# Fx.Variance

Describes how an `Fx` varies in its value, error, and service channels.

## Signatures

```ts
interface Variance<A, E, R> {
    readonly _A: Types.Covariant<A>;
    readonly _E: Types.Covariant<E>;
    readonly _R: Types.Covariant<R>;
}
```

## Why

The phantom fields preserve `Fx<A, E, R>` relationships through TypeScript's
structural type system and power channel extraction without runtime sampling.

## Ownership and lifetime

Variance markers are type-level metadata and retain no values or resources.

## Property: _A

Covariant marker for emitted values.

## Property: _A: Why

Preserves the `A` relationship without retaining an emitted value.

## Property: _A: Ownership and lifetime

This phantom field starts no work and owns no resource.

## Property: _E

Covariant marker for typed failures.

## Property: _E: Why

Preserves the `E` relationship without manufacturing a failure.

## Property: _E: Ownership and lifetime

This phantom field starts no work and owns no resource.

## Property: _R

Covariant marker for required services.

## Property: _R: Why

Preserves the `R` relationship without reading an Effect context.

## Property: _R: Ownership and lifetime

This phantom field starts no work and owns no resource.

## Examples

```ts
import type { Fx } from "@typed/fx/Fx"

type NumberFxVariance = Fx.Variance<number, never, never>
```