400-running-effects.mdx

title	excerpt	bottomNavigation
Running Effects	Explore various "run" functions in the Effect module to execute effects. Learn about `runSync` for synchronous execution, `runSyncExit` for obtaining results as `Exit`, `runPromise` for executing with a Promise result, and `runPromiseExit` for Promise results with `Exit`. Understand their use cases and considerations. Check out a cheatsheet summarizing available functions for executing effects in different contexts.	pagination

To execute an Effect, we can utilize a variety of "run" functions provided by the Effect module.

runSync

The Effect.runSync function is used to execute an Effect synchronously, which means it runs immediately and returns the result.

import { Effect } from "effect"

const program = Effect.sync(() => {
  console.log("Hello, World!")
  return 1
})

const result = Effect.runSync(program)
// Output: Hello, World!

console.log(result)
// Output: 1

If you check the console, you will see the message "Hello, World!" printed.

`Effect.runSync` will throw an error if your Effect fails or performs any asynchronous tasks. In the latter case, the execution will not proceed beyond that asynchronous task.

import { Effect } from "effect"

Effect.runSync(Effect.fail("my error")) // throws

Effect.runSync(Effect.promise(() => Promise.resolve(1))) // throws

runSyncExit

The Effect.runSyncExit function is used to execute an Effect synchronously, which means it runs immediately and returns the result as an Exit (a data type used to describe the result of executing an Effect workflow).

import { Effect } from "effect"

const result1 = Effect.runSyncExit(Effect.succeed(1))
console.log(result1)
/*
Output:
{
  _id: "Exit",
  _tag: "Success",
  value: 1
}
*/

const result2 = Effect.runSyncExit(Effect.fail("my error"))
console.log(result2)
/*
Output:
{
  _id: "Exit",
  _tag: "Failure",
  cause: {
    _id: "Cause",
    _tag: "Fail",
    failure: "my error"
  }
}
*/

`Effect.runSyncExit` will throw an error if your Effect performs any asynchronous tasks and the execution will not proceed beyond that asynchronous task.

import { Effect } from "effect"

Effect.runSyncExit(Effect.promise(() => Promise.resolve(1))) // throws

runPromise

The Effect.runPromise function is used to execute an Effect and obtain the result as a Promise.

import { Effect } from "effect"

Effect.runPromise(Effect.succeed(1)).then(console.log) // Output: 1

`Effect.runPromise` will reject with an error if your Effect fails

import { Effect } from "effect"

Effect.runPromise(Effect.fail("my error")) // rejects

runPromiseExit

The Effect.runPromiseExit function is used to execute an Effect and obtain the result as a Promise that resolves to an Exit (a data type used to describe the result of executing an Effect workflow).

import { Effect } from "effect"

Effect.runPromiseExit(Effect.succeed(1)).then(console.log)
/*
Output:
{
  _id: "Exit",
  _tag: "Success",
  value: 1
}
*/

Effect.runPromiseExit(Effect.fail("my error")).then(console.log)
/*
Output:
{
  _id: "Exit",
  _tag: "Failure",
  cause: {
    _id: "Cause",
    _tag: "Fail",
    failure: "my error"
  }
}
*/

runFork

The Effect.runFork function serves as a foundational building block for running effects. In fact, all other run functions are built upon it. Unless you have a specific need for a Promise or a synchronous operation, Effect.runFork is the recommended choice. It returns a fiber that you can observe or interrupt as needed.

import { Effect, Console, Schedule, Fiber } from "effect"

const program = Effect.repeat(
  Console.log("running..."),
  Schedule.spaced("200 millis")
)

const fiber = Effect.runFork(program)

setTimeout(() => {
  Effect.runFork(Fiber.interrupt(fiber))
}, 500)

In this example, the program continuously logs "running..." with each repetition spaced 200 milliseconds apart. You can learn more about repetitions and scheduling in our Introduction to Scheduling guide.

To stop the execution of the program, we use Fiber.interrupt on the fiber returned by Effect.runFork. This allows you to control the execution flow and terminate it when necessary.

For a deeper understanding of how fibers work and how to handle interruptions, check out our guides on Fibers and Interruptions.

Cheatsheet

The recommended approach is to design your program with the majority of its logic as Effects. It's advisable to use the `run*` functions closer to the "edge" of your program. This approach allows for greater flexibility in executing your program and building sophisticated Effects.

The table provides a summary of the available run* functions, along with their input and output types, allowing you to choose the appropriate function based on your needs.

Name	Given	To
runSync	Effect<A, E>	A
runSyncExit	Effect<A, E>	Exit<A, E>
runPromise	Effect<A, E>	Promise<A>
runPromiseExit	Effect<A, E>	Promise<Exit<A, E>>
runFork	Effect<A, E>	RuntimeFiber<A, E>

You can find the complete list of run* functions here.