Inspiration drawn much from The Little Schemer by Daniel P. Friedman and Mathias Felleisen

This little Python program tries to evaluate Scheme (a Lispy language) code. When run using python scheme.py, it provides a (rudimentary) Read-Eval-Print Loop.

• Truth values, i.e. #t and #f
• Numbers, i.e. integers and floats
• Atoms (as strings)
• Lists (as Python lists)
• Primitive functions/forms:

  • List manipulation:

      (cons a b)
      (car l)
      (cdr l)
    

  • Additions implemented with the sum function:

      (+ <arg1> <arg2> ...)
    

  • Multiplications:

      (* <arg1> <arg2> ...)
    

  • Difference which takes at least 1 argument:

      (- <arg1> <arg2> ...)
    

  • Divisions which takes at least 1 argument:

      (/ <arg1> <arg2> ...)
    

  • Predicates: >, <, =, not, eq?, null?, atom?, zero?

  • Connectives: and, or

  • Quotes

      (quote <arg>)
      '<arg>
    

  • Begins which will evaluate to the value of the last s-expression:

      (begin <arg1> <arg2> ...)
    

  • Lambdas

      (lambda (<arglist>) <sexps>)
    

  • Conditions

      (cond (<cond1> <sexps1>) (<cond1> <sexps1>) ...)
    

1. Currently, the number of provided arguments isn't checked, except for lambdas (which will raise a TypeError). Thus,

    (quote a b) --> 'a'
    (/) --> TypeError: Expected at least 1 arguments (0 provided)
   

2. Because of Python's duck typing, this is possible:

    (* 2 (quote a)) --> 'aa'