Introduction to Lark Parser

Lark is a parsing library for Python that provides a wide range of features and a flexible interface for handling various grammar contexts. It is efficient, easy to use, and can parse both context-free and context-sensitive languages. This guide covers some of the essential APIs of the Lark parser along with examples to help you get started.

Installation

pip install lark-parser

Creating a Simple Parser

The basic usage of Lark involves creating a parser with a specific grammar. Here’s an example:

from lark import Lark

parser = Lark('''
    start: "hello" "world"
''', start='start')

parse_tree = parser.parse('hello world')
print(parse_tree) 

Custom Transformers

Lark allows you to transform the parse tree into a more useful form. By subclassing the `Transformer` you can apply transformations to each node:

from lark import Lark, Transformer

class MyTransformer(Transformer):
    def start(self, items):
        return " ".join(items)

parser = Lark('''
    start: WORD+ 
    WORD: /[a-zA-Z]+/ 
''', start='start', parser='lalr')

transformer = MyTransformer()
result = transformer.transform(parser.parse('hello world'))
print(result)  # Output: hello world

Error Handling

With Lark, you can handle syntax errors gracefully using custom error messages:

from lark import Lark, UnexpectedInput

parser = Lark('''
    start: "hello" "world"
''', start='start')

try:
    parser.parse('hello')
except UnexpectedInput as e:
    print("Syntax Error: ", e)

Advanced Grammar

Lark supports advanced grammar definitions including regular expressions and custom tokens:

from lark import Lark

parser = Lark('''
    start: item ("," item)*
    item: WORD
    WORD: /[a-zA-Z]+/
    
    %import common.WS
    %ignore WS
''', start='start', parser='lalr')

parse_tree = parser.parse('apple, banana, cherry')
print(parse_tree)

Creating an App with Lark

Below is an example of a simple calculator application using Lark’s parsing capabilities:

from lark import Lark, Transformer

class CalculateTree(Transformer):
    def add(self, items):
        return items[0] + items[1]
    def sub(self, items):
        return items[0] - items[1]
    def number(self, items):
        return int(items[0])

calc_grammar = '''
    ?start: add
    add  : add "+" mul -> add
         | add "-" mul -> sub
         | mul
    mul  : NUMBER
    %import common.NUMBER
    %ignore " "
'''

parser = Lark(calc_grammar, parser='lalr', transformer=CalculateTree())

expr = '3 + 5 - 2'
print(f'{expr} = {parser.parse(expr)}')

Conclusion

Lark is a powerful and flexible parsing library for many parsing needs. It is easy to set up and allows for extensive customization. If you need to parse complex data or languages, Lark is an excellent choice.

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