Transducers in TypeScript

transducers.ts

type Reducer<Memo, Item> = (memo: Memo, item: Item) => Memo
type Transducer<Memo, Item, ToItem> = (reducing: Reducer<Memo, ToItem>) => Reducer<Memo, Item> 


type Mapper<A, B> = (a: A) => B;
const mapping = <Memo, Item, ToItem>(mapper: Mapper<Item, ToItem>): Transducer<Memo, Item, ToItem> =>
    reducing => (memo, item) =>
    reducing(memo, mapper(item))


type Predicate<T> = (item: T) => boolean;
const filtering = <Memo, Item>(predicate: Predicate<Item>): Transducer<Memo, Item, Item> =>
    reducing => (memo, item) =>
    predicate(item) ? reducing(memo, item) : memo