:: Ord a => [a] -> [a] -package:list-t -is:exact -package:imports -package:mono-traversable -package:incipit-base -package:safe -is:exact -package:base -package:ghc -package:errors -package:Cabal -package:Cabal-syntax 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.