tutorials 12 production patterns

Production patterns

Production lclang code is easiest to reason about when parsing, per-run values, host capabilities, limits, diagnostics, and cleanup have explicit owners. The same strong structure used in small examples scales to services and tools.

What you will learn

• how to parse policy once and create independent runs through a factory;
• how presets and call values express stable versus transient context;
• how to place limits and error handling at application boundaries;
• how to test policy without external side effects.

Package reusable runtime policy

A FrameFactory retains immutable construction policy. Presets hold stable host bindings; create(values=...) adds or replaces per-run inputs.

import asyncio

import lclang


ORDER = lclang.define_module(
    "order",
    {
        "subtotal": "unit_price * quantity",
        "discount": "subtotal * discount_rate if vip else 0",
        "total": "subtotal - discount",
        "result": 'f"{region}/{currency} {total:.2f}"',
    },
)
DEFAULTS = lclang.Preset(
    "order-defaults",
    {"discount_rate": 0.10, "region": "global", "currency": "USD"},
)
FACTORY = lclang.FrameFactory(
    ORDER,
    DEFAULTS,
    lclang.EvaluationLimits(
        max_depth=100,
        max_steps=10_000,
        max_collection_items=1_000,
    ),
)


async def price(*, quantity: int, vip: bool, region: str | None = None) -> str:
    values: dict[str, object] = {
        "unit_price": 25,
        "quantity": quantity,
        "vip": vip,
    }
    if region is not None:
        values["region"] = region
    async with FACTORY.create(values=values) as frame:
        result = await frame.get("result")
        assert isinstance(result, str)
        return result


async def main() -> None:
    assert await price(quantity=2, vip=False) == "global/USD 50.00"
    assert await price(quantity=4, vip=True, region="eu") == "eu/USD 90.00"


asyncio.run(main())

The regular order uses preset region and currency, computes 25 * 2, and skips the VIP discount. The second call overrides only region; 25 * 4 produces 100, the true VIP branch subtracts ten percent, and result formats the remaining 90 with the EU label.

The Module, Preset, factory, and limits are immutable reusable policy. Every created Frame still has fresh caches, tasks, traces, and lifecycle state. A raw factory has no canonical builtin parent unless one is supplied, so this example uses only its explicit values.

Load once, test many realistic scenarios

File-backed policy can use the same factory pattern. The test controls file I/O with a temporary directory and covers sunny, alternate, and rainy inputs.

import asyncio
from pathlib import Path
from tempfile import TemporaryDirectory

import lclang
from lclang.config import load_config


async def main() -> None:
    with TemporaryDirectory(prefix="lclang-production-tutorial-") as directory:
        path = Path(directory) / "capacity.lclcfg"
        path.write_text(
            "requested: replicas * capacity_per_replica\n"
            "accepted: requested <= hard_limit\n"
            'decision: "accept" if accepted else "reject"\n',
            encoding="utf-8",
        )
        config = await load_config(path)
        factory = config.frame_factory(
            limits=lclang.EvaluationLimits(max_collection_items=100)
        )

        scenarios = [
            ({"replicas": 2, "capacity_per_replica": 20, "hard_limit": 100}, "accept"),
            ({"replicas": 6, "capacity_per_replica": 20, "hard_limit": 100}, "reject"),
        ]
        for values, expected in scenarios:
            async with factory.create(values=values) as frame:
                assert await frame.get("decision") == expected
                assert frame.inspect_variable("requested").definition is not None

        async with factory.create(
            values={"replicas": 2, "capacity_per_replica": 20}
        ) as missing:
            try:
                await missing.get("decision")
            except lclang.LclNameError as error:
                assert "hard_limit" in str(error)
            else:
                raise AssertionError("missing host input was accepted")


asyncio.run(main())

The first scenario requests 40, so 40 <= 100 selects accept. The second requests 120, making the comparison false and selecting reject. The final Frame cannot resolve hard_limit while evaluating accepted; the resulting LclNameError is caught at the scenario boundary and checked for the missing input name.

Production checklist

1. Parse Modules or load Configs once during application setup.
2. Create one Frame per independent request, command, tenant, or scenario.
3. Pass plain values and narrow purpose-built host callables.
4. Mock external connectivity and isolate file inputs in tests.
5. Set evaluation and configuration-loading limits appropriate to the policy.
6. Inspect before evaluating when diagnosis must remain side-effect free.
7. Catch LclError at the boundary that can log, translate, or recover.
8. Treat cached values as snapshots; create a new Frame for a new run.
9. Close every owned Frame deterministically with async with.
10. Treat LCL sources and supplied host capabilities as trusted configuration.

The pattern is opinionated on purpose. Applications gain flexibility through definitions and controlled inputs, not by bypassing the runtime's ownership, inspection, and lifecycle rules.

Previous: Business-day calendars | Next: Scoped values and Frame evaluation | Return to the series introduction

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Edit the tested source.