Dear Cafe, This is a followup question to my previous one. Also asked as a Megaparsec question issue at https://github.com/mrkkrp/megaparsec/issues/429 https://github.com/mrkkrp/megaparsec/issues/429 As I observed (have no idea where a canonical definition could live for this) that for the contract of parser combinators: fail with no input consumed will have next alternative to be tried with possible success fail with some input consumed will err out immediately regardless rest alternatives Am I right about these rules? Where is the official specification of such rules? Then I guess that empty result of a parser could mean NoMatch even after consumed some input, and want to leverage this semantic, but encountered error with current implementation https://github.com/complyue/dcp/blob/5be688396b7e2bda00ea80fd99d2a7b3ec5c788... https://github.com/complyue/dcp/blob/5be688396b7e2bda00ea80fd99d2a7b3ec5c788... artifactDecl :: Parser ArtDecl artifactDecl = do artCmt <- immediateDocComments (eof >> empty) <|> do artBody <- takeWhileP (Just "artifact body") (not . flip elem (";{" :: [Char])) if T.null $ T.strip artBody then empty -- this is not possible in real cases else return (artCmt, artBody) $ cabal run dcp < samples/full.txt Up to date dcp: 73:1: | 73 | <empty line> | ^ expecting "{#", "{##", or artifact body CallStack (from HasCallStack): error, called at src/Parser.hs:151:14 in main:Parser So how can I achieve that? Background is I'm trying to prototype an implementation of doc comment parsing as described at #428 https://github.com/mrkkrp/megaparsec/issues/428 Best regards, Compl