> The auto-generated AST data structures are really the selling point here But sometimes you want something custom ... A fun project I did a while back was im

> The auto-generated AST data structures are really the selling point here But sometimes you want something custom ... A fun project I did a while back was implementing a C subset (no structs, pointers or goto) using a directly-executable AST! Kind of a hybrid approach between a compiler and interpreter. This was built as a C++ library to load and execute C-like macros (including a domain-specific library of built-in functions). What I did (using C++) was have two base classes, Statement and Expression, with respective virtual functions Execute() and Value(). Each type of C statement and expression then derived a corresponding "AST" class from these. e.g. class IfStatement derived from Statement and implemented the Execute() method. The IfStatement constructor (following the grammar) took an Expression and two Statement base-class pointers as arguments which it stored. The implementation of IfStatement::Execute() was then just: if (expression->Value() if_statement->Execute(); else else_statement->Execute(); Similar for ForStatement, etc, etc. The root of the AST was then a CompoundStatement representing the entire program as a list of Statements, and you would then execute the program just by calling the Execute() method on this statement. In this case I used a recursive descent parser (part of the library I built) to build this "AST", but of course you could do the same for a table driven parser, and it would make a nice demo of how little code you need to write to not only parse but also execute/evaluate whatever you are parsing.