Check and repair a shipping request

A warehouse must dispatch a request before its deadline. The supplied request claims a three-day route window that exceeds that deadline. You will identify the violated rule, examine a repair and run the check again.

Install GND, activate its environment and start in an empty working directory. All inputs below ship in the package. After installation, this exercise needs no API key, network or native compiler.

Get the model and facts

from importlib.resources import files
from shutil import copytree
copytree(str(files("gnd") / "examples" / "dispatch"), "dispatch-demo")

The folder contains a model, two populations and a fixed reference catalogue. The model specifies the rule; the populations describe requests; the catalogue supplies reference route facts. Start by checking the model:

gnd validate dispatch-demo/model.orm.json --format json

Exit 0 means the model validates. Now check the first population:

gnd check dispatch-demo/model.orm.json dispatch-demo/seed.pop.json --admission --format json

This command intentionally exits 1. In report.verdicts, the failed entry has constraint_ref: constraints[1], the model's dispatch_deadline rule. The report is explaining a semantic violation, rather than a parsing failure. A script should expect this exit for this example; do not hide it with || true.

Inspect the difference and apply the supplied repair

import json
from pathlib import Path
root = Path("dispatch-demo")
before = json.loads((root / "seed.pop.json").read_text())
after = json.loads((root / "repaired.pop.json").read_text())
for fact, rows in before["facts"].items():
    if rows != after["facts"].get(fact):
        print(fact, "before:", rows, "after:", after["facts"].get(fact))
assert before != after

The repair changes the declared request facts; it does not change the policy or rewrite the fixed catalogue. Check it against the same model:

gnd check dispatch-demo/model.orm.json dispatch-demo/repaired.pop.json --admission --format json

Exit 0 means this supplied repaired population passes. Removing or modifying the required reference data instead would refuse the check with exit 3; that would not be a repair.

Put the check in Python

from gnd import api
bad = api.check("dispatch-demo/model.orm.json", "dispatch-demo/seed.pop.json", admission=True)
good = api.check("dispatch-demo/model.orm.json", "dispatch-demo/repaired.pop.json", admission=True)
assert bad["exit_code"] == 1 and good["exit_code"] == 0
failed = [v for v in bad["report"]["verdicts"] if v["verdict"] == "fail"]
assert len(failed) == 1 and failed[0]["constraint_ref"] == "constraints[1]"
print(failed[0]["constraint_ref"], failed[0]["message"])

Your application can use this result before publishing proposed facts. GND checks the model you supplied; deciding whether a proposed repair is the right real-world action remains part of your application workflow.

Exercise: inspect the deadline rule and the two population files. Explain why changing the facts passes while removing the catalogue refuses. Then build BookRec to create an application that produces its own candidate facts, or try release readiness in another domain.