Producing Expressions from This Grammar with Recursive Descent

Answer 1

I haven't tried to analyze it in detail, but: "From or To expression enclosed in parentheses" starts to sound a lot like "context dependent", which recursive descent can't handle directly. To avoid context dependence you'll probably need a separate production for a From or To in parentheses vs. a From or To without the parens.

Edit: Though it may be too late to do any good, if my understanding of what you want to match is correct, I think I'd write it more like this:

Graph := 
       | List Sep Graph
       ;

Sep := "->"
     | "<-"
     ;

List :=
      | Value List
      ;

Value := Number 
      | Identifier 
      | String 
      | '(' Graph ')'
      ;

It's hard to be certain, but I think this should at least be close to matching (only) the inputs you want, and should make it reasonably easy to generate an AST that reflects the input correctly.