Re: [Haskell-cafe] Reason about a Megaparsec error

During the parsing of doc comments, backtracking is off on purpose, as it won't consume syntactic contents, merely whitespaces (while doc comment is a special form of block comment), the desired result is `Nothing` or `Just docCmt` without backtracking of whitespaces.

On 2020-10-25, at 18:53, Jaro Reinders wrote:

I think you have to insert a 'try' manually inside the optional, because megaparsec doesn't do backtracking by default.

On 10/25/20 11:27 AM, YueCompl via Haskell-Cafe wrote:

...

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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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