create-type-safe-errors

description: Use Data.TaggedError to create typed, distinguishable errors for your domain. globs: "**/*.ts" alwaysApply: true

Create Type-Safe Errors

Rule: Use Data.TaggedError to create typed, distinguishable errors for your domain.

Example

import { Effect, Data } from "effect"

// ============================================
// 1. Define tagged errors for your domain
// ============================================

class UserNotFoundError extends Data.TaggedError("UserNotFoundError")<{
  readonly userId: string
}> {}

class InvalidEmailError extends Data.TaggedError("InvalidEmailError")<{
  readonly email: string
  readonly reason: string
}> {}

class DuplicateUserError extends Data.TaggedError("DuplicateUserError")<{
  readonly email: string
}> {}

// ============================================
// 2. Use in Effect functions
// ============================================

interface User {
  id: string
  email: string
  name: string
}

const validateEmail = (email: string): Effect.Effect<string, InvalidEmailError> => {
  if (!email.includes("@")) {
    return Effect.fail(new InvalidEmailError({
      email,
      reason: "Missing @ symbol"
    }))
  }
  return Effect.succeed(email)
}

const findUser = (id: string): Effect.Effect<User, UserNotFoundError> => {
  // Simulate database lookup
  if (id === "123") {
    return Effect.succeed({ id, email: "[email protected]", name: "Alice" })
  }
  return Effect.fail(new UserNotFoundError({ userId: id }))
}

const createUser = (
  email: string,
  name: string
): Effect.Effect<User, InvalidEmailError | DuplicateUserError> =>
  Effect.gen(function* () {
    const validEmail = yield* validateEmail(email)

    // Simulate duplicate check
    if (validEmail === "[email protected]") {
      return yield* Effect.fail(new DuplicateUserError({ email: validEmail }))
    }

    return {
      id: crypto.randomUUID(),
      email: validEmail,
      name,
    }
  })

// ============================================
// 3. Handle errors by tag
// ============================================

const program = createUser("[email protected]", "Alice").pipe(
  Effect.catchTag("InvalidEmailError", (error) =>
    Effect.succeed({
      id: "fallback",
      email: "[email protected]",
      name: `${error.email} was invalid: ${error.reason}`,
    })
  ),
  Effect.catchTag("DuplicateUserError", (error) =>
    Effect.fail(new Error(`Email ${error.email} already registered`))
  )
)

// ============================================
// 4. Match on all errors
// ============================================

const handleAllErrors = createUser("bad-email", "Bob").pipe(
  Effect.catchTags({
    InvalidEmailError: (e) => Effect.succeed(`Invalid: ${e.reason}`),
    DuplicateUserError: (e) => Effect.succeed(`Duplicate: ${e.email}`),
  })
)

// ============================================
// 5. Run and see results
// ============================================

Effect.runPromise(program)
  .then((user) => console.log("Created:", user))
  .catch((error) => console.error("Failed:", error))