Note

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# Creating an AST from the parse tree

> This example demonstrates how to transform a parse-tree into an AST using lark.ast_utils.

> create_transformer() collects every subclass of Ast subclass from the module, and creates a Lark transformer that builds the AST with no extra code.

> This example only works with Python 3.

    import sys
    from typing import List
    from dataclasses import dataclass

    from lark import Lark, ast_utils, Transformer, v_args
    from lark.tree import Meta

    this_module = sys.modules[__name__]

    #
    #   Define AST
    #
    class _Ast(ast_utils.Ast):
        # This will be skipped by create_transformer(), because it starts with an underscore
        pass

    class _Statement(_Ast):
        # This will be skipped by create_transformer(), because it starts with an underscore
        pass

    @dataclass
    class Value(_Ast, ast_utils.WithMeta):
        "Uses WithMeta to include line-number metadata in the meta attribute"
        meta: Meta
        value: object

    @dataclass
    class Name(_Ast):
        name: str

    @dataclass
    class CodeBlock(_Ast, ast_utils.AsList):
        # Corresponds to code_block in the grammar
        statements: List[_Statement]

    @dataclass
    class If(_Statement):
        cond: Value
        then: CodeBlock

    @dataclass
    class SetVar(_Statement):
        # Corresponds to set_var in the grammar
        name: str
        value: Value

    @dataclass
    class Print(_Statement):
        value: Value

    class ToAst(Transformer):
        # Define extra transformation functions, for rules that don't correspond to an AST class.

        def STRING(self, s):
            # Remove quotation marks
            return s[1:-1]

        def DEC_NUMBER(self, n):
            return int(n)

        @v_args(inline=True)
        def start(self, x):
            return x

    #
    #   Define Parser
    #

    parser = Lark("""
        start: code_block

        code_block: statement+

        ?statement: if | set_var | print

        if: "if" value "{" code_block "}"
        set_var: NAME "=" value ";"
        print: "print" value ";"

        value: name | STRING | DEC_NUMBER
        name: NAME

        %import python (NAME, STRING, DEC_NUMBER)
        %import common.WS
        %ignore WS
        """,
        parser="lalr",
    )

    transformer = ast_utils.create_transformer(this_module, ToAst())

    def parse(text):
        tree = parser.parse(text)
        return transformer.transform(tree)

    #
    #   Test
    #

    if __name__ == '__main__':
        print(parse("""
            a = 1;
            if a {
                print "a is 1";
                a = 2;
            }
        """))

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