# mergeLeft

Merges two Fx streams and emits only values from the left stream.
Both streams run concurrently; completion when **both** complete.

## Signatures

```ts
export declare const mergeLeft: {
    <A2, E2, R2>(that: Fx<A2, E2, R2>): <A, E, R>(self: Fx<A, E, R>) => Fx<A, E | E2, R | R2>;
    <A, E, R, A2, E2, R2>(self: Fx<A, E, R>, that: Fx<A2, E2, R2>): Fx<A, E | E2, R | R2>;
};
```

## Why

`mergeLeft` keeps the right producer's lifetime and failure signal while suppressing its values.
Every left value is emitted once; right emissions only affect scheduling.

## Ownership and lifetime

Both sources are acquired together and owned by the consuming run. Interruption cancels both and
normal completion waits for both. A Cause from either source is delivered to the Sink but does not
intrinsically cancel its sibling; right successes remain suppressed by the filter.

## Examples

```ts
import { Effect, Ref } from "effect"
import { Fx, Sink } from "@typed/fx"

const program = Effect.gen(function* () {
  const failures = yield* Ref.make(0)
  yield* Fx.mergeLeft(Fx.succeed("visible"), Fx.fail("hidden side failed")).run(
    Sink.make(() => Ref.update(failures, (n) => n + 1), () => Effect.void)
  )
  return yield* Ref.get(failures) // 1; failure is still delivered
})
```