import { List } from "../../list";
import { NoInfer, Satisfies, Unreachable } from "../../type";
import { $, HKT, _, I } from "..";
declare const TS_TYPE_ERROR: unique symbol;
type TS_TYPE_ERROR<Message extends string> = {
    [TS_TYPE_ERROR]: Message;
};
type ValidateAccumulatorHkt<F> = [
    F
] extends [HKT<[any, any], any>] ? [F] extends [HKT<[infer A, infer B], NoInfer<infer A>>] ? HKT<[A, B], A> : TS_TYPE_ERROR<'Accumulator HKT output must be assignable to input accumulated value'> : TS_TYPE_ERROR<'expected HKT of the form (A, B) => A'>;
type __$Fold2<F extends HKT<[A, any], A>, L extends List, A, Acc extends A> = L extends readonly [infer H, ...infer T] ? __$Fold2<F, T, A, $<F, [Acc, H]>> : Acc;
/**
 * folds over `L` with function `F` and initial value `Initial`.
 *
 * @warning you probably want to use `$Fold` instead.
 */
type _$Fold2<F extends HKT<[A, any], A>, Initial extends A, L extends List, A> = __$Fold2<F, L, A, Initial>;
/**
 * folds over `L` with function `F` and initial value `Initial`.
 *
 * @since 0.0.5
 */
export type $Fold<F extends ValidateAccumulatorHkt<F>, Initial extends (F extends HKT<[infer A, any], NoInfer<infer A>> ? A : Unreachable), L extends (F extends HKT<[infer A, infer B], NoInfer<infer A>> ? List<B> : Unreachable)> = F extends HKT<[infer A, infer B], NoInfer<infer A>> ? Initial extends A ? L extends List<B> ? _$Fold2<F, Initial, L, A> : Unreachable : Unreachable : Unreachable;
/**
 * @since 0.0.5
 *
 * @todo how to add custom constraints to hkt input? (e.g. ensure only nonvariadics passed in to $<Fold, whatever> and error if not)
 * @todo any way to clean up / reduce redundancy in the code to for custom typechecks here?
 * @todo curry this so can pass initial as separate hkt, this will make it more composable
 */
export interface Fold<F extends ValidateAccumulatorHkt<F>, Initial extends (F extends HKT<[infer A, any], NoInfer<infer A>> ? A : Unreachable)> extends HKT {
    [HKT.i]: Satisfies<_<this>, List<F extends HKT<[infer A, infer B], NoInfer<infer A>> ? B : any>>;
    [HKT.o]: F extends HKT<[infer A, infer B], NoInfer<infer A>> ? Initial extends A ? I<this> extends List<B> ? _$Fold2<F, Initial, I<this>, A> : Unreachable : Unreachable : Unreachable;
}
export {};
//# sourceMappingURL=fold.d.ts.map