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Effect Documentation

SynchronizedRef

SynchronizedRef<A> serves as a mutable reference to a value of type A. With it, we can store immutable data and perform updates atomically and effectfully.

The distinctive function in SynchronizedRef is updateEffect. This function takes an effectful operation and executes it to modify the shared state. This is the key feature setting SynchronizedRef apart from Ref.

In real-world applications, SynchronizedRef is useful when you need to execute effects, such as querying a database, and then update shared state based on the result. It ensures that updates happen sequentially, preserving consistency in concurrent environments.

Example (Concurrent Updates with SynchronizedRef)

In this example, we simulate fetching user ages concurrently and updating a shared state that stores the ages:

import { 
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect, 
import SynchronizedRef
SynchronizedRef } from "effect"


// Simulated API to get user age
const 
const getUserAge: (userId: number) => Effect.Effect<number, never, never>
getUserAge = (
userId: number
userId: number) =>
  
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const succeed: <number>(value: number) => Effect.Effect<number, never, never>
Creates an Effect that always succeeds with a given value.


When to Use


Use this function when you need an effect that completes successfully with a
specific value without any errors or external dependencies.
@see ― fail to create an effect that represents a failure.
@example 
// Title: Creating a Successful Effect
import { Effect } from "effect"


// Creating an effect that represents a successful scenario
//
//      ┌─── Effect<number, never, never>
//      ▼
const success = Effect.succeed(42)
@since ― 2.0.0
succeed(
userId: number
userId * 10).
Pipeable.pipe<Effect.Effect<number, never, never>, Effect.Effect<number, never, never>>(this: Effect.Effect<...>, ab: (_: Effect.Effect<number, never, never>) => Effect.Effect<number, never, never>): Effect.Effect<...> (+21 overloads)
pipe(
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const delay: (duration: DurationInput) => <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R> (+1 overload)
Returns an effect that is delayed from this effect by the specified
Duration.
@since ― 2.0.0
delay(10 - 
userId: number
userId))


const 
const meanAge: Effect.Effect<number[], never, never>
meanAge = 
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const gen: <YieldWrap<Effect.Effect<SynchronizedRef.SynchronizedRef<number[]>, never, never>> | YieldWrap<Effect.Effect<number[], never, never>> | YieldWrap<...>, number[]>(f: (resume: Effect.Adapter) => Generator<...>) => Effect.Effect<...> (+1 overload)
Provides a way to write effectful code using generator functions, simplifying
control flow and error handling.


When to Use


gen allows you to write code that looks and behaves like synchronous
code, but it can handle asynchronous tasks, errors, and complex control flow
(like loops and conditions). It helps make asynchronous code more readable
and easier to manage.


The generator functions work similarly to async/await but with more
explicit control over the execution of effects. You can yield* values from
effects and return the final result at the end.
@example 
import { Effect } from "effect"


const addServiceCharge = (amount: number) => amount + 1


const applyDiscount = (
  total: number,
  discountRate: number
): Effect.Effect<number, Error> =>
  discountRate === 0
    ? Effect.fail(new Error("Discount rate cannot be zero"))
    : Effect.succeed(total - (total * discountRate) / 100)


const fetchTransactionAmount = Effect.promise(() => Promise.resolve(100))


const fetchDiscountRate = Effect.promise(() => Promise.resolve(5))


export const program = Effect.gen(function* () {
  const transactionAmount = yield* fetchTransactionAmount
  const discountRate = yield* fetchDiscountRate
  const discountedAmount = yield* applyDiscount(
    transactionAmount,
    discountRate
  )
  const finalAmount = addServiceCharge(discountedAmount)
  return `Final amount to charge: ${finalAmount}`
})
@since ― 2.0.0
gen(function* () {
  // Initialize a SynchronizedRef to hold an array of ages
  const 
const ref: SynchronizedRef.SynchronizedRef<number[]>
ref = yield* 
import SynchronizedRef
SynchronizedRef.
const make: <number[]>(value: number[]) => Effect.Effect<SynchronizedRef.SynchronizedRef<number[]>, never, never>
@since ― 2.0.0
make<number[]>([])


