Apply improved messages for error string interpolation, add chain option

Author: Jeehut

Sources/ErrorKit/ErrorKit.swift

@@ -168,7 +168,7 @@ public enum ErrorKit {
          } else {
             // For enums, include the full case description with type name
             if let enclosingType {
-               return "\(enclosingType).\(error)"
+               return "\(enclosingType).\(String(describing: error))"
             } else {
                return String(describing: error)
             }

Sources/ErrorKit/Helpers/String+ErrorKit.swift

@@ -17,3 +17,38 @@ extension String {
    }
    #endif
 }
+
+extension String.StringInterpolation {
+   /// Interpolates an error using its user-friendly message.
+   ///
+   /// Uses ``ErrorKit.userFriendlyMessage(for:)`` to provide clear, actionable error descriptions
+   /// suitable for displaying to users. For nested errors, returns the message from the root cause.
+   ///
+   /// ```swift
+   /// showAlert("Operation failed: \(error)")
+   /// ```
+   ///
+   /// - Parameter error: The error to interpolate using its user-friendly message
+   mutating public func appendInterpolation(_ error: some Error) {
+      self.appendInterpolation(ErrorKit.userFriendlyMessage(for: error))
+   }
+
+   /// Interpolates an error using its complete chain description for debugging.
+   ///
+   /// Uses ``ErrorKit.errorChainDescription(for:)`` to show the full error hierarchy,
+   /// type information, and nested structure. Ideal for logging and debugging.
+   ///
+   /// ```swift
+   /// Logger().error("Operation failed with:\n\(chain: error)")
+   /// // Operation failed with:
+   /// // DatabaseError
+   /// // └─ FileError
+   /// //    └─ PermissionError.denied(permission: "~/Downloads/Profile.png")
+   /// //       └─ userFriendlyMessage: "Access to ~/Downloads/Profile.png was declined..."
+   /// ```
+   ///
+   /// - Parameter error: The error to interpolate using its complete chain description
+   mutating public func appendInterpolation(chain error: some Error) {
+      self.appendInterpolation(ErrorKit.errorChainDescription(for: error))
+   }
+}

Tests/ErrorKitTests/ErrorKitTests.swift

@@ -40,6 +40,45 @@ enum ErrorKitTests {
       }
    }
 
+   enum StringInterpolation {
+      @Test
+      static func implicitWithStruct() async throws {
+         #expect("\(SomeThrowable())" == "Something failed hard.")
+      }
+
+      @Test
+      static func implicitWithNestedError() async throws {
+         let nestedError = DatabaseError.caught(FileError.caught(PermissionError.denied(permission: "~/Downloads/Profile.png")))
+         #expect("\(nestedError)" == "Access to ~/Downloads/Profile.png was declined. To use this feature, please enable the permission in your device Settings.")
+      }
+
+      @Test
+      static func chainWithStruct() async throws {
+         #expect(
+            "\(chain: SomeThrowable())"
+            ==
+            """
+            SomeThrowable [Struct]
+            └─ userFriendlyMessage: "Something failed hard."
+            """
+         )
+      }
+
+      @Test
+      static func chainWithNestedError() async throws {
+         let nestedError = DatabaseError.caught(FileError.caught(PermissionError.denied(permission: "~/Downloads/Profile.png")))
+         #expect(
+            "\(chain: nestedError)"
+            ==
+            """
+            DatabaseError
+            └─ FileError
+               └─ PermissionError.denied(permission: "~/Downloads/Profile.png")
+                  └─ userFriendlyMessage: "Access to ~/Downloads/Profile.png was declined. To use this feature, please enable the permission in your device Settings."
+            """)
+      }
+   }
+
    enum ErrorChainDescription {
       @Test
       static func localizedError() {