Running Effects
On this page
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.
ts
import {Effect } from "effect"constprogram =Effect .sync (() => {console .log ("Hello, World!")return 1})constresult =Effect .runSync (program )// Output: Hello, World!console .log (result )// Output: 1
ts
import {Effect } from "effect"constprogram =Effect .sync (() => {console .log ("Hello, World!")return 1})constresult =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.
ts
import {Effect } from "effect"Effect .runSync (Effect .fail ("my error")) // throwsEffect .runSync (Effect .promise (() =>Promise .resolve (1))) // throws
ts
import {Effect } from "effect"Effect .runSync (Effect .fail ("my error")) // throwsEffect .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).
ts
import {Effect } from "effect"constresult1 =Effect .runSyncExit (Effect .succeed (1))console .log (result1 )/*Output:{_id: "Exit",_tag: "Success",value: 1}*/constresult2 =Effect .runSyncExit (Effect .fail ("my error"))console .log (result2 )/*Output:{_id: "Exit",_tag: "Failure",cause: {_id: "Cause",_tag: "Fail",failure: "my error"}}*/
ts
import {Effect } from "effect"constresult1 =Effect .runSyncExit (Effect .succeed (1))console .log (result1 )/*Output:{_id: "Exit",_tag: "Success",value: 1}*/constresult2 =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.
ts
import {Effect } from "effect"Effect .runSyncExit (Effect .promise (() =>Promise .resolve (1))) // throws
ts
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
.
ts
import {Effect } from "effect"Effect .runPromise (Effect .succeed (1)).then (console .log ) // Output: 1
ts
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
ts
import {Effect } from "effect"Effect .runPromise (Effect .fail ("my error")) // rejects
ts
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).
ts
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"}}*/
ts
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.
ts
import {Effect ,Console ,Schedule ,Fiber } from "effect"constprogram =Effect .repeat (Console .log ("running..."),Schedule .spaced ("200 millis"))constfiber =Effect .runFork (program )setTimeout (() => {Effect .runFork (Fiber .interrupt (fiber ))}, 500)
ts
import {Effect ,Console ,Schedule ,Fiber } from "effect"constprogram =Effect .repeat (Console .log ("running..."),Schedule .spaced ("200 millis"))constfiber =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.