Effect-TS Expert

Expert guidance for functional programming with the Effect library, covering error handling, dependency injection, composability, and testing patterns.

Prerequisites Check

Before starting any Effect-related work, verify the Effect-TS source code exists at ~/.effect .

If missing, stop immediately and inform the user. Clone it before proceeding:

git clone https://github.com/Effect-TS/effect.git ~/.effect

Research Strategy

Effect-TS has many ways to accomplish the same task. Proactively research best practices using the Task tool to spawn research agents when working with Effect patterns, especially for moderate to high complexity tasks.

Research Sources (Priority Order)

Codebase Patterns First — Examine similar patterns in the current project before implementing. If Effect patterns exist in the codebase, follow them for consistency. If no patterns exist, skip this step.

Effect Source Code — For complex type errors, unclear behavior, or implementation details, examine the Effect source at ~/.effect/packages/effect/src/ . This contains the core Effect logic and modules.

When to Research

HIGH Priority (Always Research):

• Implementing Services, Layers, or complex dependency injection

• Error handling with multiple error types or complex error hierarchies

• Stream-based operations and reactive patterns

• Resource management with scoped effects and cleanup

• Concurrent/parallel operations and performance-critical code

• Testing patterns, especially unfamiliar test scenarios

MEDIUM Priority (Research if Complex):

• Refactoring imperative code (try-catch, promises) to Effect patterns

• Adding new service dependencies or restructuring service layers

• Custom error types or extending existing error hierarchies

• Integrations with external systems (databases, APIs, third-party services)

Research Approach

• Spawn multiple concurrent Task agents when investigating multiple related patterns

• Focus on finding canonical, readable, and maintainable solutions rather than clever optimizations

• Verify suggested approaches against existing codebase patterns for consistency (if patterns exist)

• When multiple approaches are possible, research to find the most idiomatic Effect-TS solution

Codebase Pattern Discovery

When working in a project that uses Effect, check for existing patterns before implementing new code:

• Search for Effect imports — Look for files importing from 'effect' to understand existing usage

• Identify service patterns — Find how Services and Layers are structured in the project

• Note error handling conventions — Check how errors are defined and propagated

• Examine test patterns — Look at how Effect code is tested in the project

If no Effect patterns exist in the codebase, proceed using canonical patterns from the Effect source and examples. Do not block on missing codebase patterns.

Effect Principles

Apply these core principles when writing Effect code:

Error Handling

• Use Effect's typed error system instead of throwing exceptions

• Define descriptive error types with proper error propagation

• Use Effect.fail , Effect.catchTag , Effect.catchAll for error control flow

• See ./references/critical-rules.md for forbidden patterns

Dependency Injection

• Implement dependency injection using Services and Layers

• Define services with Context.Tag

• Compose layers with Layer.merge , Layer.provide

• Use Effect.provide to inject dependencies

Composability

• Leverage Effect's composability for complex operations

• Use appropriate constructors: Effect.succeed , Effect.fail , Effect.tryPromise , Effect.try

• Apply proper resource management with scoped effects

• Chain operations with Effect.flatMap , Effect.map , Effect.tap

Code Quality

• Write type-safe code that leverages Effect's type system

• Use Effect.gen for readable sequential code

• Implement proper testing patterns using Effect's testing utilities

• Prefer Effect.fn() for automatic telemetry and better stack traces

Critical Rules

Read and internalize ./references/critical-rules.md before writing any Effect code. Key guidelines:

• INEFFECTIVE: try-catch in Effect.gen (Effect failures aren't thrown)

• AVOID: Type assertions (as never/any/unknown)

• RECOMMENDED: return yield* pattern for errors (makes termination explicit)

Common Failure Modes

Quick links to patterns that frequently cause issues:

• SubscriptionRef version mismatch — unsafeMake is not a function → Quick Reference

• Cancellation vs Failure — Interrupts aren't errors → Error Taxonomy

• Option vs null — Use Option internally, null at boundaries → option-null.md

• Stream backpressure — Infinite streams hang → streams.md

Explaining Solutions

When providing solutions, explain the Effect-TS concepts being used and why they're appropriate for the specific use case. If encountering patterns not covered in the documentation, suggest improvements while maintaining consistency with existing codebase patterns (when they exist).

