effect-ts-errors

Effect-TS Error Handling

Overview

Effect-TS provides a type-safe error channel (E in Effect<A, E, R>) that tracks potential failures. Use Tagged Errors for distinguishability and specialized combinators for handling or accumulating errors.

When to Use

• Defining custom error types for domain logic.
• Handling specific errors without catching everything.
• Validating multiple fields and collecting all failures.

When NOT to use:

• Simple try/catch for non-Effect code.
• When error types don't need to be distinguished (use Effect.fail(message)).

Core Pattern

Before (Generic Errors):

if (!user) throw new Error("Not found");

After (Tagged Errors):

if (!user) return yield* new UserNotFoundError({ userId: id });

Quick Reference

Feature	Method	Purpose
Define Error	Data.TaggedError	Create distinguishable, typed errors.
Catch One	Effect.catchTag	Handle a specific tagged error by its _tag.
Catch All	Effect.catchAll	Handle all errors in the channel.
Accumulate	Effect.all(..., { mode: 'either' })	Collect multiple results/errors into Either[].

Implementation

1. Tagged Errors

import { Data, Effect } from 'effect';

export class UserNotFoundError extends Data.TaggedError('UserNotFoundError')<{
  userId: string;
}> {}

export class DatabaseError extends Data.TaggedError('DatabaseError')<{
  message: string;
}> {}

const getUser = (id: string): Effect.Effect<User, UserNotFoundError | DatabaseError> =>
  Effect.gen(function* () {
    const result = yield* queryDatabase(id);
    if (!result) return yield* new UserNotFoundError({ userId: id });
    return result;
  });

2. Catching Specific Errors

const handled = getUser(id).pipe(
  // catchTag only works on objects with a _tag property
  Effect.catchTag('UserNotFoundError', (e) => 
    Effect.succeed({ id: e.userId, name: 'Guest' })
  )
);
// Result: Effect<User, DatabaseError>

3. Error Accumulation

import { Either, Effect } from 'effect';

const validateAll = (data: Input) =>
  Effect.gen(function* () {
    const results = yield* Effect.all([
      validateEmail(data.email),
      validateAge(data.age)
    ], { mode: 'either' });
    
    const errors = results.filter(Either.isLeft).map(e => e.left);
    if (errors.length > 0) return yield* Effect.fail(errors);
    return results.map(e => (e as Either.Right<any, any>).right);
  });

Common Mistakes

• Using throw: Breaks the error channel. Always use Effect.fail or yield* new TaggedError().
• Generic catchAll: Swallows errors you didn't intend to handle. Prefer catchTag.
• Short-circuiting: Effect.all stops at the first error by default. Use { mode: 'either' } or { mode: 'validate' } for accumulation.

REQUIRED BACKGROUND: Use effect-ts-fundamentals for core concepts.