effect-io-ai

Package: effect
Module: Ref

Ref.modifySome

Computes a result atomically and optionally updates the value of the Ref.

When to use

Use to compute a return value while optionally updating a plain Ref.

Details

The callback receives the current value and returns [result, nextValue], where nextValue is an Option. If nextValue is Option.some(value), the Ref is updated to value; if it is Option.none(), the Ref is left unchanged. The returned effect always succeeds with result.

Example (Conditionally modifying a value)

import { Effect, Option, Ref } from "effect"

const program = Effect.gen(function*() {
  const counter = yield* Ref.make(5)

  // Only modify if value is greater than 3
  const result1 = yield* Ref.modifySome(
    counter,
    (n) =>
      n > 3
        ? [`incremented ${n}`, Option.some(n + 10)]
        : ["no change", Option.none()]
  )

  console.log(result1) // "incremented 5"

  const current1 = yield* Ref.get(counter)
  console.log(current1) // 15

  // Try to modify with a condition that fails
  const result2 = yield* Ref.modifySome(
    counter,
    (n) =>
      n < 10
        ? [`decremented ${n}`, Option.some(n - 5)]
        : ["no change", Option.none()]
  )

  console.log(result2) // "no change"

  const current2 = yield* Ref.get(counter)
  console.log(current2) // 15 (unchanged)
})

See

• modify for always storing a new value
• updateSome for optional updates without a separate return value

Signature

declare const modifySome: { <B, A>(pf: (a: A) => readonly [B, Option.Option<A>]): (self: Ref<A>) => Effect.Effect<B>; <A, B>(self: Ref<A>, pf: (a: A) => readonly [B, Option.Option<A>]): Effect.Effect<B>; }

Source

Since v2.0.0