What is the use of Visitor pattern?
The Visitor pattern lets you execute an operation over a set of objects with different classes by having a visitor object implement several variants of the same operation, which correspond to all target classes. Use the Visitor to clean up the business logic of auxiliary behaviors.
What is a drawback of the Visitor pattern?
A drawback to this pattern, however, is that it makes extensions to the class hierarchy more difficult, as new classes typically require a new visit method to be added to each visitor.
Why do we use visitor pattern in AST?
The Visitor pattern was introduced to address the above scenario. Instead of spreading all the code for a given traversal throughout the nodes’ classes, the code is concentrated in a particular traversal class. That code is called by arranging for each node to Let’s suppose we wish to traverse our AST to print out its nodes.
Which is a classic use of the visitor pattern?
A classic use of the visitor pattern (often quoted), is the processing of an abstract syntax tree in a compiler. Indeed, the structure of the tree is only known at runtime (once the program is parsed), and one want to traverse the tree modifying the nodes according to semantic passes implemented as visitor.
Why do we need to print the AST?
We may want to print the AST, perform semantic analysis, or generate code. Each of these could be accomplished by refining the notion of tree traversal in extensions of some common superclass. Thus, the actual work performed upon a node depends on two things: the node type and the traversal type.
How is the visitor pattern implemented in Common Lisp?
For languages whose object systems support multiple dispatch, not only single dispatch, such as Common Lisp or C# via the Dynamic Language Runtime (DLR), implementation of the visitor pattern is greatly simplified (a.k.a. Dynamic Visitor) by allowing use of simple function overloading to cover all the cases being visited.