| Transformers & Visitors |
| ======================= |
|
|
| Transformers & Visitors provide a convenient interface to process the |
| parse-trees that Lark returns. |
|
|
| They are used by inheriting from the correct class (visitor or transformer), |
| and implementing methods corresponding to the rule you wish to process. Each |
| method accepts the children as an argument. That can be modified using the |
| ``v_args`` decorator, which allows to inline the arguments (akin to ``*args``), |
| or add the tree ``meta`` property as an argument. |
|
|
| See: `visitors.py`_ |
|
|
| .. _visitors.py: https://github.com/lark-parser/lark/blob/master/lark/visitors.py |
|
|
| Visitor |
| ------- |
|
|
| Visitors visit each node of the tree, and run the appropriate method on it according to the node's data. |
|
|
| They work bottom-up, starting with the leaves and ending at the root of the tree. |
|
|
| There are two classes that implement the visitor interface: |
|
|
| - ``Visitor``: Visit every node (without recursion) |
| - ``Visitor_Recursive``: Visit every node using recursion. Slightly faster. |
|
|
| Example: |
| :: |
| |
| class IncreaseAllNumbers(Visitor): |
| def number(self, tree): |
| assert tree.data == "number" |
| tree.children[0] += 1 |
|
|
| IncreaseAllNumbers().visit(parse_tree) |
|
|
| .. autoclass:: lark.visitors.Visitor |
|
|
| .. autoclass:: lark.visitors.Visitor_Recursive |
|
|
| Interpreter |
| ----------- |
|
|
| .. autoclass:: lark.visitors.Interpreter |
|
|
|
|
| Example: |
| :: |
| |
| class IncreaseSomeOfTheNumbers(Interpreter): |
| def number(self, tree): |
| tree.children[0] += 1 |
|
|
| def skip(self, tree): |
| # skip this subtree. don't change any number node inside it. |
| pass |
|
|
| IncreaseSomeOfTheNumbers().visit(parse_tree) |
|
|
| Transformer |
| ----------- |
|
|
| .. autoclass:: lark.visitors.Transformer |
| :members: __default__, __default_token__ |
|
|
| Example: |
| :: |
| |
| from lark import Tree, Transformer |
|
|
| class EvalExpressions(Transformer): |
| def expr(self, args): |
| return eval(args[0]) |
|
|
| t = Tree('a', [Tree('expr', ['1+2'])]) |
| print(EvalExpressions().transform( t )) |
|
|
| # Prints: Tree(a, [3]) |
|
|
| Example: |
| :: |
| |
| class T(Transformer): |
| INT = int |
| NUMBER = float |
| def NAME(self, name): |
| return lookup_dict.get(name, name) |
|
|
| T(visit_tokens=True).transform(tree) |
|
|
|
|
| v_args |
| ------ |
|
|
| .. autofunction:: lark.visitors.v_args |
|
|
| Discard |
| ------- |
|
|
| .. autoclass:: lark.visitors.Discard |
