README.md

============================== PyScheme, a scheme for pythonistas

It is a small scheme written in Python. Its goals are not to implement any specific lisp dialect, but to explain how to implement an interpretor. There is a my blog to explain how these codes been produced.

Required

The program build and test in python 2.7.6. DID NOT support Python 3 yet.

How to play

Download or git out them, then run python main.py. It's an interact command line. Play the simple Scheme script. Enjoy it.

Also, you can run

python scope.py
python parser.py
python expression.py

and see the test results for each module.

How to read these codes

builtin.py

This file includes the defines for list, func and the operator for calculation or logic: +,-,>=,if, and,or, etc.

expression.py

This file implement a Expression Class that contains the logic to evaluate a statement.

parser.py

This file is easy to understand. It define a class analysis the words and build a tree for expression to evaluate.

scope.py

This file define a class to implement the life cycles of variables.

That is all. Sorry for my poor english writings.

Some test code

Now it ONLY supports such types: integer, func, list, NO string and else. If you are interesting, you can try to add other type. And I hasn't implement the COMMENT ; single line comment or # multiple lines comments # yet. So if you try to execute the command follows, please DON'T copy the comment. Here are some test code and their expected results. tuple[0] is the code, and tuple[1] is its expected output.

>> (+ 1 2 3) ; add
6
>> (if 1 2 4) ; if 1 then 2 else 4
2
>> (if 0 2) ; if 0 then 2 else None
None
>> (def a 3) ; define a variable 'a'
3
>> a ; show the variable 'a'
3
>> (list 1  2  3  4) ; a list
(list 1 2 3 4)
>> (func (x) (+ x x) ; a function require an argument 'x'
(func (x) (+ x x))
>> (begin (def x 3) (* 1 2)) ; begin statement, it will return the result of the last statement
2
>> (def li (list 9 8 7 6)) ; define a variable 'li' of a list
(list 9 8 7 6)
>> (first li) ; first operation for list 'li'
9
>> (rest li) ; rest operation for list 'li'
(list 8 7 6)
>> (def sqr (func (x) (* x x))) ; define a function 'sqr' for square
(func (x) (* x x))
>> (sqr 5) ; execute function 'sqr' by 5
25
>> (def mul (func (x y) (* x y))) ; define a function 'mul' for mutiple
(func (x y) (* x y))
>> (def mul7 (mul 7)) ; define a partial function 'mul7' which require an argument and return a result equals the argument multiple by 7 
(func (x y) (* x y))
>> (mul7 9) ; try to execute the partial function 'mul7'
63
>> (> 3 2 1) ; 3 > 2 and 3 > 1
True
>> (> 2 2 1) ; 2 > 2 and 2 > 1
False
>> (= 3 3 3) ; 3 == 3 and 3 == 3
True
>> (= 3 3 2) ; 3 == 3 and 3 > 2
False
>> (and 1 2 3) ; 1 and 2 and 3
True
>> (and 1 2 (or 0 0 0 1)) ; 1 and 2 and (0 or 0 or 0 or 1)
True
>> ( begin 1 (and 9 8 7) (def gx 3) (sqr 9)) ; try a complex begin statement
81
>> (xor 1 1) ; 1 xor 1
False
>> (xor 0 0)
False
>> (xor 1 0)
True
>> (xor 0 1)
True
>> wtf ; call a undefined variable
error: 'wtf' is undefined.
>> (unfunc 1 2) ; call a undfined function
error: 'unfunc' is undefined.