Basic parser grammar question

Denis Washington <dwashington@gmx.net> wrote:
  > This piece of grammar works fine if I have simple additive expressions
  > like "a + b", but fails to function if having longer arrays of additive
  > expressions, e.g. "a + b + c". Why is that? Have I missed something? I
  > hope you can help me.


I think your problem is that your productions aren't recursive. Let's
look at the following example, extracted from
http://www.dabeaz.com/ply/example.html :


expression
  : expression PLUS expression
  | expression MINUS expression
  | expression TIMES expression
  | expression DIVIDE expression


Note the 'expression' production is recursive. That's why this grammar
can handle repetitions, like the ones you want your parser to grasp: a +
b + c is actually (a + b) + c.


I hope it helps, I am not a user of PLY myself, not even a regular
Python user.


[]s


O JuciĘ

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