pure-py-mb

PurePy in MoonBit: a tokenizer and parser for Python 3.12 producing a CPython-shaped abstract syntax tree, a sieve that rejects the Python PurePy excludes, a well-formedness checker, an interpreter, a printer, a command-line tool and an embedding interface.

PurePy is a pure functional subset of Python with a small-step operational semantics and a static well-formedness judgement, specified in pure-py/pure-py-spec. This is a port of that specification. The reference checker is the specification for what check decides; CPython is the oracle for what run prints.

Playground: https://marianoguerra.github.io/pure-py-mb/ — the whole library in a browser tab, with 27 examples. The page is a real embedder: the host module its examples import is supplied through the embedding interface.

What is here

lib/	marianoguerra/pure-py	the library — readme
cli/	marianoguerra/pure-py-cli	the pure-py command
docs/		embedding guide, examples compiled as tests
playground/		the page, built to wasm-gc
test/		the conformance suite, the goldens, the corpora
tools/		the differential harness and the generators

The repository root is a development module that is never published. It exists so that the harness, the corpora, the documentation and the playground reach the two real modules through their public API only, which is what keeps that API honest — and so that publishing the library does not publish half a megabyte of test fixtures.

moon add marianoguerra/pure-py       # the library
moon add marianoguerra/pure-py-cli   # the command

Conformance

Eight oracles, every one compared against something outside this port, over the conformance suite of specification v0.23.0.

oracle	compared with	result
tokens	CPython's tokenize	1005/1005
trees, no positions	CPython's ast	1005/1005
trees, with positions	CPython's ast	1005/1005
printer round trip	this parser and CPython's	1004/1004
parse	the reference checker	982/982
check	the reference checker	982/982
check-program	the reference checker	93/93
run	CPython's output	321/321

The conformance suite under test/conformance/ is the reference's own, copied verbatim, and the expected answers under test/golden/ are the reference's answers. Both are committed, so the suite runs with nothing but MoonBit and Python 3.

Development

just lists every task, grouped. The ones that matter:

just quick        type-check, format, unit tests, layering, conformance
just conform      every oracle, held to its ratchet
just one PATTERN  the tests whose name contains PATTERN, in full
just diffrun      generated programs, compared against CPython
just playground   build the page and serve it on :8000
just publish-dry  what would go to mooncakes, without sending it

Regenerating the goldens needs the reference checkout: just reference-fetch, then just goldens. Neither runs in CI.

License

Apache-2.0.