Importing Effect

Effect's modules are already in scope in .effeck files: the prelude, and every module that effect exports from its root, such as Console, Option, Stream, and Schedule. Import modules from effect's subpath packages, such as effect/http, as you need them.

Install the core package

npm install effect@4.0.5

The core package supplies Effect's data types, schedules, concurrency tools, and other modules. See Installation for platform packages, effeck, and effeck-runtime.

Module and namespace imports

function pauseBeforeRetry() {
  const started = Clock.currentTimeMillis()
  sleep(Duration.millis(25))
  return started
}

const backoff = Schedule.exponential("50 millis")

Effect, the Effect<A, E, R> type, Schema, Layer, Scope, TaggedError, and Control need no import. In a contract, Scope means Effect's Scope.Scope, as in File!IOError!Scope. The prelude also includes pipe, wire, fork, handle, resume, resumeWith, and abandon; the boundary hatches sync, try, and callback; the runners runPromise, runFork, runSync, and runCallback; and the Effect module's most used functions without the Effect. prefix: sleep, log, logDebug, logInfo, logWarning, logError, retry, repeat, timeout, all, forEach, race, withSpan, and die. The other Effect functions keep the prefix, as in Effect.result(work()). Clock, Duration, Schedule, and every other module of effect's root need no import either, and neither do flow and identity.

JavaScript's own globals keep their names. Array, String, Number, Boolean, Symbol, BigInt, Function, and RegExp mean JavaScript's, so import Effect's modules with those names under another name:

import { Array as Arr } from "effect"

function firstTwo(values: ReadonlyArray<number>) {
  return Arr.take(values, 2)
}

A host's globals do not shadow Effect's modules: with the DOM in your lib, Option is still Effect's. An import or a local declaration with the same name as a module shadows it, as import * as Duration from "effect/Duration" would. Value constructors such as Duration.millis build data; Clock.currentTimeMillis() and Effect.sleep(...) run computations. Effect's own packages are trusted, so their data functions need no foreign-call boundary; exceptions from those functions are defects.

Compose standalone functions

function expirationWindow() {
  const short = Duration.seconds(15)
  return short.toMillis()
}
// number!!

Effect exposes operations as module functions. Call them with their data argument or use their curried form in pipe where available. An Effect pipeline starts with a program, as in pipe(() => work(), Effect.timeout("1 second")).

Bundling unused code

Named module imports and direct namespace imports both expose the library without adding methods to your objects. Bundlers can eliminate unused exports when their module analysis supports it. Check the generated bundle when size matters; the import spelling alone does not guarantee a particular size.

Effeck's global names are source conveniences. They refer to Effect and language support code, which still participates in the emitted module graph.

Reusable operators

function withinSecond<A, E, R>(program: () => A!E!R) {
  return timeout(program, "1 second")
}

function example() {
  return withinSecond {
    sleep("10 millis")
    return "ready"
  }
}

A helper can accept a program and apply library operators to it. Its contract preserves the program's errors and services and adds TimeoutError here. You can extend your application without changing Effect's prototypes.

Coming from Effect

Skip imports from effect's root in .effeck. In .ts integration files, import Effect, Layer, and the other names you use as usual.

Upstream Effect page