[Python-Dev] funcdef grammar production

[Greg Ewing]

"Jewett, Jim J" <jim.jewett at eds.com>:

> one possible path is
> 
> 	(defparameter ",")* (defparameter [","])

Even that seems unnecessarily obtuse. I would write it as

  defparameter ("," defparameter)* [","]

> Under my formulation, it would be ambiguous whether "a" was a
> defparameter or a (regular) parameter, which are different
> productions.  Looking ahead for the "=" violates a no-lookahead rule.

If the grammar is only for documentation, none of that matters -- it
doesn't have to be LL(1) or anything in particular, as long as it
unambiguously specifies the language.

By the way, it occurs to me that BNF on its own doesn't seem
to be up to the task of specifying this sort of thing clearly
to a human reader. If it weren't for the commas, we could
say something like

  defparams ::= name* (name "=" expr)* ["*" name] ["**" name]

All we need to say on top of that is "separate this list of
things with commas", which could perhaps be expressed as

  defparams ::= 
    (name* (name "=" expr)* ["*" name] ["**" name]) separated_by ","

with some sort of meta-rule like

  ITEM* separated_by SEP ::= [ITEM ("," ITEM*) [","]]

Hmmm, this is starting to look dangerously like a two-level
grammar...

Greg Ewing, Computer Science Dept, +--------------------------------------+
University of Canterbury,	   | A citizen of NewZealandCorp, a	  |
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greg at cosc.canterbury.ac.nz	   +--------------------------------------+