+        }
+        XCTAssertThrowsError(try (["foo": 42] as JSON).getString("bar")) { (error) in
+            switch error {
+            case let error as JSONError: XCTAssertEqual(error.path, "bar")
+            default: XCTFail("Expected JSONError, got \(error)")
+            }
+        }
+        XCTAssertThrowsError(try (["foo": 42] as JSON).getString("foo")) { (error) in
+            switch error {
+            case let error as JSONError: XCTAssertEqual(error.path, "foo")
+            default: XCTFail("Expected JSONError, got \(error)")
+            }
+        }
+        XCTAssertThrowsError(try (["foo": ["bar": 42]] as JSON).getObject("foo", { try $0.getString("bar") })) { (error) in
+            switch error {
+            case let error as JSONError: XCTAssertEqual(error.path, "foo.bar")
+            default: XCTFail("Expected JSONError, got \(error)")
+            }
+        }
+        XCTAssertThrowsError(try (["foo": ["bar": [1, 2]]] as JSON).getObject("foo", { try $0.mapArray("bar", { try $0.getString() }) })) { (error) in
+            switch error {
+            case let error as JSONError: XCTAssertEqual(error.path, "foo.bar[0]")
+            default: XCTFail("Expected JSONError, got \(error)")
+            }
+        }
+    }
 }