Base interface for defining Higher-Kinded Type parameters.

When to use

Use to encode a type constructor for higher-kinded generic programming.

Details

A TypeLambda encodes the "shape" of a type constructor, specifying how many type parameters it takes and their variance (contravariant, covariant, or invariant). The four parameters are In for contravariant input, Out2 for covariant output often used for errors, Out1 for covariant output often used for context or environment, and Target for the invariant main type.

Example (Defining type lambdas)

import type { Effect, HKT } from "effect"

// TypeLambda for Array<A>
interface ArrayTypeLambda extends HKT.TypeLambda {
  readonly type: Array<this["Target"]>
}

// TypeLambda for Effect<A, E, R>
interface EffectTypeLambda extends HKT.TypeLambda {
  readonly type: Effect.Effect<this["Target"], this["Out2"], this["Out1"]>
}

// TypeLambda for function (A) => B
interface FunctionTypeLambda extends HKT.TypeLambda {
  readonly type: (a: this["In"]) => this["Target"]
}