import { Option } from "./Option";

export interface Foldable<T> {

    /**
     * Reduces the collection to a single value using the
     * associative binary function you give. Since the function
     * is associative, order of application doesn't matter.
     *
     * Example:
     *
     *     HashSet.of(1,2,3).fold(0, (a,b) => a + b);
     *     => 6
     */
    fold(zero:T, fn:(v1:T,v2:T)=>T): T;

    /**
     * Reduces the collection to a single value.
     * Left-associative.
     *
     * Example:
     *
     *     Vector.of("a", "b", "c").foldLeft("!", (xs,x) => x+xs);
     *     => "cba!"
     *
     * @param zero The initial value
     * @param fn A function taking the previous value and
     *           the current collection item, and returning
     *           an updated value.
     */
    foldLeft<U>(zero: U, fn:(soFar:U,cur:T)=>U): U;

    /**
     * Reduces the collection to a single value.
     * Right-associative.
     *
     * Example:
     *
     *     Vector.of("a", "b", "c").foldRight("!", (x,xs) => xs+x)
     *     => "!cba"
     *
     * @param zero The initial value
     * @param fn A function taking the current collection item and
     *           the previous value , and returning
     *           an updated value.
     */
    foldRight<U>(zero: U, fn:(cur:T, soFar:U)=>U): U;

    /**
     * Reduces the collection to a single value by repeatedly
     * calling the combine function.
     * No starting value. The order in which the elements are
     * passed to the combining function is undetermined.
     */
    reduce(combine: (v1:T,v2:T)=>T): Option<T>;
}