  // Helper function to log state before each effect
  const 
const log: <R, E, A>(label: string, effect: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>
log = <
function (type parameter) R in <R, E, A>(label: string, effect: Effect.Effect<A, E, R>): Effect.Effect<A, E, R>
R, 
function (type parameter) E in <R, E, A>(label: string, effect: Effect.Effect<A, E, R>): Effect.Effect<A, E, R>
E, 
function (type parameter) A in <R, E, A>(label: string, effect: Effect.Effect<A, E, R>): Effect.Effect<A, E, R>
A>(
label: string
label: string, 
effect: Effect.Effect<A, E, R>
effect: 
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
interface Effect<out A, out E = never, out R = never>
The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.


Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.


To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
@since ― 2.0.0
@since ― 2.0.0
Effect<
function (type parameter) A in <R, E, A>(label: string, effect: Effect.Effect<A, E, R>): Effect.Effect<A, E, R>
A, 
function (type parameter) E in <R, E, A>(label: string, effect: Effect.Effect<A, E, R>): Effect.Effect<A, E, R>
E, 
function (type parameter) R in <R, E, A>(label: string, effect: Effect.Effect<A, E, R>): Effect.Effect<A, E, R>
R>) =>
    
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const gen: <YieldWrap<Effect.Effect<A, E, R>> | YieldWrap<Effect.Effect<void, never, never>>, A>(f: (resume: Effect.Adapter) => Generator<...>) => Effect.Effect<...> (+1 overload)
Provides a way to write effectful code using generator functions, simplifying
control flow and error handling.


When to Use


gen allows you to write code that looks and behaves like synchronous
code, but it can handle asynchronous tasks, errors, and complex control flow
(like loops and conditions). It helps make asynchronous code more readable
and easier to manage.


The generator functions work similarly to async/await but with more
explicit control over the execution of effects. You can yield* values from
effects and return the final result at the end.
@example 
import { Effect } from "effect"


const addServiceCharge = (amount: number) => amount + 1


const applyDiscount = (
  total: number,
  discountRate: number
): Effect.Effect<number, Error> =>
  discountRate === 0
    ? Effect.fail(new Error("Discount rate cannot be zero"))
    : Effect.succeed(total - (total * discountRate) / 100)


const fetchTransactionAmount = Effect.promise(() => Promise.resolve(100))


const fetchDiscountRate = Effect.promise(() => Promise.resolve(5))


export const program = Effect.gen(function* () {
  const transactionAmount = yield* fetchTransactionAmount
  const discountRate = yield* fetchDiscountRate
  const discountedAmount = yield* applyDiscount(
    transactionAmount,
    discountRate
  )
  const finalAmount = addServiceCharge(discountedAmount)
  return `Final amount to charge: ${finalAmount}`
})
@since ― 2.0.0
gen(function* () {
      const 
const value: number[]
value = yield* 
import SynchronizedRef
SynchronizedRef.
const get: <number[]>(self: SynchronizedRef.SynchronizedRef<number[]>) => Effect.Effect<number[], never, never>
@since ― 2.0.0
get(
const ref: SynchronizedRef.SynchronizedRef<number[]>
ref)
      yield* 
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const log: (...message: ReadonlyArray<any>) => Effect.Effect<void, never, never>
Logs one or more messages or error causes at the current log level, which is INFO by default.
This function allows logging multiple items at once and can include detailed error information using Cause instances.


