Re: LPEG S-expression grammar

[Chris Smith <space.dandy@...>]

> On 10 Aug 2023, at 23:07, Sean Conner <sean@conman.org> wrote:
> 
> It was thus said that the Great Chris Smith once stated:
>> Hi,
>> 
>> Has anyone written an LPEG grammar for parsing S-expressions?
> 
>  Yes, and very simplistic S-expression grammar is:
> 
> local lisp = lpeg.P {
>  'lisp',
>  lisp  = lpeg.V"atom"  + lpeg.V"list",
>  atom  = lpeg.V"space" * lpeg.C(lpeg.R("!'","*~")^1),   
>  list  = lpeg.V"space" * lpeg.P'(' * lpeg.Ct((lpeg.V'list' + lpeg.V'atom')^0) * lpeg.P')',
>  space = lpeg.S" \t\v\r\n"^0
> }

Thank you, that is a bit simpler than the version I was hacking together, although I see it suffers from the same issue that mine had in that it is intolerant of trailing spaces.  For example ‘(a b c )’ won’t match.

I solved this in my version using the following:

space = lpeg.S" \t\v\r\n”
opt_space = space^0
delim = #(space^1 + ‘)’) * opt_space

Then adding *delim to the end of atom and list, but this seems a bit clumsy to me.  Is there a better way?  Is there a way to get LPeg to do a two-stage pass, so you can effectively say match all atoms first, then match lisp ignoring all spaces?

Regards,
Chris
—
Chris Smith <space.dandy@icloud.com>