Yantra is a C++ parser generator: lexer, parser, and AST walker all generated from one tool. It builds the whole AST first, then walks it.
Most LALR parser generators (Yacc, Bison, Lemon) run your semantic actions during parsing, as each rule reduces, bottom-up.
That means at the time a rule's action runs, you don't yet know what its parent looks like. This pushes a lot of grammars toward hand-built AST classes and a separate walking pass whenever you need to look ahead into siblings or defer a decision until more context is available.
On the other hand, Yantra always builds the whole AST first, then walks it top-down in a separate pass, calling your semantic actions as it goes. A parent rule's action can run before its children are visited.
