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;
        }
    """))