1 Examples

Erez Shinan edited this page 2017-10-31 19:07:44 +02:00

Table of Contents

• Tiny Calculator
• Fruit Flies Like Bananas

Here are some tiny examples of using Lark.

For more examples, check out the examples folder.

Tiny Calculator

from lark import Lark, InlineTransformer
parser = Lark('''?sum: product
                     | sum "+" product   -> add
                     | sum "-" product   -> sub

                 ?product: item
                     | product "*" item  -> mul
                     | product "/" item  -> div

                 ?item: NUMBER           -> number
                      | "-" item         -> neg
                      | "(" sum ")"

                 %import common.NUMBER
                 %import common.WS
                 %ignore WS
         ''', start='sum')

class CalculateTree(InlineTransformer):
    from operator import add, sub, mul, truediv as div, neg
    number = float

def calc(expr):
    return CalculateTree().transform( parser.parse(expr) )

In the grammar, we shape the resulting tree. The '->' operator renames branches, and the '?' prefix tells Lark to inline single values. (see the tutorial for a more in-depth explanation)

Then, the transformer calculates the tree and returns a number:

>>> calc("(200 + 3*-3) * 7")
1337.0

Fruit Flies Like Bananas

Lark can automatically resolve ambiguity by choosing the simplest solution. Or, you can ask it to return all the possible parse trees, wrapped in a meta "_ambig" node.

Here's a toy example to parse the famously ambiguous phrase: "fruit flies like bananas"

from lark import Lark

grammar = """
    sentence: noun verb noun        -> simple
            | noun verb "like" noun -> comparative

    noun: adj? NOUN
    verb: VERB
    adj: ADJ

    NOUN: "flies" | "bananas" | "fruit"
    VERB: "like" | "flies"
    ADJ: "fruit"

    %import common.WS
    %ignore WS
"""

parser = Lark(grammar, start='sentence', ambiguity='explicit')  # Explicit ambiguity in parse tree!

tree = parser.parse('fruit flies like bananas')

from lark.tree import pydot__tree_to_png    # Just a neat utility function
pydot__tree_to_png(tree, "examples/fruitflies.png")