Lesson 06
AST vs parse tree, thiết kế node, duyệt pre/in/post-order, visitor pattern (eval/print/typecheck).
Lesson 07
Bảng ký hiệu (hash map), global/local/block, lexical vs dynamic, scope chain, shadowing.
Lesson 08
Hệ thống kiểu, static vs dynamic, type rules & inference bottom-up, coercion, type error.
Lesson 09
eval(node, env), statement vs expression, if/while, runtime error — ngôn ngữ đồ chơi chạy được.
Lesson 10
Gọi hàm & environment mới, call stack & đệ quy, closure (captured env), loop variable bug.