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

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

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]

// 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

// 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"))

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")
)

// 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
}

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

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

// 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

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

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

// 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

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" })

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,
  })
)

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)

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