LARK Introduction to Parsing

Lark is the parsing tool used by the domain specific language team to enable algorithmic, grammar based conversion of dsl files, which are largely written in plaintext, to a WDL++.

from lark import Lark, Transformer, v_args
  
try:
    input = raw_input
except NameError:
    pass
 
my_grammar_calc = """
    %import common.CNAME -> NAME
    %import common.NUMBER
    %import common.WS_INLINE
    %ignore WS_INLINE
 
    ?start: value -> number
    ?value: add
        | SIGNED_NUMBER -> assign_var
    ?add: sub
        | add "+" sub -> add
    ?sub: mult
        | sub "-" mult -> sub
    ?mult: div
        | mult "*" div -> mul
    ?div: neg
        | div "/" neg -> div
    ?neg: parenth
        | "-" parenth -> neg
    ?parenth: value
        |"(" value ")"
"""
 
@v_args(inline = True)
class my_calc_tree(Transformer):
    from operator import add, sub, mul, truediv as div, neg
    number = float
    def __init__(self):
        self.vars = {}
    def assign_var(self, nm, val):
        self.vars[name] = val
 
parse_my_calc = Lark(my_grammar_calc, parser = 'lalr', transformer=my_calc_tree())
calculator = parse_my_calc.parse
 
def test():
    print(calculator("a = 3+4"));
    print(calculator("a = 3/4"));
    print(calculator("a = 3*4"));
    print(calculator("a = 3-4"));
 
test()