Result Pattern

Learn how to use explicit error handling with the Result / AsyncResult pattern from unthrown.

Overview

temporal-contract uses unthrown's Result<T, E> and AsyncResult<T, E> types throughout its public surface:

• Activities return AsyncResult<T, ApplicationFailure>.
• Workflows await activities and child workflows; the framework unwraps the Result for activities (so a workflow sees a plain value or a thrown error) and surfaces Result directly for child workflows.
• Clients await AsyncResult<T, E>; the resolved value is a Result<T, E> that you destructure with result.match({ ok, err, defect }) or the free functions isOk(result) / isErr(result) / isDefect(result).

A single library covers every context — the same import works inside activities, workflows, and clients.

Installation

pnpm add unthrown

AsyncResult<T, E> is awaitable: await asyncResult resolves to Result<T, E>. The underlying Promise is constructed when the chain runs, so the type behaves like a lazy task — call sites that already await the value before checking isOk(result) / isErr(result) need no changes.

TIP

Narrow before touching .value / .error / .cause. Both forms are type guards: the result.isOk() / result.isErr() / result.isDefect()methods and the matching free functions isOk(result) / isErr(result) / isDefect(result) imported from "unthrown". This codebase uses the methods; either works.

Basic Usage

Activities

Activities return AsyncResult<T, ApplicationFailure>. The cleanest shape is fromPromise(promise, mapError):

import { declareActivitiesHandler, ApplicationFailure } from "@temporal-contract/worker/activity";
import { fromPromise } from "unthrown";
import { orderContract } from "./contract";

export const activities = declareActivitiesHandler({
  contract: orderContract,
  activities: {
    processPayment: ({ amount }) =>
      fromPromise(paymentGateway.charge(amount), (error) =>
        ApplicationFailure.create({
          type: "PAYMENT_FAILED",
          message: error instanceof Error ? error.message : "Payment failed",
          cause: error instanceof Error ? error : undefined,
        }),
      ).map((txId) => ({ transactionId: txId, success: true })),

    sendEmail: ({ to, body }) =>
      fromPromise(emailService.send({ to, body }), (error) =>
        ApplicationFailure.create({
          type: "EMAIL_FAILED",
          message: error instanceof Error ? error.message : "Email failed",
          cause: error instanceof Error ? error : undefined,
        }),
      ).map(() => ({ sent: true })),
  },
});

Workflows

Workflows await activities and child workflows directly. Activities return plain values inside the workflow body (the framework unwraps the Result); child workflows surface Result so the workflow can branch on success vs. failure without throwing:

import { declareWorkflow } from "@temporal-contract/worker/workflow";
import { orderContract } from "./contract";

export const processOrder = declareWorkflow({
  workflowName: "processOrder",
  contract: orderContract,
  activityOptions: { startToCloseTimeout: "1 minute" },
  implementation: async (context, args) => {
    // Process payment - activities return plain values
    const payment = await context.activities.processPayment({ amount: args.amount });

    // Send confirmation email
    await context.activities.sendEmail({
      to: "customer@example.com",
      body: `Order ${args.orderId} confirmed`,
    });

    return {
      success: true,
      transactionId: payment.transactionId,
    };
  },
});

Clients

Clients receive an AsyncResult<T, E> from executeWorkflow / startWorkflow. Awaiting it yields a Result<T, E>:

import { TypedClient } from "@temporal-contract/client";
import { Client } from "@temporalio/client";
import { orderContract } from "./contract";

const temporalClient = new Client({ connection });

const client = TypedClient.create(orderContract, temporalClient);
const result = await client.executeWorkflow("processOrder", {
  workflowId: "order-123",
  args: { orderId: "ORD-123", amount: 100 },
});

// Handle result with pattern matching (object form, three channels)
result.match({
  ok: (value) => {
    console.log("Order processed:", value.transactionId);
  },
  err: (error) => {
    console.error("Order failed:", error);
  },
  defect: (cause) => {
    console.error("Unexpected failure:", cause);
  },
});

