# modify

Modifies a `RefSubject` using a pure function that returns both a result and a new value.

## Signatures

```ts
export declare const modify: {
    <A, B>(f: (value: A) => readonly [
        B,
        A
    ]): <E, R>(ref: RefSubject<A, E, R>) => Effect.Effect<B, E, R>;
    <A, E, R, B>(ref: RefSubject<A, E, R>, f: (value: A) => readonly [
        B,
        A
    ]): Effect.Effect<B, E, R>;
};
```

## Why

Modifies a `RefSubject` using a pure function that returns both a result and a new value. The
transition is serialized at the RefSubject rather than coordinated by callers or UI components.

## Ownership and lifetime

Running modify performs one serialized subject transition and resolves with its committed value.
It acquires no resource; failures and services remain those of the source ref.

## Examples

```ts
import { Effect } from "effect"
import * as RefSubject from "@typed/fx/RefSubject"

const program = Effect.gen(function* () {
  const count = yield* RefSubject.make(5)

  // Get the old value and increment, returning the old value
  const oldValue = yield* RefSubject.modify(count, (value) => [value, value + 1] as const)

  console.log(oldValue) // 5
  const newValue = yield* count
  console.log(newValue) // 6
})
```