On Tue, May 06, 2014 at 07:28:22PM +0100, John Beard wrote: > I think it would be a good idea to allow unknown fields in the > s-expression formats so that an older KiCad doesn't choke on things it > doesn't understand, and doesn't need to.
Good luck convincing Dick on this :D Jokes aside, the current parser is more or less a recursive descent one, and it needs to know the grammar of the expression beforehand. That's also because the specctra expression (the ones implemented by kicad) are not fully self-typing; for example, given the sexp (type round) you don't know (without some known syntax) if type is a keyword (or symbol, in sexp parlance), while round could be a symbol or a string. (type "round") could be unequivocally a symbol/string (*if* we agree that strings are always quoted). Also something like 12ab would be something hard to handle even for a lisp reader (it has a name: maybe-number; it's simply forbidden:D) Modifying the sexp writer to generate unambiguos sexps is easy (done it and can be almost directly (read) by common lisp...) but the patch wasn't accepted. In fact these always quoted expressions are compatibles with the current reader, anyway. The other big difference is that typically sexps are slurped by the (lisp) reader and then processed in core (more or less like DOM); kicad insteads read them token by token and 'expects' something, depending on the grammar (like SAX, but needs a grammar for the sexp). This is IMHO a good decision anyway, since doing otherwise in C++ would be... well, uneasy:P That said, I think that *if* the sexp reader can traverse automatically a generic sexp (without having a grammar, only tokenising) we could decide that it could 'skip' each subexpression starting with an unknown token. Maybe with warnings:D I don't know if the current parser is flexible enough to handle a thing like that (the common lisp reader is actually quite simple, if you exclude macro characters and programmable readtables), Dick can explain better the reason/problems behind it. Or maybe it simply can be done without problems... -- Lorenzo Marcantonio Logos Srl _______________________________________________ Mailing list: https://launchpad.net/~kicad-developers Post to : [email protected] Unsubscribe : https://launchpad.net/~kicad-developers More help : https://help.launchpad.net/ListHelp
