ts4023-effect-errors

name	ts4023-effect-errors
description	Fix TS4023 errors when exporting Effect-based functions. Use when TypeScript reports "has or is using name 'X' from external module but cannot be named" for Effect error types, or when knip flags error type exports as unused.

TS4023 with Effect Error Types

Problem

Exporting function returning Effect → TypeScript generates .d.ts → error types in Effect's error channel not exported from source package → TS4023.

TS4023 message is misleading: mentions internal Effect types (Channel, Sink, Stream from effect/Cause), not actual missing errors.

Solution

Export ALL error types that appear in any Effect's error channel - including non-exported class definitions.

1. Find ALL TaggedError classes (not just exported ones)

# Find Data.TaggedError and Schema.TaggedError (both can appear in Effect error channels)
rg "class \w+Error extends (Data|Schema)\.TaggedError" packages/<package-name>/src

Critical: Include classes WITHOUT export keyword. Example:

// This ALSO needs to be exported if used in any Effect's error channel
class EmptyComponentSetError extends Data.TaggedError('EmptyComponentSetError')<{...}> {}

2. For non-exported errors, add export to source file first

// Before
class EmptyComponentSetError extends Data.TaggedError('EmptyComponentSetError')<{...}> {}

// After
export class EmptyComponentSetError extends Data.TaggedError('EmptyComponentSetError')<{...}> {}

3. Then export from index.ts

export type { EmptyComponentSetError } from './core/componentSetService';

4. Verify

npx tsc --build packages/<package-name> --force

Why non-exported errors matter

If a service method like ensureNonEmptyComponentSet can fail with EmptyComponentSetError, that error type appears in the Effect's error channel. Any exported function calling that method inherits the error in its type signature. TypeScript needs to name it in .d.ts.

Knip false positives

Knip flags these as "unused exports" when the error class is defined and used within the same file but exported only for TS4023 reasons. Ignore these warnings - TypeScript needs the exports for declaration emit, not runtime imports.

Errors exported for cross-package consumption (e.g. re-exported from a service package's index.ts and imported by another package) are NOT false positives - knip correctly sees them as used, so leave them as-is.

Checklist

• rg "class.*TaggedError" - find ALL errors (with AND without export)
• Add export to any non-exported error classes used in Effect chains
• Add export type { ErrorName } to services index.ts
• npx tsc --build <package> --force passes
• Ignore knip "unused" warnings for these exports