Awaiting and inspecting

AsyncResult<T, E> is a thin wrapper around a Promise<Result<T, E>>. You can await it once and then inspect synchronously, or chain with .map, .mapErr, .flatMap, .orElse before awaiting:

import { isErr } from "unthrown";

const result = await client.executeWorkflow("processOrder", {
  workflowId: "order-123",
  args: { orderId: "ORD-123", amount: 100 },
});

if (isErr(result)) {
  console.error(result.error);
  return;
}

console.log(result.value);

Pattern Matching

Activities return plain values when called from workflows. If an activity fails, the framework rethrows the ApplicationFailure so workflow code can catch it like any other Temporal failure:

export const processOrder = declareWorkflow({
  workflowName: "processOrder",
  contract: orderContract,
  activityOptions: { startToCloseTimeout: "1 minute" },
  implementation: async (context, args) => {
    try {
      // Activity returns plain value (Result is unwrapped internally)
      const payment = await context.activities.processPayment({ amount: 100 });
      console.log("Payment succeeded:", payment.transactionId);

      return { success: true, transactionId: payment.transactionId };
    } catch (error) {
      // Activity errors are thrown
      console.error("Payment failed:", error);
      return { success: false, transactionId: "" };
    }
  },
});

NOTE

For child workflows, you do get Result objects. See the Child Workflows section below.

Chaining Activities

When calling multiple activities, use standard async/await with try/catch:

export const processOrder = declareWorkflow({
  workflowName: "processOrder",
  contract: orderContract,
  activityOptions: { startToCloseTimeout: "1 minute" },
  implementation: async (context, args) => {
    try {
      // Activities return plain values
      const payment = await context.activities.processPayment({ amount: 100 });

      // Next activity only runs if payment succeeded
      await context.activities.sendEmail({
        to: "customer@example.com",
        body: `Payment ${payment.transactionId} processed`,
      });

      // Update database
      await context.activities.updateDatabase({
        status: "completed",
      });

      return { success: true };
    } catch (error) {
      console.error("Workflow failed:", error);
      return { success: false };
    }
  },
});

Error Types

Define typed errors in your activities:

import { fromPromise } from "unthrown";
import { ApplicationFailure } from "@temporal-contract/worker/activity";

type PaymentError =
  | { type: "InsufficientFunds" }
  | { type: "CardDeclined" }
  | { type: "NetworkError"; message: string };

type EmailError = { type: "InvalidEmail" } | { type: "ServiceUnavailable" };

// Activities return AsyncResult with typed errors
processPayment: ({ amount }) =>
  fromPromise(paymentGateway.charge(amount), (error) => {
    // Wrap domain errors in ApplicationFailure so Temporal applies the
    // configured retry policy; set `nonRetryable: true` for permanent
    // failures.
    return ApplicationFailure.create({
      type: "PAYMENT_FAILED",
      message: error instanceof Error ? error.message : "Payment failed",
      ...(error instanceof Error ? { cause: error } : {}),
    });
  }).map((txId) => ({ transactionId: txId }));

Benefits

1. Explicit Error Handling

Activities use the AsyncResult pattern internally, while workflows use try/catch:

import { fromPromise } from "unthrown";

// Activity implementation (uses AsyncResult)
const processPayment = ({ amount }) =>
  fromPromise(paymentGateway.charge(amount), (error) =>
    ApplicationFailure.create({
      type: "PAYMENT_FAILED",
      message: "Payment failed",
      cause: error instanceof Error ? error : undefined,
    }),
  ).map((txId) => ({ transactionId: txId }));

// Workflow (uses standard try/catch for activities)
export const processOrder = declareWorkflow({
  workflowName: "processOrder",
  contract: myContract,
  activityOptions: { startToCloseTimeout: "1 minute" },
  implementation: async (context, args) => {
    try {
      // Activity returns plain value
      const payment = await context.activities.processPayment({ amount: 100 });
      return { success: true, transactionId: payment.transactionId };
    } catch (error) {
      // Handle activity error
      return { success: false };
    }
  },
});

2. No Hidden Exceptions in Activities

Activities explicitly return AsyncResult instead of throwing:

// ✅ Clear - activity returns AsyncResult<T, ApplicationFailure>
const processPayment = ({ amount }) =>
  fromPromise(paymentGateway.charge(amount), (error) =>
    ApplicationFailure.create({
      type: "PAYMENT_FAILED",
      message: "Payment failed",
      cause: error instanceof Error ? error : undefined,
    }),
  ).map((txId) => ({ transactionId: txId }));