To adjust the log level, use the Logger.withMinimumLogLevel function.
@example 
import { Cause, Effect } from "effect"


const program = Effect.log(
  "message1",
  "message2",
  Cause.die("Oh no!"),
  Cause.die("Oh uh!")
)


// Effect.runFork(program)
// Output:
// timestamp=... level=INFO fiber=#0 message=message1 message=message2 cause="Error: Oh no!
// Error: Oh uh!"
@since ― 2.0.0
log(
label: string
label, 
const value: number[]
value)
      return yield* 
effect: Effect.Effect<A, E, R>
effect
    })


  const 
const task: (id: number) => Effect.Effect<void, never, never>
task = (
id: number
id: number) =>
    
const log: <never, never, void>(label: string, effect: Effect.Effect<void, never, never>) => Effect.Effect<void, never, never>
log(
      `task ${
id: number
id}`,
      
import SynchronizedRef
SynchronizedRef.
const updateEffect: <number[], never, never>(self: SynchronizedRef.SynchronizedRef<number[]>, f: (a: number[]) => Effect.Effect<number[], never, never>) => Effect.Effect<void, never, never> (+1 overload)
@since ― 2.0.0
updateEffect(
const ref: SynchronizedRef.SynchronizedRef<number[]>
ref, (
sumOfAges: number[]
sumOfAges) =>
        
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const gen: <YieldWrap<Effect.Effect<number, never, never>>, number[]>(f: (resume: Effect.Adapter) => Generator<YieldWrap<Effect.Effect<number, never, never>>, number[], never>) => Effect.Effect<...> (+1 overload)
Provides a way to write effectful code using generator functions, simplifying
control flow and error handling.


When to Use


gen allows you to write code that looks and behaves like synchronous
code, but it can handle asynchronous tasks, errors, and complex control flow
(like loops and conditions). It helps make asynchronous code more readable
and easier to manage.


The generator functions work similarly to async/await but with more
explicit control over the execution of effects. You can yield* values from
effects and return the final result at the end.
@example 
import { Effect } from "effect"


const addServiceCharge = (amount: number) => amount + 1


const applyDiscount = (
  total: number,
  discountRate: number
): Effect.Effect<number, Error> =>
  discountRate === 0
    ? Effect.fail(new Error("Discount rate cannot be zero"))
    : Effect.succeed(total - (total * discountRate) / 100)


const fetchTransactionAmount = Effect.promise(() => Promise.resolve(100))


const fetchDiscountRate = Effect.promise(() => Promise.resolve(5))


export const program = Effect.gen(function* () {
  const transactionAmount = yield* fetchTransactionAmount
  const discountRate = yield* fetchDiscountRate
  const discountedAmount = yield* applyDiscount(
    transactionAmount,
    discountRate
  )
  const finalAmount = addServiceCharge(discountedAmount)
  return `Final amount to charge: ${finalAmount}`
})
@since ― 2.0.0
gen(function* () {
          const 
const age: number
age = yield* 
const getUserAge: (userId: number) => Effect.Effect<number, never, never>
getUserAge(
id: number
id)
          return 
sumOfAges: number[]
sumOfAges.
Array<number>.concat(...items: (number | ConcatArray<number>)[]): number[] (+1 overload)
Combines two or more arrays.
This method returns a new array without modifying any existing arrays.
@param ― items Additional arrays and/or items to add to the end of the array.
concat(
const age: number
age)
        })
      )
    )


  // Run tasks concurrently with a limit of 2 concurrent tasks
  yield* 
import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const all: <readonly [Effect.Effect<void, never, never>, Effect.Effect<void, never, never>, Effect.Effect<void, never, never>, Effect.Effect<void, never, never>], {
    ...;
}>(arg: readonly [...], options?: {
    ...;
} | undefined) => Effect.Effect<...>
Combines multiple effects into one, returning results based on the input
structure.


When to Use


Use Effect.all when you need to run multiple effects and combine their
results into a single output. It supports tuples, iterables, structs, and
records, making it flexible for different input types.


For instance, if the input is a tuple:


//         ┌─── a tuple of effects
//         ▼
Effect.all([effect1, effect2, ...])


the effects are executed sequentially, and the result is a new effect
containing the results as a tuple. The results in the tuple match the order
of the effects passed to Effect.all.


Concurrency


You can control the execution order (e.g., sequential vs. concurrent) using
the concurrency option.


