defineErrors

Related Skills: See error-handling for trySync/tryAsync usage and toast-on-error patterns. See services-layer for service architecture and namespace exports.

When to Apply This Skill

Use this pattern when you need to:

• Define or refactor domain error variants using defineErrors.
• Add error variants that include structured fields and cause: unknown.
• Centralize extractErrorMessage(cause) inside variant factories.
• Infer union and single-variant types via InferErrors/InferError.
• Replace old createTaggedError and split Err-pair patterns.

Import

import {
  defineErrors,
  extractErrorMessage,
  type InferErrors,
  type InferError,
} from 'wellcrafted/error';

Core Rules

1. All variants for a domain live in one defineErrors call — never spread them across multiple calls
2. The factory function returns { message, ...fields } — that is the entire API; no .withMessage(), .withContext(), or .withCause() chains
3. cause: unknown is just a field like any other — accept it in the input and forward it in the return object
4. Call extractErrorMessage(cause) inside the factory, never at the call site
5. Each call like MyError.Variant({ ... }) returns Err(...) automatically — no separate FooErr pair
6. Shadow the const with a same-name type using InferErrors — const FooError / type FooError
7. Use InferError<typeof FooError.Variant> to extract a single variant's type when needed
8. Variant names describe the specific failure mode — never use generic names like Service, Error, or Failed
9. Aim for 2–5 variants per domain, each named by failure mode
10. Write .message for end-user readability — toastOnError shows .message as the muted toast description below the bold title. Write messages that make sense to users, not just developers. Avoid raw paths, status codes, or stack traces as the primary message. Include them after a human-readable prefix:

// ✅ GOOD — human-readable prefix, technical detail after
message: `Could not save recording: ${extractErrorMessage(cause)}`

// ❌ BAD — raw technical output as the entire message
message: `POST /api/recordings 500: ${extractErrorMessage(cause)}`

Patterns

1. Simple variant — no input, static message

export const RecorderError = defineErrors({
  AlreadyRecording: () => ({
    message: 'A recording is already in progress',
  }),
});
export type RecorderError = InferErrors<typeof RecorderError>;

// Call site
return RecorderError.AlreadyRecording();

2. Variant with structured fields — message computed from input

export const DbError = defineErrors({
  NotFound: ({ table, id }: { table: string; id: string }) => ({
    message: `${table} '${id}' not found`,
    table,
    id,
  }),
});
export type DbError = InferErrors<typeof DbError>;

// Call site
return DbError.NotFound({ table: 'users', id: '123' });
// error.message → "users '123' not found"
// error.table   → "users"
// error.id      → "123"

3. Variant with cause — extractErrorMessage inside the factory

import { extractErrorMessage } from 'wellcrafted/error';

export const FfmpegError = defineErrors({
  CompressFailed: ({ cause }: { cause: unknown }) => ({
    message: `Failed to compress audio: ${extractErrorMessage(cause)}`,
    cause,
  }),
  VerifyFailed: ({ cause }: { cause: unknown }) => ({
    message: `Failed to verify temp file: ${extractErrorMessage(cause)}`,
    cause,
  }),
});
export type FfmpegError = InferErrors<typeof FfmpegError>;

// Call site — pass the raw caught error, never call extractErrorMessage here
catch: (error) => FfmpegError.CompressFailed({ cause: error }),

4. Multiple variants in one object — discriminated union built-in

export const DeviceStreamError = defineErrors({
  PermissionDenied: ({ cause }: { cause: unknown }) => ({
    message: `Microphone permission denied. ${extractErrorMessage(cause)}`,
    cause,
  }),
  DeviceConnectionFailed: ({
    deviceId,
    cause,
  }: {
    deviceId: string;
    cause: unknown;
  }) => ({
    message: `Unable to connect to device '${deviceId}'. ${extractErrorMessage(cause)}`,
    deviceId,
    cause,
  }),
  NoDevicesFound: () => ({
    message: "No microphones found. Check your connections and try again.",
  }),
});
export type DeviceStreamError = InferErrors<typeof DeviceStreamError>;
// DeviceStreamError is automatically the union of all three variants

// Extracting a single variant type
type NoDevicesFoundError = InferError<typeof DeviceStreamError.NoDevicesFound>;

