simple_lisp.py

env = {}                                                                                       
                                                                                               
                                                                                               
def lisp(sexpression):                                                                         
    if isinstance(sexpression, list):                                                          
        if sexpression[0] == 'quote':                                                          
            return sexpression[1:]                                                             
        elif sexpression[0] == 'defn':                                                         
            env[sexpression[1]] = lisp(sexpression[2])                                         
            # Hack for variable assignment.                                                    
            return lambda *x: x[1]                                                             
        return lisp(sexpression[0])(*[lisp(s) for s in sexpression[1:]])                       
    return env.get(sexpression, sexpression)                                                   
                                                                                               
                                                                                               
if __name__ == "__main__":                                                                     
    import operator                                                                            
    assert 4 == lisp([operator.mul, 2, 2])                                                     
    assert 24 == lisp([operator.mul, 2, [operator.mul, 3, 4]])                                 
    assert 'a-b-c' == lisp(['-'.join, ['quote', 'a', 'b', 'c']])                               
    from random import choice                                                                  
    assert lisp([                                                                              
        [choice, ['quote', operator.add, operator.sub]], 10, 1]) in [9, 11]                    
    assert 3 == lisp([                                                                         
        ['defn', 'x', [operator.add, 0, 1]],                                                   
        ['defn', 'y', 2],                                                                      
        [operator.add, 'x', 'y']])                                                             
    assert 10 == lisp([                                                                        
        ['defn', 'double', lambda x: 2*x],                                                     
        ['defn', 'pi', '3.14'],                                                                
        ['double', 5]])