// ❌ Unclear - might throw anything
async function processPayment({ amount }) {
  const txId = await paymentGateway.charge(amount);
  return { transactionId: txId };
}

3. Railway-Oriented Programming (Activities)

Activity implementations can chain operations that short-circuit on error using .flatMap (unthrown's bind/chain operator):

// Activity implementation with chaining
const processOrder = ({ orderId }) =>
  validateOrderId(orderId)
    .flatMap((validId) => fetchOrder(validId))
    .flatMap((order) => processPayment(order))
    .flatMap((payment) => updateDatabase(payment))
    .mapErr((error) =>
      ApplicationFailure.create({
        type: "ORDER_FAILED",
        message: "Order processing failed",
        cause: error instanceof Error ? error : undefined,
      }),
    );
// Stops at first error

4. Partial Success Handling

Track partial success in complex workflows using try/catch blocks:

export const processOrder = declareWorkflow({
  workflowName: "processOrder",
  contract: orderContract,
  activityOptions: { startToCloseTimeout: "1 minute" },
  implementation: async (context, args) => {
    let paymentTransactionId: string | undefined;

    try {
      // Step 1: Process payment
      const payment = await context.activities.processPayment({ amount: args.amount });
      paymentTransactionId = payment.transactionId;

      // Step 2: Schedule shipment
      await context.activities.scheduleShipment({ orderId: args.orderId });

      return { success: true, transactionId: paymentTransactionId };
    } catch (error) {
      // Payment succeeded but shipment failed - can handle specially
      if (paymentTransactionId) {
        // Rollback payment
        await context.activities.refundPayment({ transactionId: paymentTransactionId });

        return {
          success: false,
          message: "Shipment failed, payment refunded",
          completedSteps: { payment: paymentTransactionId },
        };
      }

      return { success: false, message: "Payment failed" };
    }
  },
});

Combining results

unthrown exposes all([...]) to fan in a list of Results into a single Result<T[], E> that fails on the first error. Destructure the combined array, or call .match on the result:

import { all } from "unthrown";

const combined = all([validateA(a), validateB(b), validateC(c)]);

return combined.match({
  ok: ([resA, resB, resC]) => proceed({ resA, resB, resC }),
  err: (error) => fail(error),
  defect: (cause) => fail(cause),
});

Child Workflows

Child workflows return AsyncResult for consistent error handling:

Execute and Wait

import { declareWorkflow } from "@temporal-contract/worker/workflow";

export const parentWorkflow = declareWorkflow({
  workflowName: "parentWorkflow",
  contract: myContract,
  activityOptions: { startToCloseTimeout: "1 minute" },
  implementation: async (context, args) => {
    // Execute child workflow and wait for result
    const result = await context.executeChildWorkflow(myContract, "processPayment", {
      workflowId: `payment-${args.orderId}`,
      args: { amount: args.totalAmount },
    });

    // Workflows return plain objects, not Result
    return result.match({
      ok: (output) => ({
        success: true,
        transactionId: output.transactionId,
      }),
      err: (error) => ({
        success: false,
        error: error.message,
      }),
      defect: (cause) => ({
        success: false,
        error: cause instanceof Error ? cause.message : "Unexpected failure",
      }),
    });
  },
});

Start Without Waiting

export const parentWorkflow = declareWorkflow({
  workflowName: "parentWorkflow",
  contract: myContract,
  activityOptions: { startToCloseTimeout: "1 minute" },
  implementation: async (context, args) => {
    // Start child workflow without waiting
    const handleResult = await context.startChildWorkflow(myContract, "sendNotification", {
      workflowId: `notification-${args.orderId}`,
      args: { message: "Order received" },
    });

    handleResult.match({
      ok: async (handle) => {
        // Child started successfully
        // Can wait for result later if needed
        const result = await handle.result();
      },
      err: (error) => {
        console.error("Failed to start child:", error);
      },
      defect: (cause) => {
        console.error("Unexpected failure starting child:", cause);
      },
    });

    // Workflows return plain objects, not Result
    return { success: true };
  },
});

