result-types

name: result-types description: Models expected failures as values with Result<T, E> instead of throwing, and composes them with railway-oriented workflows. Use when a function can fail in expected ways (not found, validation, conflict, timeout), when designing error types, when chaining fallible operations, when mapping errors to HTTP status codes, or when deciding whether to throw or return a Result. version: 1.1.0 libraries: ["@jagreehal/workflow"]

Typed Errors: Never Throw

Overview

Exceptions are invisible (they don't appear in a function's signature, so callers can't see them coming), they bypass composition (a throw unwinds the stack past every intermediate step), and they conflate unrelated failures (a catch block can't tell a 404 from a database outage). For expected failures (not found, validation, conflict, timeout) return a Result<T, E> value instead, so the failure is visible in the type, exhaustively handled by the compiler, and composable.

throw is reserved for the exceptional: programmer error, invariant violation, corrupted state. Everything a caller might reasonably want to handle is a Result.

This is the third leg of the architecture: fn-args-deps functions return Results, validation-boundary produces the VALIDATION_FAILED variant, and resilience wraps Result-returning steps with retries and timeouts.

When to Use

• Any function with an expected failure mode (not found, unauthorized, conflict, validation)
• Chaining several fallible operations where one failure should short-circuit the rest
• Designing an error type for a module or domain
• Mapping internal failures to HTTP status codes at the boundary
• Bridging throwing third-party code into a typed pipeline

When NOT to use: Don't return a Result for programmer errors or impossible states. Use throw (or asserts) for those, since no caller can sensibly recover. See When Throwing Is Still Right.

Related: fn-args-deps (the functions that return Results), validation-boundary (source of validation errors), resilience (retry/timeout around Result steps), api-design (the error-envelope shape), observability (logging failures without throwing).

For how this layer fits the whole system, see references/architecture.md.

Core Principle

Exceptions are invisible, bypass composition, and conflate different failures. Return Result<T, E> instead.

// WRONG - Signature lies
async function getUser(args): Promise<User> {
  const user = await deps.db.findUser(args.userId);
  if (!user) throw new Error('User not found');  // Hidden!
  return user;
}

// CORRECT - Signature tells the truth
async function getUser(args, deps): Promise<Result<User, 'NOT_FOUND' | 'DB_ERROR'>> {
  try {
    const user = await deps.db.findUser(args.userId);
    return user ? ok(user) : err('NOT_FOUND');
  } catch {
    return err('DB_ERROR');
  }
}

The Result Type

type Result<T, E> =
  | { ok: true; value: T }
  | { ok: false; error: E };

type AsyncResult<T, E> = Promise<Result<T, E>>;

const ok = <T>(value: T): Result<T, never> => ({ ok: true, value });
const err = <E>(error: E): Result<never, E> => ({ ok: false, error });

Required Behaviors

1. Business Functions Return Results

async function getUser(
  args: { userId: string },
  deps: GetUserDeps
): Promise<Result<User, 'NOT_FOUND' | 'DB_ERROR'>> {
  try {
    const user = await deps.db.findUser(args.userId);
    if (!user) return err('NOT_FOUND');
    return ok(user);
  } catch {
    return err('DB_ERROR');
  }
}

2. Use createWorkflow() for Composition

Avoid verbose if-checking with railway-oriented programming:

import { createWorkflow } from '@jagreehal/workflow';

// Declare dependencies -> error union computed automatically
const loadUserData = createWorkflow({ getUser, getPosts, enrichUser });

const result = await loadUserData(async (step) => {
  const user = await step(() => getUser({ userId }, deps));
  const posts = await step(() => getPosts({ userId: user.id }, deps));
  const enriched = await step(() => enrichUser({ user, posts }, deps));

  return { user: enriched };
});

// result: Result<{ user: EnrichedUser }, 'NOT_FOUND' | 'DB_ERROR' | 'FETCH_ERROR' | ...>

The step() function:

• Unwraps ok results and continues on happy path
• On err, immediately short-circuits and skips remaining steps

3. Use step.try() for Throwing Code

Bridge between throwing code and Result pipeline:

const workflow = createWorkflow({ getUser });

const result = await workflow(async (step) => {
  const user = await step(() => getUser({ userId }, deps));

  // Throwing function: use step.try() with error mapping
  const config = await step.try(
    () => JSON.parse(user.configJson),
    { error: 'INVALID_CONFIG' as const }
  );

  return { user, config };
});

• step(): For functions that already return Result (your code)
• step.try(): For functions that throw (third-party, built-in)
• step.fromResult(): For Result-returning functions where you need to map errors

For Result-returning functions: Use step.fromResult() to preserve typed errors:

// callProvider returns Result<Response, ProviderError>
const callProvider = async (input: string): AsyncResult<Response, ProviderError> => { ... };

const response = await step.fromResult(
  () => callProvider(input),
  {
    onError: (e) => ({
      type: 'PROVIDER_FAILED' as const,
      provider: e.provider,  // TypeScript knows e is ProviderError
      code: e.code,
    })
  }
);

4. Map Results to HTTP at Boundary

const errorToStatus: Record<string, number> = {
  NOT_FOUND: 404,
  UNAUTHORIZED: 401,
  FORBIDDEN: 403,
  VALIDATION_FAILED: 400,
  CONFLICT: 409,
};

function resultToResponse<T, E extends string>(
  result: Result<T, E>,
  res: Response
): Response {
  if (result.ok) {
    return res.status(200).json(result.value);
  }

  const status = errorToStatus[result.error] ?? 500;
  return res.status(status).json({
    error: result.error,
    code: result.error,
  });
}

