peye

peye — reasoning you can see.

A standalone, dependency-free Python rule language with forward and backward reasoning and checkable proofs.

peye turns explicit facts and rules into conclusions whose derivations can be inspected and checked. An answer can arrive together with a certificate, and that certificate can be verified against the program that produced it.

Facts and rules are Python:

from peye import *

fact(human('socrates'))
forward(mortal(X), human(X))

There is nothing to declare: a name the program uses without defining it is a variable when it starts with a capital or an underscore, like X, and a predicate otherwise, like human. Atoms are strings.

forward(Head, *Body) materializes conclusions until a fixpoint. backward(Head, *Body) defines a predicate evaluated when called. The two compose: a forward body may call backward definitions, and a backward goal may use facts that forward reasoning established.

The thread

peye is built around one idea: reasoning you can see. You write facts and rules; peye draws conclusions, forward until nothing new follows and backward on request, and every answer can come with a proof. A separate checker verifies that proof against the program, independently of the reasoner, and does more than a classic proof checker:

• The derivation: every step is an instance of a program clause (C1), nothing is circular (C2), and every claim and every use is justified (C4).
• The form (C3): every step has exactly one justification of an allowed kind, in the right shape, so the report shows at once whether a proof fails on form or on content.
• Recomputation (C5): built-in calculations are redone rather than trusted.
• Honesty about absence: "there is nothing that …" and "these are all the answers" cannot be proved; they become explicit obligations, which the checker tries to refute with the evidence at hand (C6).
• The right question (C7): the proof answers the question asked and contains nothing beside it.

The report is itself Python data, and every generated proof is checked before it is returned. Proofs are read with Python's ast module and never executed, so checking a proof from someone else runs none of their code. Around that core, 59 examples grew, from Socrates and the zebra puzzle to a hospital research portal decided under today's EU rules and under the Commission's Digital Omnibus proposal, and package holiday cancellations under the 2015 and the revised Package Travel Directive. Each has a deck for a wide audience and can be run in the playground.

A proof guarantees that the conclusions follow from the rules, not that the rules say what the law or the policy says. So peye --unused shows which parts of a translation make no difference to the conclusions, and an expert knows where to look.

Run it

Python 3.9 or newer. No dependencies, no build step. From the root of a checkout:

python -m peye examples/socrates.py
python -m peye --proof examples/socrates.py
python -m peye --proof examples/socrates.py | python -m peye --check-proof - examples/socrates.py
python -m unittest discover -s tests

To use peye from anywhere, install it; an editable install keeps using the checkout, so your edits take effect at once:

pip install -e .
peye --proof examples/socrates.py
python examples/socrates.py --proof

Once peye is installed, a program is also a script: python program.py runs it with the same options as peye program.py. Run peye --help for the full command line.

Or in the browser: the playground edits, runs and checks any example, with peye running in the page through Pyodide. To run it from a checkout, serve it (python -m http.server) and open /playground/.

From Python

from peye import check_proof, load_text, run

socrates = load_text("""
from peye import *
fact(human('socrates'))
forward(mortal(X), human(X))
""")
result = run(socrates, goal='mortal(X)', proof=True)
print(result.answers)                                  # ["mortal('socrates')"]
print(check_proof(socrates, result.proof)['valid'])    # True

load('program.py') reads a program file the same way.

Read on

• Make reasoning something you can see — what the language is for, how to write it, what a checked proof does and does not establish, and how the engine works.
• Examples — 59 complete programs, each with its saved conclusions, proof and C1-C7 check report.
• Example decks — a short card deck for every example, explaining it for a wide audience: the question, what peye concludes, why, and what the proof checker confirms.
• Playground — write a program in the browser, run it, check its proof, and share a link to exactly what you see.

License

MIT