evt/src/operators/scan.ts

Type safe replacement for node's EventEmitter

import { compose } from "../lib/util/compose";

export const scan = <T, U>(
    accumulator: (acc: U, value: T, index: number) => U,
    seed: U
) => compose<T, [T, U, number], U>(
    [
        (data, [, acc, index]) =>
            [[data, accumulator(acc, data, index), index + 1]],
        [null as any, seed, 0]
    ],
    ([, acc]) => [acc]
);