Short-Circuiting Behavior


The Effect.all function stops execution on the first error it encounters,
this is called "short-circuiting". If any effect in the collection fails, the
remaining effects will not run, and the error will be propagated. To change
this behavior, you can use the mode option, which allows all effects to run
and collect results as Either or Option.


The mode option


The { mode: "either" } option changes the behavior of Effect.all to
ensure all effects run, even if some fail. Instead of stopping on the first
failure, this mode collects both successes and failures, returning an array
of Either instances where each result is either a Right (success) or a
Left (failure).


Similarly, the { mode: "validate" } option uses Option to indicate
success or failure. Each effect returns None for success and Some with
the error for failure.
@see ― forEach for iterating over elements and applying an effect.
@example 
// Title: Combining Effects in Tuples
import { Effect, Console } from "effect"


const tupleOfEffects = [
  Effect.succeed(42).pipe(Effect.tap(Console.log)),
  Effect.succeed("Hello").pipe(Effect.tap(Console.log))
] as const


//      ┌─── Effect<[number, string], never, never>
//      ▼
const resultsAsTuple = Effect.all(tupleOfEffects)


Effect.runPromise(resultsAsTuple).then(console.log)
// Output:
// 42
// Hello
// [ 42, 'Hello' ]
@example 
// Title: Combining Effects in Iterables
import { Effect, Console } from "effect"


const iterableOfEffects: Iterable<Effect.Effect> = [1, 2, 3].map(
(n) => Effect.succeed(n).pipe(Effect.tap(Console.log))
)


//      ┌─── Effect<number[], never, never>
//      ▼
const resultsAsArray = Effect.all(iterableOfEffects)


Effect.runPromise(resultsAsArray).then(console.log)
// Output:
// 1
// 2
// 3
// [ 1, 2, 3 ]
@example 
// Title: Combining Effects in Structs
import { Effect, Console } from "effect"


const structOfEffects = {
a: Effect.succeed(42).pipe(Effect.tap(Console.log)),
b: Effect.succeed("Hello").pipe(Effect.tap(Console.log))
}


//      ┌─── Effect<{ a: number; b: string; }, never, never>
//      ▼
const resultsAsStruct = Effect.all(structOfEffects)


Effect.runPromise(resultsAsStruct).then(console.log)
// Output:
// 42
// Hello
// { a: 42, b: 'Hello' }
@example 
// Title: Combining Effects in Records
import { Effect, Console } from "effect"


const recordOfEffects: Record<string, Effect.Effect> = {
key1: Effect.succeed(1).pipe(Effect.tap(Console.log)),
key2: Effect.succeed(2).pipe(Effect.tap(Console.log))
}


//      ┌─── Effect<{ [x: string]: number; }, never, never>
//      ▼
const resultsAsRecord = Effect.all(recordOfEffects)


Effect.runPromise(resultsAsRecord).then(console.log)
// Output:
// 1
// 2
// { key1: 1, key2: 2 }
@example 
// Title: Short-Circuiting Behavior
import { Effect, Console } from "effect"


const program = Effect.all([
Effect.succeed("Task1").pipe(Effect.tap(Console.log)),
Effect.fail("Task2: Oh no!").pipe(Effect.tap(Console.log)),
// Won't execute due to earlier failure
Effect.succeed("Task3").pipe(Effect.tap(Console.log))
])


Effect.runPromiseExit(program).then(console.log)
// Output:
// Task1
// {
//   _id: 'Exit',
//   _tag: 'Failure',
//   cause: { _id: 'Cause', _tag: 'Fail', failure: 'Task2: Oh no!' }
// }
@example 
// Title: Collecting Results with mode: "either"
import { Effect, Console } from "effect"


const effects = [
Effect.succeed("Task1").pipe(Effect.tap(Console.log)),
Effect.fail("Task2: Oh no!").pipe(Effect.tap(Console.log)),
Effect.succeed("Task3").pipe(Effect.tap(Console.log))
]


const program = Effect.all(effects, { mode: "either" })


