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Effect Documentation

Introduction to Streams

The Stream type is an immutable description of a potentially unbounded sequence of values, handled within a context and capable of error management.

Here is the general form of an Stream:

         ┌─── Success type
         │        ┌─── Error type
         │        │      ┌─── Requirements
         ▼        ▼      ▼
Stream<Success, Error, Requirements>

When executed, a Stream can emit zero or more values of type Success, handle errors of type Error, and operate within a context of type Requirements.

Streams are well-suited for managing sequences of values over time and are a great fit as alternatives to observables, Node streams, and AsyncIterables. You can think of a Stream as an Effect that can yield multiple values instead of a single result.

Unlike an Effect, which always resolves to one specific value, a Stream offers greater flexibility, allowing for:

  • Empty Streams: Streams that produce no values.
  • Single-Element Streams: Streams that emit only one value.
  • Finite Streams: Streams with a defined number of elements.
  • Infinite Streams: Streams that emit values continuously.

Example (Creating Different Stream Types)

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import { 
import Stream
Stream } from "effect"
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// An empty Stream
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const 
const emptyStream: Stream.Stream<never, never, never>
emptyStream = 
import Stream
Stream.
const empty: Stream.Stream<never, never, never>
The empty stream.
empty
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// A Stream with a single number
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const 
const oneNumberValueStream: Stream.Stream<number, never, never>
oneNumberValueStream = 
import Stream
Stream.
const succeed: <number>(value: number) => Stream.Stream<number, never, never>
Creates a single-valued pure stream.
succeed(3)
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// A Stream with numbers from 1 to 10
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const 
const finiteNumberStream: Stream.Stream<number, never, never>
finiteNumberStream = 
import Stream
Stream.
const range: (min: number, max: number, chunkSize?: number) => Stream.Stream<number>
Constructs a stream from a range of integers, including both endpoints.
range(1, 10)
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// An infinite Stream of numbers starting from 1 and incrementing
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const 
const infiniteNumberStream: Stream.Stream<number, never, never>
infiniteNumberStream = 
import Stream
Stream.
const iterate: <number>(value: number, next: (value: number) => number) => Stream.Stream<number, never, never>
The infinite stream of iterative function application: a, f(a), f(f(a)),
f(f(f(a))), ...
iterate(1, (
(parameter) n: number
n) => 
(parameter) n: number
n + 1)