Introduction to Lark Parser

Lark is a powerful parsing library for Python, designed to handle complex grammars with speed and efficiency. This article delves into the uses and functionalities of the lark-parser, providing numerous API examples and application use cases.

Getting Started with Lark Parser

To install Lark, you can use pip:

 $ pip install lark-parser 

Basic Usage

Create your first parser:

 from lark import Lark
grammar = ''' start: "hello" NAME NAME: /\\w+/ %import common.WS %ignore WS ''' parser = Lark(grammar)
tree = parser.parse("hello world") print(tree) 

API Examples

Using Transformers

Transformers are used to convert the parse tree into a user-friendly format:

 from lark import Lark, Transformer
class TreeToDict(Transformer):
    def NAME(self, name):
        return str(name)

grammar = ''' start: "hello" NAME NAME: /\\w+/ %import common.WS %ignore WS ''' parser = Lark(grammar, parser='lalr', transformer=TreeToDict()) tree = parser.parse("hello Lark") print(tree) 

Using Visitors

Visitors allow traversal of the parse tree:

 from lark import Lark, Visitor
class MyVisitor(Visitor):
    def NAME(self, tree):
        print("Visited:", tree)

grammar = ''' start: "greet" NAME NAME: /\\w+/ %import common.WS %ignore WS ''' parser = Lark(grammar) tree = parser.parse("greet Lark") MyVisitor().visit(tree) 

Custom Parsing

Define a custom parsing rule:

 from lark import Lark
grammar = ''' start: item ("," item)* item: "item" NUMBER %import common.NUMBER %import common.WS %ignore WS ''' parser = Lark(grammar)
tree = parser.parse("item 1, item 2, item 3") print(tree) 

Application Example

Let’s create a simple app that calculates expressions using Lark parser:

 from lark import Lark, Transformer
class CalculateTree(Transformer):
    def NUMBER(self, n):
        return int(n)
    
    def add(self, items):
        return items[0] + items[1]

grammar = ''' start: addition addition: NUMBER "+" NUMBER -> add %import common.NUMBER %import common.WS %ignore WS ''' parser = Lark(grammar, parser='earley', transformer=CalculateTree())
tree = parser.parse("3 + 4") result = tree print(result) 

With this setup, you can extend the calculator to handle more operations and complexities. Lark parser provides a flexible and powerful way to handle custom grammars in Python.

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