Effect by Example: Hello World

Tags:

core

The most basic effect program

import { 
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect } from "effect";


const 
const main: Effect.Effect<void, never, never>
main = 
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const sync: <void>(thunk: LazyArg<void>) => Effect.Effect<void, never, never>
Creates an Effect that represents a synchronous side-effectful computation.


Details


The provided function (thunk) must not throw errors; if it does, the error
will be treated as a "defect".


This defect is not a standard error but indicates a flaw in the logic that
was expected to be error-free. You can think of it similar to an unexpected
crash in the program, which can be further managed or logged using tools like


catchAllDefect


.


When to Use


Use this function when you are sure the operation will not fail.


Example (Logging a Message)


import { Effect } from "effect"


const log = (message: string) =>
  Effect.sync(() => {
    console.log(message) // side effect
  })


//      ┌─── Effect<void, never, never>
//      ▼
const program = log("Hello, World!")
@see ― try_try for a version that can handle failures.
@since ― 2.0.0
sync(() => 
var console: Console
The console module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.


The module exports two specific components:


    

  • A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
    • 

  • A global console instance configured to write to process.stdout and
process.stderr. The global console can be used without importing the node:console module.
    • 



Warning: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the note on process I/O for
more information.


Example using the global console:


console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3


const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr


Example using the Console class:


const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);


myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err


const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
@see ― source
console.
Console.log(...data: any[]): void (+1 overload)
Prints to stdout with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to util.format()).


const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout


See util.format() for more information.
@since ― v0.1.100
log("Hello World"));


import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const runSync: <void, never>(effect: Effect.Effect<void, never, never>) => void
Executes an effect synchronously, running it immediately and returning the
result.


Details


This function evaluates the provided effect synchronously, returning its
result directly. It is ideal for effects that do not fail or include
asynchronous operations. If the effect does fail or involves async tasks, it
will throw an error. Execution stops at the point of failure or asynchronous
operation, making it unsuitable for effects that require asynchronous
handling.


Important: Attempting to run effects that involve asynchronous operations
or failures will result in exceptions being thrown, so use this function with
care for purely synchronous and error-free effects.


When to Use


Use this function when:


    

  • You are sure that the effect will not fail or involve asynchronous
operations.
    • 

  • You need a direct, synchronous result from the effect.
    • 

  • You are working within a context where asynchronous effects are not
allowed.
    • 



Avoid using this function for effects that can fail or require asynchronous
handling. For such cases, consider using


runPromise


or


runSyncExit


.


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


Example (Incorrect Usage with Failing or Async Effects)


import { Effect } from "effect"


try {
  // Attempt to run an effect that fails
  Effect.runSync(Effect.fail("my error"))
} catch (e) {
  console.error(e)
}
// Output:
// (FiberFailure) Error: my error


try {
  // Attempt to run an effect that involves async work
  Effect.runSync(Effect.promise(() => Promise.resolve(1)))
} catch (e) {
  console.error(e)
}
// Output:
// (FiberFailure) AsyncFiberException: Fiber #0 cannot be resolved synchronously. This is caused by using runSync on an effect that performs async work
@see ― runSyncExit for a version that returns an Exit type instead of
throwing an error.
@since ― 2.0.0
runSync(
const main: Effect.Effect<void, never, never>
main);