Quick Reference

Creating Effects

Effect.succeed(value) // Wrap success value Effect.fail(error) // Create failed effect Effect.tryPromise(fn) // Wrap promise-returning function Effect.try(fn) // Wrap synchronous throwing function Effect.sync(fn) // Wrap synchronous non-throwing function

Composing Effects

Effect.flatMap(effect, fn) // Chain effects Effect.map(effect, fn) // Transform success value Effect.tap(effect, fn) // Side effect without changing value Effect.all([...effects]) // Run effects (concurrency configurable) Effect.forEach(items, fn) // Map over items with effects

// Collect ALL errors (not just first) Effect.all([e1, e2, e3], { mode: "validate" }) // Returns all failures

// Partial success handling Effect.partition([e1, e2, e3]) // Returns [failures, successes]

Error Handling

// Define typed errors with Data.TaggedError (preferred) class UserNotFoundError extends Data.TaggedError("UserNotFoundError")<{ userId: string }> {}

// Direct yield of errors (no Effect.fail wrapper needed) Effect.gen(function* () { if (!user) { return yield* new UserNotFoundError({ userId }) } })

Effect.catchTag(effect, tag, fn) // Handle specific error tag Effect.catchAll(effect, fn) // Handle all errors Effect.result(effect) // Convert to Exit value Effect.orElse(effect, alt) // Fallback effect

Error Taxonomy

Categorize errors for appropriate handling:

Category Examples Handling

Expected Rejections User cancel, deny Graceful exit, no retry

Domain Errors Validation, business rules Show to user, don't retry

Defects Bugs, assertions Log + alert, investigate

Interruptions Fiber cancel, timeout Cleanup, may retry

Unknown/Foreign Thrown exceptions Normalize at boundary

// Pattern: Normalize unknown errors at boundary const safeBoundary = Effect.catchAllDefect(effect, (defect) => Effect.fail(new UnknownError({ cause: defect })) )

// Pattern: Catch user-initiated cancellations separately Effect.catchTag(effect, "UserCancelledError", () => Effect.succeed(null))

// Pattern: Handle interruptions differently from failures Effect.onInterrupt(effect, () => Effect.log("Operation cancelled"))

Pattern Matching (Match Module)

Default branching tool for tagged unions and complex conditionals.

import { Match } from "effect"

// Type-safe exhaustive matching on tagged errors const handleError = Match.type<AppError>().pipe( Match.tag("UserCancelledError", () => null), // Expected rejection Match.tag("ValidationError", (e) => e.message), // Domain error Match.tag("NetworkError", () => "Connection failed"), // Retryable Match.exhaustive // Compile error if case missing )

// Replace nested catchTag chains // BEFORE: effect.pipe(catchTag("A", ...), catchTag("B", ...), catchTag("C", ...)) // AFTER: Effect.catchAll(effect, (error) => Match.value(error).pipe( Match.tag("A", handleA), Match.tag("B", handleB), Match.tag("C", handleC), Match.exhaustive ) )

// Match on values (cleaner than if/else) const describe = Match.value(status).pipe( Match.when("pending", () => "Loading..."), Match.when("success", () => "Done!"), Match.orElse(() => "Unknown") )

Services and Layers

// Pattern 1: Context.Tag (implementation provided separately via Layer) class MyService extends Context.Tag("MyService")<MyService, { ... }>() {} const MyServiceLive = Layer.succeed(MyService, { ... }) Effect.provide(effect, MyServiceLive)

// Pattern 2: Effect.Service (default implementation bundled) class UserRepo extends Effect.Service<UserRepo>()("UserRepo", { effect: Effect.gen(function* () { const db = yield* Database return { findAll: db.query("SELECT * FROM users") } }), dependencies: [Database.Default], // Optional service dependencies accessors: true // Auto-generate method accessors }) {} Effect.provide(effect, UserRepo.Default) // .Default layer auto-generated // Use UserRepo.DefaultWithoutDependencies when deps provided separately

// Effect.Service with parameters (3.16.0+) class ConfiguredApi extends Effect.Service<ConfiguredApi>()("ConfiguredApi", { effect: (config: { baseUrl: string }) => Effect.succeed({ fetch: (path: string) => ${config.baseUrl}/${path} }) }) {}

// Pattern 3: Context.Reference (defaultable tags - 3.11.0+) class SpecialNumber extends Context.Reference<SpecialNumber>()( "SpecialNumber", { defaultValue: () => 2048 } ) {} // No Layer required if default value suffices

// Pattern 4: Context.ReadonlyTag (covariant - 3.18.0+) // Use for functions that consume services without modifying the type function effectHandler<I, A, E, R>(service: Context.ReadonlyTag<I, Effect.Effect<A, E, R>>) { // Handler can use service in a covariant position }

Generator Pattern

Effect.gen(function* () { const a = yield* effectA; const b = yield* effectB; if (error) { return yield* Effect.fail(new MyError()); } return result; });

// Effect.fn - automatic tracing and telemetry (preferred for named functions) const fetchUser = Effect.fn("fetchUser")(function* (id: string) { const db = yield* Database return yield* db.query(id) }) // Creates spans, captures call sites, provides better stack traces

Resource Management

Effect.acquireUseRelease(acquire, use, release) // Bracket pattern Effect.scoped(effect) // Scope lifetime to effect Effect.addFinalizer(cleanup) // Register cleanup action

Duration

Effect accepts human-readable duration strings anywhere a DurationInput is expected:

// String syntax (preferred) - singular or plural forms work Duration.toMillis("5 minutes") // 300000 Duration.toMillis("1 minute") // 60000 Duration.toMillis("30 seconds") // 30000 Duration.toMillis("100 millis") // 100

