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What is an Effect?

An Effect is a description of a program that may produce a value of type A, fail with an error of type E, or require services of type R. It’s a lazy, composable way to model side effects and computations. The type signature is Effect<A, E, R> where:
  • A is the success type
  • E is the error type
  • R is the required services/context

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Writing Effect Code with Effect.gen

Prefer writing Effect code with Effect.gen for an imperative style similar to async/await. Use yield* to access the result of an effect. 
import { Effect, Schema } from "effect"

Effect.gen(function*() {
  yield* Effect.log("Starting the file processing...")
  yield* Effect.log("Reading file...")

  // Always return when raising an error, to ensure typescript understands that
  // the function will not continue executing.
  return yield* new FileProcessingError({ message: "Failed to read the file" })
}).pipe(
  // Add additional functionality with .pipe
  Effect.catch((error) => Effect.logError(`An error occurred: ${error}`))
)

// Use Schema.TaggedErrorClass to define a custom error
export class FileProcessingError extends Schema.TaggedErrorClass<FileProcessingError>()("FileProcessingError", {
  message: Schema.String
}) {}

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Writing Effect Functions with Effect.fn

When writing functions that return an Effect, use Effect.fn to use the generator syntax. Avoid creating functions that return an Effect.gen, use Effect.fn instead. 
import { Effect, Schema } from "effect"

// Pass a string to Effect.fn, which will improve stack traces and also
// attach a tracing span (using Effect.withSpan behind the scenes).
//
// The name string should match the function name.
//
export const effectFunction = Effect.fn("effectFunction")(
  // You can use `Effect.fn.Return` to specify the return type of the function.
  // It accepts the same type parameters as `Effect.Effect`.
  function*(n: number): Effect.fn.Return<string, SomeError> {
    yield* Effect.logInfo("Received number:", n)

    // Always return when raising an error, to ensure typescript understands that
    // the function will not continue executing.
    return yield* new SomeError({ message: "Failed to read the file" })
  },
  // Add additional functionality by passing in additional arguments
  Effect.catch((error) => Effect.logError(`An error occurred: ${error}`)),
  Effect.annotateLogs({
    method: "effectFunction"
  })
)

// Use Schema.TaggedErrorClass to define a custom error
export class SomeError extends Schema.TaggedErrorClass<SomeError>()("SomeError", {
  message: Schema.String
}) {}

​
Yielding Effects with yield*

Inside Effect.gen or Effect.fn, use yield* to unwrap the value from an Effect. This is similar to await with Promises. 
import { Effect } from "effect"

const program = Effect.gen(function*() {
  // yield* unwraps the Effect and gives you the value
  const timestamp = yield* Effect.sync(() => Date.now())
  yield* Effect.logInfo("Current timestamp:", timestamp)
  
  const config = yield* Effect.succeed({ env: "prod", retries: 3 })
  yield* Effect.logInfo("Config:", config)
  
  return timestamp
})

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Creating Effects from Common Sources

Effect provides multiple constructors for creating effects from different sources:

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From Plain Values

import { Effect } from "effect"

// `Effect.succeed` wraps values you already have in memory.
export const fromValue = Effect.succeed({ env: "prod", retries: 3 })

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From Synchronous Code

import { Effect } from "effect"

// `Effect.sync` wraps synchronous side effects that should not throw.
export const fromSyncSideEffect = Effect.sync(() => Date.now())

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From Code That May Throw

import { Effect, Schema } from "effect"

class InvalidPayload extends Schema.TaggedErrorClass<InvalidPayload>()("InvalidPayload", {
  input: Schema.String,
  cause: Schema.Defect
}) {}

// `Effect.try` wraps synchronous code that may throw.
export const parsePayload = Effect.fn("parsePayload")((input: string) =>
  Effect.try({
    try: () => JSON.parse(input) as { readonly userId: number },
    catch: (cause) => new InvalidPayload({ input, cause })
  })
)

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From Promise APIs

import { Effect, Schema } from "effect"

class UserLookupError extends Schema.TaggedErrorClass<UserLookupError>()("UserLookupError", {
  userId: Schema.Number,
  cause: Schema.Defect
}) {}

const users = new Map<number, { readonly id: number; readonly name: string }>([
  [1, { id: 1, name: "Ada" }],
  [2, { id: 2, name: "Lin" }]
])

// `Effect.tryPromise` wraps Promise-based APIs that can reject or throw.
export const fetchUser = Effect.fn("fetchUser")((userId: number) =>
  Effect.tryPromise({
    async try() {
      const user = users.get(userId)
      if (!user) {
        throw new Error(`Missing user ${userId}`)
      }
      return user
    },
    catch: (cause) => new UserLookupError({ userId, cause })
  })
)

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From Nullable Values

import { Effect, Schema } from "effect"

class MissingWorkspaceId extends Schema.TaggedErrorClass<MissingWorkspaceId>()("MissingWorkspaceId", {}) {}

const requestHeaders = new Map<string, string>([
  ["x-request-id", "req_1"]
])

// `Effect.fromNullishOr` turns nullable values into a typed effect.
export const fromNullishHeader = Effect.fromNullishOr(requestHeaders.get("x-workspace-id")).pipe(
  Effect.mapError(() => new MissingWorkspaceId())
)

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From Callback APIs

import { Effect } from "effect"

// `Effect.callback` wraps callback-style asynchronous APIs.
export const fromCallback = Effect.callback<number>((resume) => {
  const timeoutId = setTimeout(() => {
    resume(Effect.succeed(200))
  }, 10)

  // Return a finalizer so interruption can cancel the callback source.
  return Effect.sync(() => {
    clearTimeout(timeoutId)
  })
})

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Best Practices

  • Use Effect.gen for imperative-style code blocks
  • Use Effect.fn for reusable Effect functions (not Effect.gen)
  • Always return when raising an error to help TypeScript’s control flow analysis
  • Attach additional behavior with .pipe() and combinators
  • Name your Effect.fn functions to improve stack traces and tracing

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