Visitor pattern (ast)

separate data structure from algorithms run on the data structure - perform new operations on nodes of an AST without modifying the node classes themselves

basic

• Used with abstract syntax tree (AST) in compilers
• relies on double dispatch
  • AST node interface has common accept(visitor)
  • each concrete node class implements it, and calls visitor.visit(this)
  • Visitor interface defines visit() for every concrete AST node type in tree
  • concrete visitors define specific algorithm for each node type
    • a visitor could calculate the area
    • another could export tree to a different format

benefits

• separation of concerns - AST node focuses on structure and traversal, while visitor handles operations
• visitors can be added without modifying the AST node class
• all logic for a single pass (like type checking) is contained in one visitor class
• no messy type checking