// Verbose syntax (avoid) Duration.toMillis(Duration.minutes(5)) // Same result, more verbose

// Common units: millis, seconds, minutes, hours, days, weeks // Also: nanos, micros

Scheduling

Effect.retry(effect, Schedule.exponential("100 millis")) // Retry with backoff Effect.repeat(effect, Schedule.fixed("1 second")) // Repeat on schedule Schedule.compose(s1, s2) // Combine schedules

State Management

Ref.make(initialValue) // Mutable reference Ref.get(ref) // Read value Ref.set(ref, value) // Write value Deferred.make<E, A>() // One-time async value

SubscriptionRef (Reactive References)

// WARNING: Never use unsafeMake - it may not exist in your Effect version. // If you see "unsafeMake is not a function", use the safe API below.

SubscriptionRef.make(initial) // Create reactive reference (safe) SubscriptionRef.get(ref) // Read current value SubscriptionRef.set(ref, value) // Update value (notifies subscribers) SubscriptionRef.changes(ref) // Stream of value changes

// React integration (effect-atom pattern) const ref = yield* SubscriptionRef.make<User | null>(null) // Hook reads: useSubscriptionRef(ref) — returns current value or null // Handle null explicitly in components

Concurrency

Effect.fork(effect) // Run in background fiber Fiber.join(fiber) // Wait for fiber result Effect.race(effect1, effect2) // First to complete wins Effect.all([...effects], { concurrency: "unbounded" })

Configuration & Environment Variables

import { Config, ConfigProvider, Effect, Layer, Redacted } from "effect"

// Basic config values const port = Config.number("PORT") // Required number const host = Config.string("HOST").pipe( // Optional with default Config.withDefault("localhost") )

// Sensitive values (masked in logs) const apiKey = Config.redacted("API_KEY") // Returns Redacted<string> const secret = Redacted.value(yield* apiKey) // Unwrap when needed

// Nested configuration with prefix const dbConfig = Config.all({ host: Config.string("HOST"), port: Config.number("PORT"), name: Config.string("NAME"), }).pipe(Config.nested("DATABASE")) // DATABASE_HOST, DATABASE_PORT, etc.

// Using config in effects const program = Effect.gen(function* () { const p = yield* Config.number("PORT") const key = yield* Config.redacted("API_KEY") return { port: p, apiKey: Redacted.value(key) } })

// Custom config provider (e.g., from object instead of env) const customProvider = ConfigProvider.fromMap( new Map([["PORT", "3000"], ["API_KEY", "secret"]]) ) const withCustomConfig = Effect.provide( program, Layer.setConfigProvider(customProvider) )

// Config validation and transformation const validPort = Config.number("PORT").pipe( Config.validate({ message: "Port must be between 1 and 65535", validation: (n) => n >= 1 && n <= 65535, }) )

Array Operations

import { Array as Arr, Order } from "effect"

// Sorting with built-in orderings (accepts any Iterable) Arr.sort([3, 1, 2], Order.number) // [1, 2, 3] Arr.sort(["b", "a", "c"], Order.string) // ["a", "b", "c"] Arr.sort(new Set([3n, 1n, 2n]), Order.bigint) // [1n, 2n, 3n]

// Sort by derived value Arr.sortWith(users, (u) => u.age, Order.number)

// Sort by multiple criteria Arr.sortBy( users, Order.mapInput(Order.number, (u: User) => u.age), Order.mapInput(Order.string, (u: User) => u.name) )

// Built-in orderings: Order.string, Order.number, Order.bigint, Order.boolean, Order.Date // Reverse ordering: Order.reverse(Order.number)

Utility Functions

import { constVoid as noop } from "effect/Function"

// constVoid returns undefined, useful as a no-operation callback noop() // undefined

// Common use cases: Effect.tap(effect, noop) // Ignore value, just run effect Promise.catch(noop) // Swallow errors eventEmitter.on("event", noop) // Register empty handler

Deprecations

• BigDecimal.fromNumber — Use BigDecimal.unsafeFromNumber instead (3.11.0+)

• Schema.annotations() — Now removes previously set identifier annotations; identifiers are tied to the schema's ast reference only (3.17.10)

Additional Resources

Local Effect Resources

• ~/.effect/packages/effect/src/ — Core Effect modules and implementation

External Resources

• Effect-Atom — https://github.com/tim-smart/effect-atom (open in browser for reactive state management patterns)

Reference Files

• ./references/critical-rules.md — Forbidden patterns and mandatory conventions

• ./references/effect-atom.md — Effect-Atom reactive state management for React

• ./references/next-js.md — Effect + Next.js 15+ App Router integration patterns

• ./references/option-null.md — Option vs null boundary patterns

• ./references/streams.md — Stream patterns and backpressure gotchas

• ./references/testing.md — Vitest deterministic testing patterns