一个简单的编译器源码

代码说明：

这是编程语言这门课的大作业（CS 383 Course Project ）     In this project, you are going to implement an interpreter for a simple programming language called SimPL. The language is an extension of   calculus which is the basis of functional programming but includes some features imperative programming as well. Your interpreter should work like a shell, which takes in an expression and produce correct evaluation results.