[antlr-interest] anybody help with me a python grammar rule?

[John Pybus]

> On Feb 21, 2004, at 6:10 PM, Terence Parr wrote:
> 
> 
>>Folks, the formal parameter list for functions looks wacky in the
>>distribution and on the net.
>>
>>First the grammar in the distribution:
>>
>>varargslist: (fpdef ['=' test] ',')* ('*' NAME [',' '**' NAME] | '**'
>>NAME) | fpdef ['=' test] (',' fpdef ['=' test])* [',']
>>fpdef: NAME | '(' fplist ')'
>>fplist: fpdef (',' fpdef)* [',']
>>
>>The one from the doc is
>>
>>parameter_list ::=
>>              (defparameter ",")*
>>                 ("*" identifier [, "**" identifier]
>>                 | "**" identifier
>>                   | defparameter [","])
>>
>>defparameter ::=
>>              parameter ["=" expression]
>>
>>sublist ::=
>>              parameter ("," parameter)* [","]
>>
>>parameter ::=
>>              identifier | "(" sublist ")"
>>
>>My question here is why have the final '| defparameter [","]' alt in
>>parameter_list????  Surely it's redundant (might have to move a comma,
>>but...)
>>
>>Can anybody give me some examples of the various things parameters
>>might look like in the wacky cases?

def foo(x,y,z):  #GOOD

def foo(x,y,z,): #GOOD trailing comma allowed

def bar(*baz):   # STILL GOOD

def bar(*baz,):  # BAD Syntax error !!

i.e. You're allowed a trailing comma in a parameter list, unless you're
using either of the excess positional parameter forms.  Though I won't
to pretend to answer why this should be the rule ;-)

Terence Parr wrote:
  > For example, won't this rule generate the same language?
  >
  > parameter_list:
  >      defparameter (',' defparameter)*
  >      ( ','
  >        ('*' NAME [',' '**' NAME]       | '**' NAME
  >        )
  >      )
  >   | '*' NAME [',' '**' NAME]
  >   | '**' NAME
  >
  > and it's LL(1).
  >
  > arglist has the same problem.  I wonder what kinda parser they build
  > from this grammar.  Ick.
  >
  > Ter

How does this match a param_list not ending in one of the 'excess
parameter' forms?

foo(a) or foo(a,)

I think you'll need the form they've used (or something similarly
complex) to get the optional trailing comma on simple parameters, but
not on *'d ones.

Yours,

John




 
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