diff --git a/doc/python-target.md b/doc/python-target.md index 6780c4fe05..b747f34a22 100644 --- a/doc/python-target.md +++ b/doc/python-target.md @@ -71,22 +71,36 @@ _For a list of antlr4 tool options, please visit [ANTLR Tool Command Line Option python Driver.py input.txt ``` -## Visitors +## Parse tree traversal -Antlr listeners and visitors are implementations that traverse a parse tree in slightly different ways. Tree traversal is used to implement [static](https://en.wikipedia.org/wiki/Static_program_analysis) or [dynamic](https://en.wikipedia.org/wiki/Dynamic_program_analysis) program analysis. -Understanding when to choose a listener versus a visitor is a good idea. +Antlr generates two types of tree traversals: visitors and listeners. + +Understanding when to choose a visitor versus a listener is a good idea. For further information, see https://tomassetti.me/listeners-and-visitors/. -A visitor is the best choice when computing only a synthesized attribute. + +You can use this tree walker +to implement both [inherited](https://en.wikipedia.org/wiki/Attribute_grammar#Inherited_attributes) +and [synthesized attribute](https://en.wikipedia.org/wiki/Attribute_grammar#Synthesized_attributes) +analysis. + +A visitor is the best choice when computing only a single synthesized attribute. Alternatively, a listener is the best choice when computing both synthesized and inherited attributes. -But, in most situations, they are interchangeable. +But, in many situations, they are interchangeable. + +### Visitors + +Antlr visitors generally implement a post-order tree walk. If you write +`visit...` methods, the method must contain code to visit the children +in the order you want. For a post-order tree walk, visit the children first. To implement a visitor, add the `-visitor` option to the `antlr4` command. -Add a class that inherits from the generated visitor -with code that implements the analysis. +Create a class that inherits from the generated visitor, +then add `visit` methods that implement the analysis. Your driver code +should call the `visit()` method for the root of the parse tree. For example, the following code implements an expression evaluator for the Expr.g4 grammar using a visitor. @@ -157,13 +171,11 @@ For example, the following code implements an expression evaluator for the Expr. return 0 ``` -## Listeners - -Antlr listeners can be used to implement program analysis. It differs from a visitor in that -there are `enter` and `exit` methods called during the LR tranversal. You can use this tree walker -to implement both [inherited](https://en.wikipedia.org/wiki/Attribute_grammar#Inherited_attributes) -and [synthesized attribute](https://en.wikipedia.org/wiki/Attribute_grammar#Synthesized_attributes) -analysis. +### Listeners + +Antlr listeners perform an LR tree traversal. `enter` and `exit` methods are +called during the tranversal. A parse tree node is visited twice, first for +the `enter` method, then the `exit` method after all children have been walked. To implement a listener, add the `-listener` option to the `antlr4` command. Add a class that inherits from the generated listener