Implement the substring function.
Implement the string-append function.
Implement two functions of string->list and list->string.
Implement the string-copy function.
Implement the string-fill! function.
Implement REPL mode.
Implement the variable definition and evaluation.
But the functions of cond and case is comment outed for syntax error.
I will fix them later.
Fix the error in the functions of cond and case for adding Env parameter.
And uncomment them.
Add new primitives of PrimitiveFunc, Func that is used by the function definition.
Implement the showVal function for PrimitiveFunc, Func.
Fix the warning of shadowing the existing binding.
Add the ability to define any user function.
Implement the file I/O functions.
Now, we can define any function and variable in any file.