Better visitor

[Daniel63656]

Hey, coming from antlr parsing, I found the Visitor class(es) provided by lark a bit lacking. There is no way to get callbacks when exiting a rule and when terminals are encountered. Based on that, I wrote a very simple recursive top-down base visitor that has these features. I also made it so that the user can prevent further visiting a rule by returning False in an enter rule. Feel free to use this if you want:

class TopDownVisitor():
    """ Base class for visiting a parse tree top-down in a recursive manner.
        Derivations can receive callbacks when a rule is entered by implementing a function ``enter_rule(self, tree)``. Enter rules
        must return True if the rule should be further visited.
        ``exit_rule(self, tree)`` can be used to get a callback once a rule is exited. 
        Leave nodes (terminals) will call the dedicated ``visit_terminal`` function.
    """
    def __default__(self, tree):
         """ Default function that is called if there is no attribute matching ``enter_tree.data``
            or ``exit_tree.data``. Defaults to doing nothing and allow for further visiting children in the enter case
        """
        return True


    def visit_terminal(self, terminal):
        return terminal
    

    def visit(self, tree):
        if getattr(self, "enter_"+tree.data, self.__default__)(tree):
            for child in tree.children:
                if isinstance(child, Tree):
                    self.visit(child)
                else:
                    self.visit_terminal(child)
            getattr(self, "exit_"+tree.data, self.__default__)(tree)

[erezsh]

Can you give some example situation in which you would want to use this pattern?