Overview

Master type-safe error handling in Effect applications. This skill covers expected errors, error recovery, selective error handling, and error transformations using Effect's error management operators.

Expected Errors vs Defects

Effect distinguishes between two types of failures:

Expected Errors (E channel): Recoverable errors tracked in the type system
Defects: Unexpected failures (bugs, programming errors)

typescript

import { Effect } from "effect"

// Expected error - tracked in type
interface ValidationError {
  _tag: "ValidationError"
  field: string
  message: string
}

const validateEmail = (email: string): Effect.Effect<string, ValidationError, never> => {
  if (!email.includes("@")) {
    return Effect.fail({
      _tag: "ValidationError",
      field: "email",
      message: "Invalid email format"
    })
  }
  return Effect.succeed(email)
}

// Defect - throws, becomes unexpected failure
const riskyOperation = Effect.sync(() => {
  throw new Error("Unexpected error") // This is a defect
})

// Proper way - expected error
const safeOperation = Effect.try({
  try: () => {
    // Code that might throw
    return riskyParse(data)
  },
  catch: (error) => ({
    _tag: "ParseError",
    message: String(error)
  })
})

Tagged Error Types

Use tagged unions for error types to enable pattern matching:

typescript

import { Effect } from "effect"

// Define tagged error types
interface NotFoundError {
  _tag: "NotFoundError"
  id: string
}

interface UnauthorizedError {
  _tag: "UnauthorizedError"
  userId: string
}

interface NetworkError {
  _tag: "NetworkError"
  message: string
}

type AppError = NotFoundError | UnauthorizedError | NetworkError

// Functions returning typed errors
const fetchUser = (id: string): Effect.Effect<User, NotFoundError | NetworkError, never> => {
  // Implementation
}

const authenticate = (token: string): Effect.Effect<User, UnauthorizedError | NetworkError, never> => {
  // Implementation
}

Catching All Errors

Effect.catchAll - Recover from Any Error

Catches all expected errors and provides fallback:

typescript

import { Effect } from "effect"

const program = Effect.gen(function* () {
  const user = yield* fetchUser("123")
  return user
}).pipe(
  Effect.catchAll((error) =>
    Effect.succeed({ id: "default", name: "Guest" })
  )
)
// Effect<User, never, never> - Error channel is now never

// With error inspection
const programWithLogging = Effect.gen(function* () {
  const user = yield* fetchUser("123")
  return user
}).pipe(
  Effect.catchAll((error) => {
    console.error("Error occurred:", error)
    return Effect.succeed(defaultUser)
  })
)

// Fallback to another effect
const programWithFallback = pipe(
  fetchUser("123"),
  Effect.catchAll(() => fetchUserFromCache("123"))
)

Selective Error Handling

Effect.catchTag - Handle Specific Error Types

Catches errors by their _tag field:

typescript

import { Effect, pipe } from "effect"

const program = pipe(
  fetchUser("123"),
  Effect.catchTag("NotFoundError", (error) =>
    Effect.succeed({ id: error.id, name: "Not Found" })
  )
)
// Still can fail with NetworkError

// Handling multiple tags
const program2 = pipe(
  authenticatedRequest(),
  Effect.catchTag("UnauthorizedError", (error) =>
    Effect.fail({ _tag: "LoginRequired" })
  ),
  Effect.catchTag("NetworkError", (error) =>
    retryRequest()
  )
)

// Using Effect.gen with early return
const program3 = Effect.gen(function* () {
  const result = yield* riskyOperation().pipe(
    Effect.catchTag("TemporaryError", () =>
      Effect.succeed(null)
    )
  )
  return result
})

Effect.catchTags - Handle Multiple Error Types

typescript

import { Effect, pipe } from "effect"

const program = pipe(
  complexOperation(),
  Effect.catchTags({
    NotFoundError: (error) =>
      Effect.succeed(defaultValue),
    UnauthorizedError: (error) =>
      Effect.fail({ _tag: "LoginRequired" }),
    NetworkError: (error) =>
      retryOperation()
  })
)

// With different recovery strategies
const programWithStrategies = pipe(
  processPayment(amount),
  Effect.catchTags({
    InsufficientFunds: (error) =>
      Effect.fail({ _tag: "PaymentDeclined", reason: "insufficient-funds" }),
    NetworkError: () =>
      retryPayment(amount),
    ValidationError: (error) =>
      Effect.fail({ _tag: "InvalidPayment", field: error.field })
  })
)

Effect.catchIf - Conditional Error Handling

Catches errors that match a predicate:

typescript

import { Effect, pipe } from "effect"

const isRetryable = (error: AppError): boolean => {
  return error._tag === "NetworkError" || error._tag === "TimeoutError"
}

const program = pipe(
  fetchData(),
  Effect.catchIf(isRetryable, (error) =>
    retryFetchData()
  )
)

// With type narrowing
const program2 = pipe(
  operation(),
  Effect.catchIf(
    (error): error is NetworkError => error._tag === "NetworkError",
    (error) => {
      // TypeScript knows error is NetworkError here
      console.log("Network error:", error.message)
      return retry()
    }
  )
)

