| """ | |
| LALR’s contextual lexer | |
| ======================= | |
| Demonstrates the power of LALR’s contextual lexer on a toy configuration language. | |
| The tokens NAME and VALUE match the same input. A standard lexer would arbitrarily | |
| choose one over the other, which would lead to a (confusing) parse error. | |
| However, due to the unambiguous structure of the grammar, Lark's LALR(1) algorithm knows | |
| which one of them to expect at each point during the parse. | |
| The lexer then only matches the tokens that the parser expects. | |
| The result is a correct parse, something that is impossible with a regular lexer. | |
| Another approach is to discard a lexer altogether and use the Earley algorithm. | |
| It will handle more cases than the contextual lexer, but at the cost of performance. | |
| See examples/conf_earley.py for an example of that approach. | |
| """ | |
| from lark import Lark | |
| parser = Lark(r""" | |
| start: _NL? section+ | |
| section: "[" NAME "]" _NL item+ | |
| item: NAME "=" VALUE? _NL | |
| VALUE: /./+ | |
| %import common.CNAME -> NAME | |
| %import common.NEWLINE -> _NL | |
| %import common.WS_INLINE | |
| %ignore WS_INLINE | |
| """, parser="lalr") | |
| sample_conf = """ | |
| [bla] | |
| a=Hello | |
| this="that",4 | |
| empty= | |
| """ | |
| print(parser.parse(sample_conf).pretty()) | |