Dear Cafe, I find myself unable to reason about an error that `optional` in a parser err out instead of return Nothing, I asked the question at StackOverflow, and would like to seek your help here as well. https://stackoverflow.com/questions/64522568/why-optional-in-a-parser-can-er... https://stackoverflow.com/questions/64522568/why-optional-in-a-parser-can-er... https://github.com/complyue/dcp https://github.com/complyue/dcp is a minimum working example to reprod this error $ cabal run dcp:dcp < samples/basic.txt Up to date dcp: 10:1: | 10 | method doXXX() pass | ^ unexpected 'm' expecting ';' CallStack (from HasCallStack): error, called at src/Parser.hs:149:14 in main:Parser $ I believe it's optionalSemicolon causing the failure: https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa... https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa... findIt = do -- ignore leading whitespaces and an optional semicolon in between nbsc >> optionalSemicolon >> nbsc -- try get a doc comment block getIt >>= \case And it's defined like this: https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa... https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa... optionalSemicolon :: Parser Bool optionalSemicolon = fromMaybe False <$> optional (True <$ symbol ";") I can't reason about why it can fail like this. Best regards, Compl