Running Effects

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

runSync

Executes an effect synchronously, running it immediately and returning the result.

Example (Synchronous Logging)

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import { Effect } from "effect"
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const program = Effect.sync(() => {
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  console.log("Hello, World!")
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  return 1
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})
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const result = Effect.runSync(program)
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// Output: Hello, World!
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console.log(result)
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// Output: 1

Use Effect.runSync to run an effect that does not fail and does not include any asynchronous operations. If the effect fails or involves asynchronous work, it will throw an error, and execution will stop where the failure or async operation occurs.

Example (Incorrect Usage with Failing or Async Effects)

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import { Effect } from "effect"
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try {
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  // Attempt to run an effect that fails
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  Effect.runSync(Effect.fail("my error"))
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} catch (e) {
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  console.error(e)
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}
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/*
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Output:
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(FiberFailure) Error: my error
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*/
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try {
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  // Attempt to run an effect that involves async work
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  Effect.runSync(Effect.promise(() => Promise.resolve(1)))
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} catch (e) {
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  console.error(e)
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}
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/*
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Output:
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(FiberFailure) AsyncFiberException: Fiber #0 cannot be resolved synchronously. This is caused by using runSync on an effect that performs async work
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*/

runSyncExit

Runs an effect synchronously and returns the result as an Exit type, which represents the outcome (success or failure) of the effect.

Use Effect.runSyncExit to find out whether an effect succeeded or failed, including any defects, without dealing with asynchronous operations.

The Exit type represents the result of the effect:

• If the effect succeeds, the result is wrapped in a Success.
• If it fails, the failure information is provided as a Failure containing a Cause type.

Example (Handling Results as Exit)

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import { Effect } from "effect"
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console.log(Effect.runSyncExit(Effect.succeed(1)))
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/*
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Output:
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{
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  _id: "Exit",
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  _tag: "Success",
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  value: 1
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}
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*/
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console.log(Effect.runSyncExit(Effect.fail("my error")))
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/*
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Output:
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{
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  _id: "Exit",
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  _tag: "Failure",
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  cause: {
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    _id: "Cause",
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    _tag: "Fail",
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    failure: "my error"
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  }
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}
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*/

If the effect contains asynchronous operations, Effect.runSyncExit will return an Failure with a Die cause, indicating that the effect cannot be resolved synchronously.

Example (Asynchronous Operation Resulting in Die)

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

runPromise

Executes an effect and returns the result as a Promise.

Use Effect.runPromise when you need to execute an effect and work with the result using Promise syntax, typically for compatibility with other promise-based code.

Example (Running a Successful Effect as a Promise)

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import { Effect } from "effect"
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Effect.runPromise(Effect.succeed(1)).then(console.log)
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// Output: 1

If the effect succeeds, the promise will resolve with the result. If the effect fails, the promise will reject with an error.

Example (Handling a Failing Effect as a Rejected Promise)

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import { Effect } from "effect"
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Effect.runPromise(Effect.fail("my error")).catch(console.error)
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/*
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Output:
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(FiberFailure) Error: my error
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*/

runPromiseExit

Runs an effect and returns a Promise that resolves to an Exit, which represents the outcome (success or failure) of the effect.

Use Effect.runPromiseExit when you need to determine if an effect succeeded or failed, including any defects, and you want to work with a Promise.

The Exit type represents the result of the effect:

• If the effect succeeds, the result is wrapped in a Success.
• If it fails, the failure information is provided as a Failure containing a Cause type.

Example (Handling Results as Exit)

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import { Effect } from "effect"
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Effect.runPromiseExit(Effect.succeed(1)).then(console.log)
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/*
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Output:
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{
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  _id: "Exit",
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  _tag: "Success",
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  value: 1
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}
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*/
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Effect.runPromiseExit(Effect.fail("my error")).then(console.log)
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/*
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Output:
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{
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  _id: "Exit",
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  _tag: "Failure",
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  cause: {
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    _id: "Cause",
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    _tag: "Fail",
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    failure: "my error"
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  }
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}
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*/

runFork

The foundational function for running effects, returning a “fiber” that can be observed or interrupted.

Effect.runFork is used to run an effect in the background by creating a fiber. It is the base function for all other run functions. It starts a fiber that can be observed or interrupted.

Example (Running an Effect in the Background)

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import { Effect, Console, Schedule, Fiber } from "effect"
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//      ┌─── Effect<number, never, never>
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//      ▼
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const program = Effect.repeat(Console.log("running..."), Schedule.spaced("200 millis"))
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//      ┌─── RuntimeFiber<number, never>
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//      ▼
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const fiber = Effect.runFork(program)
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setTimeout(() => {
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  Effect.runFork(Fiber.interrupt(fiber))
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}, 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.

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