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Effect Documentation

Effect 3.6 (Release)

Tim Smart
Tim Smart
Founding Engineer

Effect 3.6 has been released! This release includes a number of new features and improvements. Here’s a summary of what’s new:

DateTime module

The DateTime module provides functionality for working with time, including support for time zones and daylight saving time.

It has two main data types: DateTime.Utc and DateTime.Zoned.

A DateTime.Utc represents a time in Coordinated Universal Time (UTC), and a DateTime.Zoned contains both a UTC timestamp and a time zone.

There is also a CurrentTimeZone service, for setting a time zone contextually.

import { DateTime, Effect } from "effect"


Effect.gen(function* () {
  // Get the current time in the current time zone
  const now = yield* DateTime.nowInCurrentZone


  // Math functions are included
  const tomorrow = DateTime.add(now, { days: 1 })


  // Convert to a different time zone
  // The UTC portion of the `DateTime` is preserved and only the time zone is
  // changed
  const sydneyTime = tomorrow.pipe(
    DateTime.unsafeSetZoneNamed("Australia/Sydney"),
  )
}).pipe(DateTime.withCurrentZoneNamed("America/New_York"))

Stream.asyncPush api

Stream.asyncPush can be used to create a Stream from an external push-based resource.

You can customize the buffer size and strategy by passing an object as the second argument with the bufferSize and strategy fields.

import { Effect, Stream } from "effect";


Stream.asyncPush<string>(
  (emit) =>
    Effect.acquireRelease(
      Effect.gen(function* () {
        yield* Effect.log("subscribing");
        return setInterval(() => emit.single("tick"), 1000);
      }),
      (handle) =>
        Effect.gen(function* () {
          yield* Effect.log("unsubscribing");
          clearInterval(handle);
        }),
    ),
  { bufferSize: 16, strategy: "dropping" },
);

Struct.keys api

To access the fully typed keys of a struct, you can use the Struct.keys function.

import { Struct } from "effect"


const symbol: unique symbol = Symbol()


const value = {
  a: 1,
  b: 2,
  [symbol]: 3
}


const keys: Array<"a" | "b"> = Struct.keys(value)

@effect/sql-kysely package

The @effect/sql-kysely package provides @effect/sql integration with the kysely query builder apis.

// create a Tag with your `Database` type
class KyselyDB extends Context.Tag("KyselyDB")<KyselyDB, Kysely<Database>>() {}


Effect.gen(function*() {
  // access the service and execute queries
  const db = yield* KyselyDB


  yield* db.schema
    .createTable("users")
    .addColumn("id", "integer", (c) => c.primaryKey().autoIncrement())
    .addColumn("userName", "text", (c) => c.notNull())


  const inserted = yield* db.insertInto("users").values({ userName: "Alice" }).returningAll()
  const selected = yield* db.selectFrom("users").selectAll()
  const updated = yield* db.updateTable("users").set({ userName: "Bob" }).returningAll()
  const deleted = yield* db.deleteFrom("users").returningAll()
})

Random.choice api

This api allows you to randomly select an item from an Iterable.

Unless the Iterable is “NonEmpty”, then the Effect can fail with a Cause.NoSuchElementException.

import { Random } from "effect"


Effect.gen(function* () {
  const randomItem = yield* Random.choice([1, 2, 3])
  console.log(randomItem)
})

onlyEffect option for Effect.tap

If the onlyEffect option for Effect.tap is set to true, then it will ensure the side effect only uses Effect’s.

This can be useful when you want to add strictness to your program.

Refinement support for Predicate.tuple/struct

Refinements can now be used with Predicate.tuple and Predicate.struct to narrow the resulting type.

import { Predicate } from "effect"


const isTrue = (u: unknown): u is true => u === true


// will narrow the type to { isTrue: true }
Predicate.struct({ isTrue })

Stream hook apis

Some new lifetime hook apis have been added to the Stream module:

  • Stream.onStart - run an effect when the stream starts
  • Stream.onEnd - run an effect when the stream ends without error

Other changes

There were several other smaller changes made. Take a look through the CHANGELOG to see them all: CHANGELOG.

Don’t forget to join our Discord Community to follow the last updates and discuss every tiny detail!

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