feat: allow returning Error instances in tryCatch

Author: maxmorozoff

.changeset/tender-dogs-tie.md

@@ -0,0 +1,7 @@
+---
+"@maxmorozoff/try-catch-tuple": minor
+---
+
+feat: allow returning Error instances in tryCatch
+
+- Enhanced `tryCatch` to support returning an `Error` instance instead of throwing it.

packages/try-catch-tuple/README.md

@@ -103,6 +103,26 @@ console.log(nullError.cause); // null
 
 This ensures tryCatch always provides a proper error object.
 
+#### Return Error
+
+You can also return an `Error` instance instead of throwing it:
+
+```ts
+const [result, error] = tryCatch(() => {
+  if (Math.random() < 0.5) {
+    return new Error("Too small"); // Return Error instead of throwing it
+  }
+
+  return { message: "ok" };
+});
+if (!error) return result;
+//                 ^? const result: { message: string }
+error;
+// ^? const error: Error
+result;
+// ^? const result: null
+```
+
 #### Extending Error types
 
 ##### Option 1: Manually Set Result and Error Type

packages/try-catch-tuple/src/tryCatch.ts

@@ -10,7 +10,7 @@ type DataErrorTuple<T, E> = Branded<
 /**
  * Represents a successful result where `data` is present and `error` is `null`.
  */
-export type Success<T> = DataErrorTuple<T, null>;
+export type Success<T> = DataErrorTuple<Exclude<T, Error>, null>;
 
 /**
  * Represents a failure result where `error` contains an error instance and `data` is `null`.
@@ -20,7 +20,9 @@ export type Failure<E extends Error> = DataErrorTuple<null, E | Error>;
 /**
  * Represents the result of an operation that can either succeed with `T` or fail with `E`.
  */
-export type Result<T, E extends Error> = Success<T> | Failure<E>;
+export type Result<T, E extends Error> =
+  | Success<T>
+  | Failure<E | Extract<T, Error>>;
 
 /**
  * Resolves the return type based on whether `T` is a promise:
@@ -107,7 +109,7 @@ export const tryCatch: TryCatch = <T, E extends Error = Error>(
     if (result instanceof Promise)
       return tryCatchAsync(result, operationName) as TryCatchResult<T, E>;
 
-    return [result, null] as TryCatchResult<T, E>;
+    return handleResult<T, E>(result, operationName) as TryCatchResult<T, E>;
   } catch (rawError) {
     return handleError(rawError, operationName) as TryCatchResult<T, E>;
   }
@@ -119,7 +121,7 @@ export const tryCatchSync: TryCatch["sync"] = <T, E extends Error = Error>(
 ) => {
   try {
     const result = fn();
-    return [result, null] as Result<T, E>;
+    return handleResult<T, E>(result, operationName);
   } catch (rawError) {
     return handleError(rawError, operationName);
   }
@@ -135,7 +137,7 @@ export const tryCatchAsync: TryCatch["async"] = async <
   try {
     const promise = typeof fn === "function" ? fn() : fn;
     const result = await promise;
-    return [result, null] as Result<Awaited<T>, E>;
+    return handleResult<Awaited<T>, E>(result, operationName);
   } catch (rawError) {
     return handleError(rawError, operationName);
   }
@@ -148,6 +150,22 @@ tryCatch.sync = tryCatchSync;
 tryCatch.async = tryCatchAsync;
 tryCatch.errors = tryCatchErrors;
 
+// Handles a result which might be an error, annotates if needed
+function handleResult<T, E extends Error>(
+  result: T,
+  operationName?: string,
+): Result<T, E> {
+  if (!isError(result)) {
+    return [result, null] as Success<T>;
+  }
+
+  if (operationName) {
+    annotateErrorMessage(result, operationName);
+  }
+
+  return [null, result] as Failure<Extract<T, E>>;
+}
+
 // Handles a raw unknown error, ensures it's wrapped and annotated
 function handleError(rawError: unknown, operationName?: string) {
   const processedError = isError(rawError)

packages/try-catch-tuple/tests/tryCatch.test.ts

@@ -400,6 +400,66 @@ describe("tryCatch", () => {
     });
   });
 
+  describe("handleResult", () => {
+    test("should handle returned Error", () => {
+      const [result, error] = tryCatch(() => {
+        return new Error("test");
+      });
+      expect(result).toBeNil();
+      if (!error) expect.unreachable();
+      expect(error).toBeInstanceOf(Error);
+      expect(error.message).toBe("test");
+      error satisfies Error;
+    });
+
+    test("should handle returned async Error", async () => {
+      const [result, error] = await tryCatch(async () => {
+        return new Error("test");
+      });
+      expect(result).toBeNil();
+      if (!error) expect.unreachable();
+      expect(error).toBeInstanceOf(Error);
+      expect(error.message).toBe("test");
+      error satisfies Error;
+    });
+
+    describe("should handle multiple errors", () => {
+      /**
+       * @note Explicit return type is required!
+       *
+       * TypeScript automatically collapses `SyntaxError` into `Error` because `SyntaxError` extends `Error`.
+       * Without an explicit return type, TypeScript infers `Error | number`, discarding `SyntaxError`.
+       *
+       * By explicitly declaring `Error | SyntaxError | number`, we force TypeScript to retain `SyntaxError`
+       * instead of generalizing it into `Error`.
+       *
+       * Use of `.error<E>()` helper also resolves this.
+       */
+      const multipleErrorTypes = (n: number): Error | SyntaxError | number => {
+        if (n === 0) {
+          return new SyntaxError("test");
+        }
+        if (n === 1) {
+          return new Error("test");
+        }
+        return 1;
+      };
+
+      test.each([
+        [0, SyntaxError],
+        [1, Error],
+      ] as const)("should handle %s", (n, errorClass) => {
+        const [result, error] = tryCatch(() => multipleErrorTypes(n));
+        //             ^?
+        expect(result).toBeNil();
+        if (!error) expect.unreachable();
+        expect(error).toBeInstanceOf(errorClass);
+        expect(error.message).toBe("test");
+        error satisfies InstanceType<typeof errorClass>;
+      });
+    });
+  });
+
   describe("handleError", () => {
     const now = new Date();
     test.each([