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 → runtime.md

• Cancellation vs Failure — Interrupts aren't errors → [Error Taxonomy](#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)

When you need to use Effect's Match module for pattern matching, see references/pattern-matching.md.

Services and Layers / Generator Pattern

For service definition patterns (Context.Tag, Effect.Service, Context.Reference, Context.ReadonlyTag) and the generator pattern (Effect.gen, Effect.fn), see references/services-layers.md.

Runtime Patterns (Resource Management, Duration, Scheduling, State, SubscriptionRef, Concurrency)

For resource lifecycles, durations, scheduling, state management, reactive refs, and concurrency primitives, see references/runtime.md.

Configuration & Environment Variables

When you need to read configuration with Config, handle secrets via Redacted, or wire custom config providers, see references/config.md.

Quick Utilities (Array Operations, Utility Functions, Deprecations)

For Effect's Array/Order sorting helpers, small utility functions like constVoid, and the running list of deprecations, see references/quick-utils.md.

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/config.md** — Config, Redacted, and custom config providers

• **./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/pattern-matching.md** — Match module for tagged unions and conditionals

• **./references/quick-utils.md** — Array/Order, utility helpers, deprecations

• **./references/runtime.md** — Resource management, Duration, Scheduling, State, SubscriptionRef, Concurrency

• **./references/services-layers.md** — Services, Layers, generator (Effect.gen / Effect.fn)

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

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