effect-io-ai

Package: effect
Module: Utils

Utils.Variance

Type-level marker encoding the variance of a TypeLambda’s type parameters.

When to use

Use to define variance constraints for a higher-kinded type so that Gen can correctly infer R, O, and E from yielded values.

Details

F is invariant and must match exactly. R is contravariant in the input or environment position. O and E are covariant in the output and error positions. This is a pure type-level construct with no runtime representation.

Example (Declaring variance for a TypeLambda)

import type { Option, Utils } from "effect"

declare const variance: Utils.Variance<
  Option.OptionTypeLambda,
  never,
  never,
  never
>

See

• Gen for the type-level signature that uses Variance

Signature

export interface Variance<in out F extends TypeLambda, in R, out O, out E> {
  readonly _F: Types.Invariant<F>
  readonly _R: Types.Contravariant<R>
  readonly _O: Types.Covariant<O>
  readonly _E: Types.Covariant<E>
}

Source

Since v2.0.0