"""An interpreter for the Scheme-Syntax Calculator Language An interpreter for a calculator language that uses prefix-order call syntax. Operator expressions must be operator symbols. Operand expressions are separated by spaces. Examples: > (* 1 2 3) 6 > (+) 0 > (+ 2 (/ 4 8)) 2.5 > (+ 2 2) (* 3 3) 4 9 > (+ 1 (- 23) (* 4 2.5)) -12 > ) SyntaxError: unexpected token: ) > 2.3.4 ValueError: invalid numeral: 2.3.4 > + TypeError: + is not a number or call expression > (/ 5) TypeError: / requires exactly 2 arguments > (/ 1 0) ZeroDivisionError: division by zero """ from ucb import trace, main, interact from operator import add, sub, mul from scheme_reader import Pair, nil, scheme_read, buffer_input # Eval & Apply def calc_eval(exp): """Evaluate a Calculator expression. >>> calc_eval(as_scheme_list('+', 2, as_scheme_list('*', 4, 6))) 26 >>> calc_eval(as_scheme_list('+', 2, as_scheme_list('/', 40, 5))) 10 """ if type(exp) in (int, float): return simplify(exp) elif isinstance(exp, Pair): arguments = exp.second.map(calc_eval) return simplify(calc_apply(exp.first, arguments)) else: raise TypeError(exp + ' is not a number or call expression') def calc_apply(operator, args): """Apply the named operator to a list of args. >>> calc_apply('+', as_scheme_list(1, 2, 3)) 6 >>> calc_apply('-', as_scheme_list(10, 1, 2, 3)) 4 >>> calc_apply('*', nil) 1 >>> calc_apply('*', as_scheme_list(1, 2, 3, 4, 5)) 120 >>> calc_apply('/', as_scheme_list(40, 5)) 8.0 """ if not isinstance(operator, str): raise TypeError(str(operator) + ' is not a symbol') if operator == '+': return reduce(add, args, 0) elif operator == '-': if len(args) == 0: raise TypeError(operator + ' requires at least 1 argument') elif len(args) == 1: return -args[0] else: return reduce(sub, args.second, args.first) elif operator == '*': return reduce(mul, args, 1) elif operator == '/': if len(args) != 2: raise TypeError(operator + ' requires exactly 2 arguments') numer, denom = args return numer/denom else: raise TypeError(operator + ' is an unknown operator') def simplify(value): """Return an int if value is an integer, or value otherwise. >>> simplify(8.0) 8 >>> simplify(2.3) 2.3 >>> simplify('+') '+' """ if isinstance(value, float) and int(value) == value: return int(value) return value def reduce(fn, scheme_list, start): """Reduce a recursive list of Pairs using fn and a start value. >>> reduce(add, as_scheme_list(1, 2, 3), 0) 6 """ if scheme_list is nil: return start return reduce(fn, scheme_list.second, fn(start, scheme_list.first)) def as_scheme_list(*args): """Return a recursive list of Pairs that contains the elements of args. >>> as_scheme_list(1, 2, 3) Pair(1, Pair(2, Pair(3, nil))) """ if len(args) == 0: return nil return Pair(args[0], as_scheme_list(*args[1:])) @main def read_eval_print_loop(): """Run a read-eval-print loop for Calculator.""" while True: try: src = buffer_input() while src.more_on_line: expression = scheme_read(src) print(calc_eval(expression)) except (SyntaxError, TypeError, ValueError, ZeroDivisionError) as err: print(type(err).__name__ + ':', err) except (KeyboardInterrupt, EOFError): # -D, etc. print('Calculation completed.') return