Re: [LPeg] How can I signal parsing errors with LPeg?

["Soni L." <fakedme@...>]

On 30/07/16 01:20 AM, Sean Conner wrote:
> It was thus said that the Great Soni L. once stated:
> > As seen in http://stackoverflow.com/q/38664815/3691554
> >
> > `Line` and `Data` are the things that when `:match()`ed should return
> > `nil, errmsg` on parsing error. Note that only `Line` decides whether a
> > line is valid or not, so that's where parsing errors happen. `Data` only
> > extracts the data from a (valid) line.
>    Make sure Line always matches the entirety of a valid line.  Then you can
> modify it to return nil,errmsg, like:
>
> local Line = lpeg.C((wsc + (backslashEscaped + 1 - nl))^0)
>             / function(x) return x end
>             * nl
>             * lpeg.Cp()
>             + lpeg.Cc(nil) * lpeg.Cc("Invalid parse of a line")
>
>
> local Data = lpeg.S(" \t")^0
>             * lpeg.Cs((ws / " " + inlineComment / "" + backslashEscaped + 1 - (lpeg.S(" \t")^0 * nl))^0)
>             * lpeg.S(" \t")^0
>             * nl
>             + lpeg.Cc(nil) * lpeg.Cc("Invalid data")
Data isn't needed :P because Line already validates the line.
>    If you want more specific errors, then you can do something like this:
>
> local escape = lpeg.P"\\ "  / " "
>               + lpeg.P"\\\\" / "\\"
>               + lpeg.P"\\#"  / "#"
>               + lpeg.P"\\>"  / ">"
>               + lpeg.P"\\"  / ""
>               + lpeg.P"\\n"  / "\n"
>               + lpeg.P"\\" * lpeg.Cmt(lpeg.Carg(1) * lpeg.C(1),
>                                        function(subject,pos,e,c)
>                                          table.insert(e,"Bad escape " .. c)
>                                          return nil
>                                        end)
> local char    = lpeg.R(" [","]~")

Right. Does the "utf-8" on the MDXML spec[1] not mean anything to you?

> local cleanup = lpeg.Cs((escape + char)^0) * lpeg.P"\n"
>
> local function test(s)
>    local e = {}
>    local r = cleanup:match(s,1,e)
>    if not r then
>      print(e[1])
>    end
> end
>
> test [[Hello\ There\# foob
> ]]
> test [[Hello\aThere\bfoob
> ]]
>
>    I'm using lpeg.Carg() to pass in a table to accumulate the errors (this
> keeps this reentrant).  lpeg.Cmt() does a match-time capture and if this
> returns nil, then that particular capture fails.  I've set up this example
> such that if cleanup returns nil, then the passed in table will contain the
> error.

I see. I'd still prefer :match itself to emit nil, errmsg with a 
dynamically generated errmsg.

>    -spc

[1] https://github.com/SoniEx2/MDXML#encoding

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