:: Ord a => [a] -> [a] -package:list-t -package:incipit-base -package:extra -package:base-prelude -package:ghc -package:base is:exact -package:containers -package:lifted-base -package:generic-deriving -package:witherable -package:vector-algorithms package:parsers
Take a parser that may consume input, and on failure, go back to where
we started and fail as if we didn't consume input.