[Haskell-cafe] Reason about a Megaparsec error

YueCompl compl.yue at icloud.com
Sun Oct 25 10:27:45 UTC 2020

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-err-out <https://stackoverflow.com/questions/64522568/why-optional-in-a-parser-can-err-out>

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/1df7ad590d78d4fa9a017eb53f9f265e291bdfa7/src/Parser.hs#L50-L54 <https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa7/src/Parser.hs#L50-L54>
  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/1df7ad590d78d4fa9a017eb53f9f265e291bdfa7/src/Parser.hs#L31-L32 <https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa7/src/Parser.hs#L31-L32>
optionalSemicolon :: Parser Bool
optionalSemicolon = fromMaybe False <$> optional (True <$ symbol ";")
I can't reason about why it can fail like this.

Best regards,

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20201025/f05b3ee3/attachment.html>

More information about the Haskell-Cafe mailing list