// Handler becomes simple
app.get('/users/:id', async (req, res) => {
  const result = await getUser({ userId: req.params.id }, deps);
  return resultToResponse(result, res);
});

5. Exhaustive Error Handling

TypeScript enforces handling all error cases:

if (!result.ok) {
  switch (result.error) {
    case 'NOT_FOUND':
      return res.status(404).json({ error: 'User not found' });
    case 'DB_ERROR':
    case 'FETCH_ERROR':
      return res.status(500).json({ error: 'Internal error' });
    // TypeScript will error if you miss a case!
  }
}

Error Type Patterns

String Literals (Simple)

type AppError = 'NOT_FOUND' | 'UNAUTHORIZED' | 'DB_ERROR';

Discriminated Unions (Rich)

type AppError =
  | { type: 'NOT_FOUND'; resource: string }
  | { type: 'VALIDATION'; field: string; message: string }
  | { type: 'DB_ERROR'; query: string };

Const Objects (Runtime + Type)

const Errors = {
  NOT_FOUND: 'NOT_FOUND',
  DB_ERROR: 'DB_ERROR',
} as const;

type AppError = (typeof Errors)[keyof typeof Errors];

return err(Errors.NOT_FOUND);  // Runtime value available

Error Grouping at Scale

As applications grow, error unions become unwieldy:

// This becomes a "Type Wall"
type AllErrors =
  | 'NOT_FOUND'
  | 'DB_ERROR'
  | 'DB_CONNECTION_FAILED'
  | 'DB_TIMEOUT'
  | 'FETCH_ERROR'
  | 'HTTP_TIMEOUT'
  | 'RATE_LIMITED'
  | 'CIRCUIT_OPEN'
  | 'VALIDATION_FAILED'
  // ... 20 more errors

Solution: Group related errors into categories:

// Group by domain
type DatabaseError = 'DB_ERROR' | 'DB_CONNECTION_FAILED' | 'DB_TIMEOUT';
type NetworkError = 'FETCH_ERROR' | 'HTTP_TIMEOUT' | 'RATE_LIMITED';
type BusinessError = 'NOT_FOUND' | 'VALIDATION_FAILED' | 'UNAUTHORIZED';

type AppError = DatabaseError | NetworkError | BusinessError;

// Or use discriminated unions for richer context
type AppError =
  | { type: 'DATABASE'; code: 'CONNECTION_FAILED' | 'TIMEOUT' | 'QUERY_FAILED' }
  | { type: 'NETWORK'; code: 'TIMEOUT' | 'RATE_LIMITED' | 'UNREACHABLE' }
  | { type: 'BUSINESS'; code: 'NOT_FOUND' | 'VALIDATION_FAILED' };

This keeps error types manageable while preserving type safety.

When Throwing Is Still Right

Throw only for:

• Invariant violation (programmer error, impossible state)
• Corrupted process state (can't recover)
• Truly unrecoverable situations

// Good: throw for impossible states
if (!user) throw new Error('Unreachable: user should exist after insert');

Using asserts for Type Narrowing

The asserts keyword creates runtime checks that also narrow types:

// Assert function: throws if condition fails, narrows type if succeeds
function assertUser(user: User | null): asserts user is User {
  if (!user) throw new Error('Invariant violated: user must exist');
}

function assertDefined<T>(value: T | undefined, name: string): asserts value is T {
  if (value === undefined) throw new Error(`${name} must be defined`);
}

// Usage: TypeScript narrows the type after the assertion
const user = await deps.db.findUser(userId);
assertUser(user);  // Throws if null
// TypeScript now knows `user` is `User`, not `User | null`
console.log(user.name);  // Safe access

When to use asserts:

• After database inserts (record MUST exist)
• After config loading (values MUST be present)
• After state transitions (state MUST be valid)

Don't use for: Normal business logic failures (use Result instead)

Quick Reference

Architecture Layer

Handlers / Routes
  -> map Result -> HTTP response

Business Logic
  -> createWorkflow({ ... })(async (step) => { ... })

Core Functions
  -> fn(args, deps): Result<T, E>

Infrastructure
  -> catch exceptions, return Results

Common Rationalizations

Red Flags

• A function whose signature says Promise<User> but throws on the not-found path
• try/catch blocks scattered through business logic instead of at infrastructure edges
• Returning null/undefined to signal distinct failures the caller needs to tell apart
• A single catch (e) swallowing several unrelated failure modes
• throw new Error('not found') for a recoverable, caller-handleable condition
• Result errors mapped to HTTP status codes in multiple handlers instead of one shared mapper
• Error unions that grow unbounded with no grouping by domain

Verification

After implementing fallible logic:

• Functions with expected failures return Result<T, E>, not Promise<T> that throws
• The error type E enumerates every expected failure (no string/any)
• Chained operations compose via createWorkflow / step, short-circuiting on err
• Throwing third-party calls are bridged with step.try() and mapped to typed errors
• Results map to HTTP status in one place at the boundary
• switch on result.error is exhaustive (compiler errors on a missing case)
• throw / asserts is used only for invariant violations and impossible states