module Strain (..) where

import List


keep : (a -> Bool) -> List a -> List a
keep predicate list =
  List.foldr (consIf predicate) [] list


discard : (a -> Bool) -> List a -> List a
discard predicate list =
  List.foldr (consIf (\v -> not <| predicate v)) [] list


consIf : (a -> Bool) -> a -> List a -> List a
consIf predicate value list =
  if predicate value then
    value :: list
  else
    list