Interpreter

The parser has finally generated a rooted, structured AST tree.

Token('CODE_BLOCK', 'CODE_BLOCK')
  Token('ASSIGN', '=')
    Token('VARIABLE', 'a')
    Token('INTEGER', '1')
  Token('ASSIGN', '=')
    Token('VARIABLE', 'b')
    Token('INTEGER', '2')

The interpreter traverses the tree and visits each node. If its for example an ASSIGN token, there has to be 2 children – the variable name and the value which should be stored. This is the most important part. It is where variables get created and managed.

class Interpreter {
 
	private:
 
		Parser parser;  
 
	public:
 
		Interpreter(Parser _parser) {
			parser = _parser;
		} 
 
		~Interpreter() {};
 
		Token interpret() {	
			ASTNode tree = parser.parse();
			return visit(tree);
		}
 
		Token visit(ASTNode node) {
			if(node._token()._type() == EMPTY) {
				return Token(INTEGER, "0");
			}
 
			else if(node._token()._type() == CODE_BLOCK) { 
 
				Token token;
 
				for(auto it = node.child.begin(); it != node.child.end(); it++) {
 
					token = visit(**it);
 
				} 
 
				return token;
 
			}
 
			else if(node._token()._type() == INTEGER) {
 
				return node._token();
 
			}
 
			else if (node._token()._type() == ASSIGN) {
 
				Token token = visit(*node.child[1]); 
 
				if (token._type() == INTEGER) {
 
					int i = 0;
					try {
						i = stoi(token._value());        
					} catch(...) { 
						return Token(INTEGER, "1"); // indicating error                    
					}
					printf("Assigning Integer %d\n", i );
 
				}
 
			}
 
			return Token(INTEGER, "0");
 
		}
 
};