[Python-Dev] funcdef grammar production

[Jewett, Jim J]

Greg Ewing:
>Jewett, Jim J:

>> one possible path is

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

> Even that seems unnecessarily obtuse.

I agree, but that is the current definition.  (Except that 
I explicitly chose the last of several alternatives, 
instead of leaving in the "choice".)

> I would write it as

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

That would be incorrect.

Even the first defparameter is optional if there is an 
*args or a **kwargs.  

There does need to be something in the list.  (An empty
parameter list is handled by making it an optional part of 
funcdef.)  I assume this restriction is also due to the 
unambiguous-at-first-token rule.

That optional "," is required if you have both a defparameter
and an *args or **kwargs.  It is forbidden if there is
no 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

There is value in keeping the grammar documentation as close 
as possible to the actual coded grammar.  If a rule expressible 
in the grammar is actually implemented outside it, then either
the documented grammar and the real grammar diverge, or some 
rules must be listed only in the text.  Pick your poison.

> 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.

Agreed, but text often fails.  The advantage of BNF is that it
can be checked to ensure that the writer and the reader agree.

> If it weren't for the commas, 

Yes.  Particularly evil is the rule that allows a trailing 
comma sometimes but not always.  That said, I would rather
have a confusing grammar than have to add trailing commas
for standardization.

The sublist possibilities don't make things any easier,
particularly since they aren't full parameter lists.  
(You can't use defaults, *, or ** within a sublist, though 
you can use a default for the entire list.)  I can only 
assume that this was an attempt to meet lispers halfway;
there are people who consider destructuring-bind to be a 
valuable feature.  

-jJ