//=> The `list` of filtered elements depending on `predicate`
const filter = <Any>(list: Any[], predicate: ((elem: any) => boolean)) : Any[] =>
  list.filter (predicate)


//=> The `list` of elements less than `value`
const filterLessThan = (list: number[], value: number) : number[] =>
  filter (list, (elem) => elem < value)


//=> The `list` of elements greater or equal than `value`
const filterGreaterOrEqual = (list: number[], value: number) : number[] =>
  filter (list, (elem) => elem >= value)


//=> The sorted `list` thanks to the `quicksort` algorithm
const quicksort = ([head: number, ...tail: number[]]) : number[] => head === undefined ? [] :
  ( filterLessThan (tail, head), filterGreaterOrEqual (tail, head) )
  | (elemsLess, elemsGreater) => [...quicksort (elemsLess), head, ...quicksort (elemsGreater)]