Agent skill

define-errors

How to define and use defineErrors from wellcrafted. Use when creating new error types, updating error definitions, or reviewing error patterns. Covers variant factories, extractErrorMessage for cause, InferErrors/InferError for types, and call site patterns.

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npx add-skill https://github.com/EpicenterHQ/epicenter/tree/main/.agents/skills/define-errors

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Additional technical details for this skill

author
epicenter
version
3.0

SKILL.md

defineErrors

Related Skills: See error-handling for trySync/tryAsync usage. 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

typescript
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

Patterns

1. Simple variant — no input, static message

typescript
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

typescript
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

typescript
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

typescript
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

typescript
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

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

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

Anti-Patterns

typescript
// 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,
  }),
});

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