Cross-Contract Child Workflows

Invoke workflows from different contracts/workers:

import { orderContract, notificationContract } from "./contracts";

export const orderWorkflow = declareWorkflow({
  workflowName: "processOrder",
  contract: orderContract,
  activityOptions: { startToCloseTimeout: "1 minute" },
  implementation: async (context, args) => {
    // Child workflow from another contract
    const notifyResult = await context.executeChildWorkflow(
      notificationContract,
      "sendOrderConfirmation",
      {
        workflowId: `notify-${args.orderId}`,
        args: { orderId: args.orderId },
      },
    );

    // Workflows return plain objects, not Result
    return notifyResult.match({
      ok: () => ({ status: "completed" }),
      err: (error) => ({
        status: "failed",
        error: error.message,
      }),
      defect: (cause) => ({
        status: "failed",
        error: cause instanceof Error ? cause.message : "Unexpected failure",
      }),
    });
  },
});

The defect channel

unthrown models three outcomes, not two. Besides ok (success) and err (a deliberate, anticipated failure), there is a third channel — defect — for unanticipated failures: bugs, programmer errors, or any exception you never modeled.

• An err is a value you returned on purpose (err(...) / errAsync → err(...).toAsync(), or a rejection mapped through fromPromise(promise, errFn)). It is part of your type signature.
• A defect is captured when an unexpected throw escapes — for example a fromSafePromise(...) thunk that throws, or an unhandled exception inside a .map. It is not part of the modeled error type and carries the raw failure on result.cause.

A defect re-throws when you await/unwrap it rather than being handled as a value, so genuine bugs surface loudly instead of being silently swallowed. Inspect it with the free function isDefect(result) and result.cause, or handle all three channels at once with result.match({ ok, err, defect }):

import { isOk, isErr, isDefect } from "unthrown";

const result = await client.executeWorkflow("processOrder", {
  workflowId: "order-123",
  args: { orderId: "ORD-123", amount: 100 },
});

if (isOk(result)) {
  console.log(result.value);
} else if (isErr(result)) {
  console.error("Modeled failure:", result.error); // anticipated boundary error
} else if (isDefect(result)) {
  console.error("Unexpected failure (bug):", result.cause); // unmodeled
}

Keep deliberate boundary errors in the err channel (wrap them in ApplicationFailure for activities) and let only truly unexpected throws become defects.

TaggedError and matchTags

Error classes are built with TaggedError, which stamps each class with a _tag discriminant:

import { TaggedError } from "unthrown";

class PaymentDeclined extends TaggedError("PaymentDeclined")<{
  readonly customerId: string;
}> {}

class GatewayTimeout extends TaggedError("GatewayTimeout")<{
  readonly elapsedMs: number;
}> {}

NOTE

The worker's ValidationError subclasses are the exception — they still extend Temporal's ApplicationFailure rather than TaggedError.

NOTE

temporal-contract's own error classes namespace their tag with the package scope — e.g. _tag === "@temporal-contract/WorkflowExecutionNotFoundError" — so they never collide with a _tag from your own code or another library. Their .name stays the bare class name (e.g. "WorkflowExecutionNotFoundError") for readable logs. When folding library errors, the matchTags keys carry the prefix: matchTags(result, { "@temporal-contract/WorkflowExecutionNotFoundError": ... }).

Because every tagged error carries a _tag, unthrown's matchTags folds a Result exhaustively by tag, with dedicated Ok and Defect channels:

import { matchTags } from "unthrown";

const message = matchTags(result, {
  Ok: (value) => `charged ${value.transactionId}`,
  PaymentDeclined: (e) => `declined for ${e.customerId}`,
  GatewayTimeout: (e) => `timed out after ${e.elapsedMs}ms`,
  Defect: (cause) => `unexpected: ${String(cause)}`,
});

When to Use

Use Result / AsyncResult When:

• In Activity Implementations: Always use AsyncResult for explicit error handling
• For Child Workflows: Child workflows return Result for explicit error handling
• For Type-Safe Errors: When you need ApplicationFailure with type / nonRetryable for proper retry policies

Use Standard async/await When:

• In Workflow Logic: Use try/catch when calling activities from workflows
• For Simple Error Handling: When standard exception handling is sufficient
• For Deterministic Code: Workflows must remain deterministic

See Also

• Migrating from neverthrow
• Order Processing Example
• Worker Implementation