Modifying result type

Often times you want to filter out some specific errors from the client. Typically you'll handle this by calling NeverThrow's own mapErr() on your returned/yielded results, however sometimes it can be handy to do this globally for your SafeFn. You can modify the error type of the SafeFn by using .mapErr(). This is a function that takes in an argument of the original error type (see ), and returns a value that will set the new error type.

As an example:

// Fake function declaration
declare const generateText: (
  prompt: string,
) => ResultAsync<string, { code: "NO_CREDITS_REMAINING" }>;
 
const generateTodo = createSafeFn()
  .input(z.object({ prompt: z.string() }))
  .safeHandler(async function* (args) {
    const todo = yield* generateText(`Make todo based on ${args.input.prompt}`);
    return ok(todo);
  })
  .mapErr((e) => {
    if (e.code === "NO_CREDITS_REMAINING") {
      // Don't let em know funds are running low lmao
      return {
        code: "UNKNOWN_ERROR",
      } as const;
    }
    return e;
  });

The possible Error type when running this function will be

type Res =
  | {
      code: "UNKNOWN_ERROR";
    }
  | {
      code: "INPUT_PARSING";
      cause: ...;
    }
  | { code: "UNCAUGHT_ERROR"; cause: ... };

Modifying result type

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