effect-io-ai

Package: effect
Module: Ref

Ref.modify

Modifies the value of the Ref atomically using the given function.

When to use

Use to compute both a separate return value and the next stored Ref value in one atomic update.

Details

The function receives the current value and returns a tuple of [result, newValue]. The Ref is updated with newValue, and result is returned by the effect.

Example (Modifying a value atomically)

import { Effect, Ref } from "effect"

const program = Effect.gen(function*() {
  const counter = yield* Ref.make(10)

  // Modify the ref and return some computation result
  const result = yield* Ref.modify(counter, (n) => [
    `Previous value was ${n}`, // Return value
    n * 2 // New ref value
  ])

  console.log(result) // "Previous value was 10"

  const current = yield* Ref.get(counter)
  console.log(current) // 20
})

// Example with more complex computation
const program2 = Effect.gen(function*() {
  const state = yield* Ref.make({ count: 0, total: 0 })

  const incremented = yield* Ref.modify(state, (s) => [
    s.count, // Return previous count
    { count: s.count + 1, total: s.total + s.count + 1 } // New state
  ])

  console.log(incremented) // 0
})

See

• updateAndGet for returning the new stored value
• modifySome for optionally updating while returning a separate result

Signature

declare const modify: (<A, B>(f: (a: A) => readonly [B, A]) => (self: Ref<A>) => Effect.Effect<B>) & (<A, B>(self: Ref<A>, f: (a: A) => readonly [B, A]) => Effect.Effect<B>)

Source

Since v2.0.0