Permalink
847870f Jun 27, 2018
0 contributors

Users who have contributed to this file

76 lines (52 sloc) 1.41 KB
#
# This example shows how to write a basic calculator with variables.
#
from lark import Lark, Transformer, v_args
try:
input = raw_input # For Python2 compatibility
except NameError:
pass
calc_grammar = """
?start: sum
| NAME "=" sum -> assign_var
?sum: product
| sum "+" product -> add
| sum "-" product -> sub
?product: atom
| product "*" atom -> mul
| product "/" atom -> div
?atom: NUMBER -> number
| "-" atom -> neg
| NAME -> var
| "(" sum ")"
%import common.CNAME -> NAME
%import common.NUMBER
%import common.WS_INLINE
%ignore WS_INLINE
"""
@v_args(inline=True) # Affects the signatures of the methods
class CalculateTree(Transformer):
from operator import add, sub, mul, truediv as div, neg
number = float
def __init__(self):
self.vars = {}
def assign_var(self, name, value):
self.vars[name] = value
return value
def var(self, name):
return self.vars[name]
calc_parser = Lark(calc_grammar, parser='lalr', transformer=CalculateTree())
calc = calc_parser.parse
def main():
while True:
try:
s = input('> ')
except EOFError:
break
print(calc(s))
def test():
print(calc("a = 1+2"))
print(calc("1+a*-3"))
if __name__ == '__main__':
# test()
main()