illustrate how to implement a lisp interpreter

illustrate how to implement a  lisp interpreter,see pic

1.lexer:

lexer read and translate the input string  "(+ 1 2 )"  into tokens.(Pair)

2.parser:

parser translate the tokens into AST(abstract syntax tree)

3.evaluator:

evaluator evaluate the AST to a result.

4 return the result==>3,done. so easy!:)