[Haskell-cafe] Reason about a Megaparsec error

[Jaro Reinders]

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-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,
> Compl
> 
> 
> 
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