Introduction

In stream processing, a Sink is a construct designed to consume elements generated by a Stream.

     ┌─── Type of the result produced by the Sink
     |  ┌─── Type of elements consumed by the Sink
     |  |   ┌─── Type of any leftover elements
     │  |   |  ┌─── Type of possible errors
     │  │   |  |  ┌─── Type of required dependencies
     ▼  ▼   ▼  ▼  ▼
Sink<A, In, L, E, R>

Here’s an overview of what a Sink does:

It consumes a varying number of In elements, which may include zero, one, or multiple elements.
It can encounter errors of type E during processing.
It produces a result of type A once processing completes.
It can also return a remainder of type L, representing any leftover elements.

To process a stream using a Sink, you can pass it directly to the Stream.run function:

Example (Using a Sink to Collect Stream Elements)

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import { Stream, Sink, Effect } from "effect"
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//      ┌─── Stream<number, never, never>
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//      ▼
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const stream = Stream.make(1, 2, 3)
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// Create a sink to take the first 2 elements of the stream
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//
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//      ┌─── Sink<Array<number>, number, number, never, never>
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//      ▼
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const sink = Sink.take<number>(2)
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// Run the stream through the sink to collect the elements
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//
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//      ┌─── Effect<Array<number>, never, never>
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//      ▼
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const sum = Stream.run(stream, sink)
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await Effect.runPromise(sum) // => [1, 2]

The type of sink is as follows:

       ┌─── result
       |              ┌─── consumed elements
       |              |       ┌─── leftover elements
       │              |       |       ┌─── no errors
       │              │       |       |      ┌─── no dependencies
       ▼              ▼       ▼       ▼      ▼
Sink<Array<number>, number, number, never, never>

Here’s the breakdown:

Array<number>: The final result produced by the sink after processing elements.
number (first occurrence): The type of elements that the sink will consume from the stream.
number (second occurrence): The type of leftover elements, if any, that are not consumed.
never (first occurrence): Indicates that this sink does not produce any errors.
never (second occurrence): Shows that no dependencies are required to operate this sink.