5. Domain errors with specific operation failures

export const FsError = defineErrors({
  ReadFailed: ({ path, cause }: { path: string; cause: unknown }) => ({
    message: `Failed to read '${path}': ${extractErrorMessage(cause)}`,
    path,
    cause,
  }),
  WriteFailed: ({ path, cause }: { path: string; cause: unknown }) => ({
    message: `Failed to write '${path}': ${extractErrorMessage(cause)}`,
    path,
    cause,
  }),
  DeleteFailed: ({ path, cause }: { path: string; cause: unknown }) => ({
    message: `Failed to delete '${path}': ${extractErrorMessage(cause)}`,
    path,
    cause,
  }),
});
export type FsError = InferErrors<typeof FsError>;

// Call site
return FsError.ReadFailed({ path: '/tmp/foo.txt', cause: error });

Type Extraction

// Full union type for all variants
type HttpError = InferErrors<typeof HttpError>;

// Single variant type
type ConnectionError = InferError<typeof HttpError.Connection>;

Anti-Patterns

// WRONG — old createTaggedError API
import { createTaggedError } from 'wellcrafted/error';
const { FooError, FooErr } = createTaggedError('FooError')
  .withContext<{ id: string }>()
  .withMessage(({ context }) => `Not found: ${context.id}`);

// WRONG — calling extractErrorMessage at the call site
catch: (error) => MyError.Failed({ message: extractErrorMessage(error) });
// CORRECT — pass raw cause, call extractErrorMessage inside the factory
catch: (error) => MyError.Failed({ cause: error });

// WRONG — one defineErrors per variant (defeats the namespace grouping)
const BusyError = defineErrors({ BusyError: () => ({ message: 'Busy' }) });
const PermError = defineErrors({ PermError: () => ({ message: 'No perm' }) });
// CORRECT — all variants for a domain in one call
const RecorderError = defineErrors({
  Busy: () => ({ message: 'A recording is already in progress' }),
  PermissionDenied: () => ({ message: 'Microphone permission denied' }),
});

// WRONG — using ReturnType instead of InferErrors
type FooError = ReturnType<typeof FooError>;
// CORRECT
type FooError = InferErrors<typeof FooError>;

// WRONG — using separate Err/FooErr pair (old API)
FooErr({ context: { id: '1' } });
// CORRECT — each variant call returns Err(...) automatically
FooError.NotFound({ id: '1' });

// WRONG — generic "Service" variant name (says nothing about the failure mode)
const RecorderError = defineErrors({
  Service: ({ message }: { message: string }) => ({ message }),
});
// RecorderError.Service({ message: '...' }) — "Service" is not a failure mode
// CORRECT — name each variant by what actually went wrong
const RecorderError = defineErrors({
  AlreadyRecording: () => ({ message: 'A recording is already in progress' }),
  PermissionDenied: ({ cause }: { cause: unknown }) => ({
    message: `Microphone permission denied. ${extractErrorMessage(cause)}`,
    cause,
  }),
  DeviceNotFound: ({ deviceId }: { deviceId: string }) => ({
    message: `Device not found: ${deviceId}`,
    deviceId,
  }),
});

// WRONG — generic catch-all with operation string (hides failure modes behind a parameter)
const FfmpegError = defineErrors({
  Service: ({ operation, cause }: { operation: string; cause: unknown }) => ({
    message: `Failed to ${operation}: ${extractErrorMessage(cause)}`,
    operation,
    cause,
  }),
});
// FfmpegError.Service({ operation: 'compress audio', cause }) — variant name is meaningless
// CORRECT — each operation is its own variant
const FfmpegError = defineErrors({
  CompressFailed: ({ cause }: { cause: unknown }) => ({
    message: `Failed to compress audio: ${extractErrorMessage(cause)}`,
    cause,
  }),
  VerifyFailed: ({ cause }: { cause: unknown }) => ({
    message: `Failed to verify temp file: ${extractErrorMessage(cause)}`,
    cause,
  }),
});

// WRONG — monolithic single-variant error for a domain with many failure modes
const RecorderError = defineErrors({
  Error: ({ message }: { message: string }) => ({ message }), // Too vague
});
// CORRECT — split by failure mode
const RecorderError = defineErrors({
  AlreadyRecording: () => ({ message: 'A recording is already in progress' }),
  InitFailed: ({ cause }: { cause: unknown }) => ({
    message: `Failed to initialize recorder: ${extractErrorMessage(cause)}`,
    cause,
  }),
  StreamAcquisition: ({ cause }: { cause: unknown }) => ({
    message: `Failed to acquire recording stream: ${extractErrorMessage(cause)}`,
    cause,
  }),
});