Effect.runPromiseExit(program).then(console.log)
// Output:
// Task1
// Task3
// {
//   _id: 'Exit',
//   _tag: 'Success',
//   value: [
//     { _id: 'Either', _tag: 'Right', right: 'Task1' },
//     { _id: 'Either', _tag: 'Left', left: 'Task2: Oh no!' },
//     { _id: 'Either', _tag: 'Right', right: 'Task3' }
//   ]
// }
@example 
//Example: Collecting Results with mode: "validate"
import { Effect, Console } from "effect"


const effects = [
Effect.succeed("Task1").pipe(Effect.tap(Console.log)),
Effect.fail("Task2: Oh no!").pipe(Effect.tap(Console.log)),
Effect.succeed("Task3").pipe(Effect.tap(Console.log))
]


const program = Effect.all(effects, { mode: "validate" })


Effect.runPromiseExit(program).then((result) => console.log("%o", result))
// Output:
// Task1
// Task3
// {
//   _id: 'Exit',
//   _tag: 'Failure',
//   cause: {
//     _id: 'Cause',
//     _tag: 'Fail',
//     failure: [
//       { _id: 'Option', _tag: 'None' },
//       { _id: 'Option', _tag: 'Some', value: 'Task2: Oh no!' },
//       { _id: 'Option', _tag: 'None' }
//     ]
//   }
// }
@since ― 2.0.0
all([
const task: (id: number) => Effect.Effect<void, never, never>
task(1), 
const task: (id: number) => Effect.Effect<void, never, never>
task(2), 
const task: (id: number) => Effect.Effect<void, never, never>
task(3), 
const task: (id: number) => Effect.Effect<void, never, never>
task(4)], {
    
concurrency: number
concurrency: 2
  })


  // Retrieve the updated value
  const 
const value: number[]
value = yield* 
import SynchronizedRef
SynchronizedRef.
const get: <number[]>(self: SynchronizedRef.SynchronizedRef<number[]>) => Effect.Effect<number[], never, never>
@since ― 2.0.0
get(
const ref: SynchronizedRef.SynchronizedRef<number[]>
ref)
  return 
const value: number[]
value
})


import Effect
@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
const runPromise: <number[], never>(effect: Effect.Effect<number[], never, never>, options?: {
    readonly signal?: AbortSignal;
} | undefined) => Promise<number[]>
Executes an effect and returns the result as a Promise.


When to Use


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


If the effect succeeds, the promise will resolve with the result. If the
effect fails, the promise will reject with an error.
@see ― runPromiseExit for a version that returns an Exit type instead of rejecting.
@example 
// Title: Running a Successful Effect as a Promise
import { Effect } from "effect"


Effect.runPromise(Effect.succeed(1)).then(console.log)
// Output: 1
@example 
//Example: Handling a Failing Effect as a Rejected Promise
import { Effect } from "effect"


Effect.runPromise(Effect.fail("my error")).catch(console.error)
// Output:
// (FiberFailure) Error: my error
@since ― 2.0.0
runPromise(
const meanAge: Effect.Effect<number[], never, never>
meanAge).
Promise<number[]>.then<void, never>(onfulfilled?: ((value: number[]) => void | PromiseLike<void>) | null | undefined, onrejected?: ((reason: any) => PromiseLike<never>) | null | undefined): Promise<...>
Attaches callbacks for the resolution and/or rejection of the Promise.
@param ― onfulfilled The callback to execute when the Promise is resolved.
@param ― onrejected The callback to execute when the Promise is rejected.
@returns ― A Promise for the completion of which ever callback is executed.
then(
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(message?: any, ...optionalParams: any[]): void
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)
/*
Output:
timestamp=... level=INFO fiber=#2 message="task 1" message=[]
timestamp=... level=INFO fiber=#3 message="task 2" message=[]
timestamp=... level=INFO fiber=#2 message="task 3" message="[
  10
]"
timestamp=... level=INFO fiber=#3 message="task 4" message="[
  10,
  20
]"
[ 10, 20, 30, 40 ]
*/