Effect.catchSome - Partial Error Handling

Catches errors and optionally handles them:

typescript

import { Effect, Option, pipe } from "effect"

const program = pipe(
  fetchUser("123"),
  Effect.catchSome((error) => {
    if (error._tag === "NotFoundError") {
      return Option.some(Effect.succeed(guestUser))
    }
    return Option.none() // Don't handle, propagate error
  })
)

// Complex decision logic
const programWithDecision = pipe(
  processRequest(request),
  Effect.catchSome((error) => {
    if (error._tag === "RateLimitError" && error.retryAfter < 1000) {
      return Option.some(
        Effect.sleep(error.retryAfter).pipe(
          Effect.andThen(processRequest(request))
        )
      )
    }
    return Option.none()
  })
)

Converting Errors

Effect.either - Convert to Either<Success, Error>

Transforms an effect into one that cannot fail, wrapping result in Either:

typescript

import { Effect, Either } from "effect"

const program = Effect.gen(function* () {
  const result = yield* fetchUser("123").pipe(Effect.either)

  if (Either.isLeft(result)) {
    // Handle error
    console.error("Error:", result.left)
    return null
  } else {
    // Handle success
    return result.right
  }
})
// Effect<User | null, never, never>

// Pattern matching on Either
const program2 = pipe(
  fetchUser("123"),
  Effect.either,
  Effect.map(
    Either.match({
      onLeft: (error) => ({ success: false, error }),
      onRight: (user) => ({ success: true, data: user })
    })
  )
)

Effect.option - Convert to Option<Success>

Converts failures to None, success to Some:

typescript

import { Effect, Option } from "effect"

const program = Effect.gen(function* () {
  const maybeUser = yield* fetchUser("123").pipe(Effect.option)

  if (Option.isNone(maybeUser)) {
    return guestUser
  } else {
    return maybeUser.value
  }
})
// Effect<User, never, never>

// Using Option.match
const program2 = pipe(
  fetchUser("123"),
  Effect.option,
  Effect.map(
    Option.match({
      onNone: () => "No user found",
      onSome: (user) => `Found: ${user.name}`
    })
  )
)

Error Transformation

Effect.mapError - Transform Error Types

typescript

import { Effect, pipe } from "effect"

interface DbError {
  _tag: "DbError"
  code: string
  message: string
}

interface AppError {
  _tag: "AppError"
  message: string
  context: string
}

const program = pipe(
  queryDatabase(),
  Effect.mapError((dbError: DbError): AppError => ({
    _tag: "AppError",
    message: dbError.message,
    context: `Database operation failed: ${dbError.code}`
  }))
)

// Enriching errors with context
const enrichError = <E extends { message: string }>(
  context: string
) => (error: E) => ({
  ...error,
  message: `${context}: ${error.message}`
})

const programWithContext = pipe(
  fetchData(),
  Effect.mapError(enrichError("Failed to fetch user data"))
)

Effect.tapError - Side Effects on Error

Perform side effects when an error occurs without changing it:

typescript

import { Effect, pipe } from "effect"

const program = pipe(
  processPayment(amount),
  Effect.tapError((error) =>
    Effect.sync(() => {
      console.error("Payment failed:", error)
      logToMonitoring(error)
    })
  ),
  Effect.tapError((error) =>
    sendErrorNotification(error)
  )
)
// Error still propagates after taps

Retry and Fallback Patterns

Effect.orElse - Fallback Effect

Provide alternative effect on failure:

typescript

import { Effect, pipe } from "effect"

const program = pipe(
  fetchFromPrimarySource(),
  Effect.orElse(() => fetchFromSecondarySource())
)

// With error-specific fallbacks
const programWithCheck = pipe(
  fetchData(),
  Effect.orElse((error) => {
    if (error._tag === "NetworkError") {
      return fetchFromCache()
    }
    return Effect.fail(error)
  })
)

// Multiple fallbacks
const programWithMultipleFallbacks = pipe(
  fetchFromPrimary(),
  Effect.orElse(() => fetchFromSecondary()),
  Effect.orElse(() => fetchFromTertiary()),
  Effect.orElse(() => Effect.succeed(defaultData))
)

Effect.retry - Retry on Failure

typescript

import { Effect, Schedule, pipe } from "effect"

// Retry with schedule
const program = pipe(
  fetchData(),
  Effect.retry(Schedule.recurs(3)) // Retry up to 3 times
)

// Exponential backoff
const programWithBackoff = pipe(
  fetchData(),
  Effect.retry(
    Schedule.exponential("100 millis", 2.0) // 100ms, 200ms, 400ms, ...
  )
)

// Conditional retry
const programConditionalRetry = pipe(
  fetchData(),
  Effect.retry({
    while: (error) => error._tag === "NetworkError",
    schedule: Schedule.recurs(5)
  })
)

