Understanding Lark-Parser: Powerful Text Parsing Made Easy

Lark-parser is a comprehensive library for parsing and processing text. It enables developers to build sophisticated parsers quickly and efficiently. With dozens of useful APIs, Lark-parser provides an unparalleled solution for text parsing needs.

Installation

pip install lark-parser

Basic Usage

The following example demonstrates the basic usage of the Lark-parser:

 from lark import Lark
parser = Lark('''
    start: "hello" NAME
    NAME: /[a-z]+/
    %import common.WS
    %ignore WS
''')
tree = parser.parse("hello world") print(tree) 

Lark Configuration and API Examples

Defining Grammars

Lark supports various grammar definitions:

 grammar = '''
    start: "let" var "=" value
    var: /[a-zA-Z_][a-zA-Z0-9_]*/
    value: NUMBER
    %import common.NUMBER
    %import common.WS
    %ignore WS
''' parser = Lark(grammar) 

Tree Visitors

Tree visitors help traverse and manipulate parse trees:

 from lark import Visitor
class TransformNumbers(Visitor):
    def number(self, tree):
        tree.data = 'number'
        tree.children[0] = int(tree.children[0])

parser = Lark('''
    start: number+
    number: NUMBER
    %import common.NUMBER
    %ignore " "
''')
tree = parser.parse("1 2 3") TransformNumbers().visit(tree) print(tree) 

Error Handling

Lark provides robust error handling mechanisms:

 from lark import UnexpectedInput
try:
    parser.parse("invalid input")
except UnexpectedInput as e:
    print(e)

Embedding Actions

Actions can directly be embedded in the grammar:

 parser = Lark('''
    start: "add" NUMBER "and" NUMBER -> add
    %import common.NUMBER
    %ignore " "

    add: (lambda args: sum(map(int, args)))
''')
tree = parser.parse("add 2 and 3") print(tree.children[0].children)  # Prints 5 

Real-world Application Example: Simple Calculator

 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 mul(self, items):
        return items[0] * items[1]

    def div(self, items):
        return items[0] / items[1]

calculator_grammar = '''
    ?start: sum
    ?sum: product
        | sum "+" product   -> add
        | sum "-" product   -> sub
    ?product: atom
        | product "*" atom  -> mul
        | product "/" atom  -> div
    ?atom: NUMBER           -> number
        | "(" sum ")"
    %import common.NUMBER
    %import common.WS
    %ignore WS
'''
parser = Lark(calculator_grammar, parser='lalr', transformer=CalculateTree()) result = parser.parse("(1 + 2) * 3 - 4 / 2") print(result)  # Outputs: 8.0 

These examples demonstrate the power and flexibility of Lark-parser in various applications.

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