Running Effects

To execute an effect, you can use one of the many run functions provided by the Effect module.

Running Effects at the Program's Edge

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.

runSync

The Effect.runSync function executes an effect synchronously, meaning it runs immediately and returns the result.

Example (Synchronous Logging)

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

Effect.runSync will throw an error if the effect fails or if it involves any asynchronous operations. In such cases, execution will stop at the point where the asynchronous task occurs:

import { Effect } from "effect"

Effect.runSync(Effect.fail("my error"))
/*
throws:
[Error: my error] {
  name: '(FiberFailure) Error',
  [Symbol(effect/Runtime/FiberFailure/Cause)]: { _tag: 'Fail', error: 'my error' }
  ...
}
*/

Effect.runSync(Effect.promise(() => Promise.resolve(1)))
/*
throws:
[AsyncFiberException: Fiber #0 cannot be resolved synchronously. This is caused by using runSync on an effect that performs async work] {
  name: '(FiberFailure) AsyncFiberException',
  [Symbol(effect/Runtime/FiberFailure/Cause)]: {
    _tag: 'Die',
    defect: [Fiber #0 cannot be resolved synchronously. This is caused by using runSync on an effect that performs async work] {
      fiber: FiberRuntime {
        ...
      },
      _tag: 'AsyncFiberException',
      name: 'AsyncFiberException'
    }
  }
  ...
}
*/

runSyncExit

Effect.runSyncExit runs an effect synchronously, returning the result as an Exit, which represents the outcome (success or failure) of the effect.

Example (Handling Results as Exit)

import { Effect } from "effect"

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

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

If the effect involves any asynchronous operations, Effect.runSyncExit will return an Exit with a Die cause, indicating that the effect can’t be resolved synchronously.

Example (Asynchronous Operation Resulting in Die)

import { Effect } from "effect"

console.log(Effect.runSyncExit(Effect.promise(() => Promise.resolve(1))))
/*
Output:
{
  _id: 'Exit',
  _tag: 'Failure',
  cause: {
    _id: 'Cause',
    _tag: 'Die',
    defect: [Fiber #0 cannot be resolved synchronously. This is caused by using runSync on an effect that performs async work] {
      fiber: [FiberRuntime],
      _tag: 'AsyncFiberException',
      name: 'AsyncFiberException'
    }
  }
}
*/

runPromise

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

Example (Running an Effect as a Promise)

import { Effect } from "effect"

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

If the effect fails, Effect.runPromise will reject with an error:

import { Effect } from "effect"

Effect.runPromise(Effect.fail("my error"))
/*
throws:
[Error: my error] {
  name: '(FiberFailure) Error',
  [Symbol(effect/Runtime/FiberFailure/Cause)]: {
    _tag: 'Fail',
    error: 'my error'
  }
  ...
}
*/

runPromiseExit

The Effect.runPromiseExit function runs an effect and returns a Promise that resolves to an Exit, which represents the outcome (success or failure) of the effect.

Example (Handling Results as Exit)

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 is the foundational function for running effects. It starts a fiber that can be observed or interrupted. All other run functions are built on top of Effect.runFork.

The Default for Effect Execution

Unless you specifically need a Promise or synchronous operation, Effect.runFork is a good default choice.

Example (Running an Effect with a Fiber)

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.

Synchronous vs. Asynchronous Effects

In the Effect library, there is no built-in way to determine in advance whether an effect will execute synchronously or asynchronously. While this idea was considered in earlier versions of Effect, it was ultimately not implemented for a few important reasons:

1. Complexity: Introducing this feature to track sync/async behavior in the type system would make Effect more complex to use and limit its composability.

2. Safety Concerns: We experimented with different approaches to track asynchronous Effects, but they all resulted in a worse developer experience without significantly improving safety. Even with fully synchronous types, we needed to support a fromCallback combinator to work with APIs using Continuation-Passing Style (CPS). However, at the type level, it’s impossible to guarantee that such a function is always called immediately and not deferred.

Best Practices for Running Effects

In most cases, effects are run at the outermost parts of your application. Typically, an application built around Effect will involve a single call to the main effect. Here’s how you should approach effect execution:

• Use runPromise or runFork: For most cases, asynchronous execution should be the default. These methods provide the best way to handle Effect-based workflows.

• Use runSync only when necessary: Synchronous execution should be considered an edge case, used only in scenarios where asynchronous execution is not feasible. For example, when you are sure the effect is purely synchronous and need immediate results.

Cheatsheet

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.

API	Given	Result
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.