Combining Error Handlers

Chaining Multiple Handlers

typescript

import { Effect, pipe } from "effect"

const program = pipe(
  complexOperation(),
  Effect.catchTag("NotFoundError", () =>
    Effect.succeed(defaultValue)
  ),
  Effect.catchTag("NetworkError", () =>
    retryOperation()
  ),
  Effect.catchTag("UnauthorizedError", () =>
    Effect.fail({ _tag: "LoginRequired" })
  ),
  Effect.catchAll((unknownError) =>
    Effect.sync(() => {
      console.error("Unhandled error:", unknownError)
      return fallbackValue
    })
  )
)

Error Accumulation

typescript

import { Effect, Array } from "effect"

interface ValidationError {
  _tag: "ValidationError"
  errors: string[]
}

const validateAll = (fields: string[]) =>
  Effect.gen(function* () {
    const results = yield* Effect.all(
      fields.map(validateField),
      { mode: "either" } // Don't short-circuit on first error
    )

    const errors = results.filter(Either.isLeft)
    if (errors.length > 0) {
      return yield* Effect.fail({
        _tag: "ValidationError",
        errors: errors.map(e => e.left.message)
      })
    }

    return results.map(r => r.right)
  })

Handling Defects

Effect.catchAllCause - Handle Both Errors and Defects

typescript

import { Effect, Cause, Exit, pipe } from "effect"

const program = pipe(
  riskyOperation(),
  Effect.catchAllCause((cause) => {
    if (Cause.isFailure(cause)) {
      // Expected error
      const error = Cause.failureOption(cause)
      return handleExpectedError(error)
    } else if (Cause.isDie(cause)) {
      // Defect (unexpected error)
      const defect = Cause.dieOption(cause)
      return handleDefect(defect)
    } else {
      // Interruption
      return Effect.succeed(defaultValue)
    }
  })
)

Effect.sandbox - Expose Defects as Errors

Converts defects into the error channel for handling:

typescript

import { Effect, Cause, pipe } from "effect"

const program = pipe(
  riskyOperation(),
  Effect.sandbox,
  Effect.catchAll((cause) => {
    console.error("Failure cause:", cause)
    return Effect.succeed(fallbackValue)
  })
)

Best Practices

Use Tagged Error Types: Always tag errors with _tag for catchTag.
Keep Error Types Specific: Don't use generic Error. Define specific error types for each failure mode.
Handle Errors Close to Source: Catch errors where you have enough context to handle them properly.
Use catchTag Over catchAll: Prefer specific error handling to blanket catching.
Convert at Boundaries: Use either/option when interfacing with code that doesn't expect errors.
Log Before Catching: Use tapError to log before handling errors.
Don't Swallow Errors: Always handle errors meaningfully or propagate them.
Use Retry Strategically: Only retry transient failures, not all errors.
Enrich Errors with Context: Add context to errors as they propagate up.
Document Error Types: Clearly document what errors each function can produce.

Common Pitfalls

Using catchAll Everywhere: Over-using catchAll hides error types. Use catchTag.
Not Tagging Errors: Without tags, you can't use catchTag effectively.
Swallowing Errors: Catching errors and returning success without proper handling.
Infinite Retry: Not limiting retries or checking error types before retrying.
Losing Error Information: Transforming errors without preserving important details.
Not Handling Defects: Forgetting that some operations can throw unexpectedly.
Wrong Error Boundaries: Catching errors too early or too late in the pipeline.
Type Widening: Losing specific error types by combining with catchAll too early.
Ignoring Error Channel: Not checking the E type parameter when composing effects.
Not Testing Error Paths: Only testing happy paths, not failure scenarios.

When to Use This Skill

Use effect-error-handling when you need to:

Handle expected errors in a type-safe manner
Recover from failures with fallback logic
Implement retry strategies for transient failures
Transform errors between different layers
Log errors without stopping execution
Implement error boundaries in applications
Handle specific error types differently
Convert errors to Option or Either
Accumulate validation errors
Build resilient error recovery systems

Resources

Official Documentation

Related Skills

effect-core-patterns - Basic Effect operations
effect-testing - Testing error scenarios

📖 effect-error-handling

Overview

Expected Errors vs Defects

Tagged Error Types

Catching All Errors

Effect.catchAll - Recover from Any Error

Selective Error Handling

Effect.catchTag - Handle Specific Error Types

Effect.catchTags - Handle Multiple Error Types

Effect.catchIf - Conditional Error Handling

Effect.catchSome - Partial Error Handling

Converting Errors

Effect.either - Convert to Either<Success, Error>

Effect.option - Convert to Option<Success>

Error Transformation

Effect.mapError - Transform Error Types

Effect.tapError - Side Effects on Error

Retry and Fallback Patterns

Effect.orElse - Fallback Effect

Effect.retry - Retry on Failure

Combining Error Handlers

Chaining Multiple Handlers

Error Accumulation

Handling Defects

Effect.catchAllCause - Handle Both Errors and Defects

Effect.sandbox - Expose Defects as Errors

Best Practices

Common Pitfalls

When to Use This Skill

Resources

Official Documentation

Related Skills