lang-mu

lang-mu is a Python distribution for the mu Python package, an implementation of the Mu configuration language. It provides:

• A parser that preserves Mu syntax as an AST.
• A typed decoder that maps Mu expressions into Python dataclasses and typing constructs.
• An experimental runtime evaluator (mu.exec) for callable execution semantics.

Why this library

Mu configuration files can describe nested structures, tagged records, and mixed positional/named arguments. This package gives you a strict, testable way to parse and decode those configs into Python types.

Installation

pip install lang-mu

Quickstart: Parsing

from mu import AtomExpr, Document, GroupExpr, parse

source = """
; application plus shared includes
(app-jvm "billing-api"
  :main "billing.Main"
  :ports [8080 8443]
  :env {profile: prod, region: us-east-1}
)
(include "shared/logging.mu")
"""

doc = parse(source)
assert isinstance(doc, Document)
assert len(doc.exprs) == 2

app = doc.exprs[0]
assert isinstance(app, GroupExpr)
assert isinstance(app.values[0], AtomExpr)
assert app.values[0].value == "app-jvm"

Parser Literals

The parser supports both single- and double-quoted strings, and parses numeric literals into dedicated AST node types:

• SInt for integers
• SReal for real numbers and percentages
• SRational for rational values

from mu import parse
from mu.types import AtomExpr, SInt, SRational, SReal, StringExpr

doc = parse("'hi' 42 1.5 50% 2/3 first'")
assert doc.exprs == [
    StringExpr("hi"),
    SInt(42),
    SReal(1.5),
    SReal(0.5),
    SRational((2, 3)),
    AtomExpr("first'"),
]

Quickstart: Typed decoding

from dataclasses import dataclass

from mu import parse_one


@dataclass
class Http:
    port: int


@dataclass
class Worker:
    queue: str


@dataclass
class Service:
    name: str
    mode: Http | Worker


cfg = parse_one('(service "api" :mode (http :port 8080))', Service)
assert cfg == Service(name="api", mode=Http(port=8080))

Error handling

Typed decoding raises DecodeError with structured context:

• path: decode path (for example $.field[0])
• expected: human-readable expected target/type
• got: actual Mu expression description
• span: optional source span/token information
• cause: optional underlying exception

from dataclasses import dataclass
from mu import DecodeError, parse_one


@dataclass
class Counter:
    value: int


try:
    parse_one('(counter :value "not-an-int")', Counter)
except DecodeError as e:
    print(e.path, e.expected, e.got)

API contract

Stable API (from mu import ...)

• Stable symbol reference is generated from code: docs/api-stable.md.
• Main entry points:
  • parse, ParseError
  • parse_one, parse_many, decode
  • DecodeError, DecoderRegistry, Quoted

Experimental API (from mu.exec import ...)

• Experimental symbol reference is generated from code: docs/api-experimental.md.

The experimental runtime API is available but not considered stable yet. Non-exported internals (for example mu.arg_match and parser private helpers) are unsupported and may change without notice.

Python support

• Python >=3.10

License

This project is licensed under AGPL-3.0-or-later. See LICENSE.md for full text.

Development and release checks

python scripts/generate_api_docs.py --check
python scripts/check_docs_links.py
pytest -q tests/test_docs_snippets.py
codespell README.md docs CHANGELOG.md CONTRIBUTING.md --ignore-words=.codespell-ignore-words.txt
mkdocs build --strict
pytest -q
ruff check .
python -m build --sdist --wheel
python -m twine check dist/*
./scripts/check_wheel_contents.sh