Write a cancellation contract — Boundary detective

PART I

Warm-up

Reconnect with a useful idea before starting write a cancellation contract. In one or two sentences, describe the input, output or host boundary from the previous exercise that this lesson builds on. If you are arriving directly, skim the worked example first; there is no locked prerequisite.

A rough note is enough. You can revise it after the example.

PART II

Instruction & guided example

Check cancellation at the publication race

Cancellation is a request to stop or detach work according to a host contract. It is not automatically rollback, instant interruption or successful completion. Define accepted states and side-effect boundaries before selecting syntax; the source leaves exact cancellation signatures and delivery rules unresolved.

Exercise jobJ builds a read-only report through states requested→admitted→working→ready→published. Cancellation before published prevents publication; after published it does not erase the already accepted result. Host-owned cleanup still has an accountable owner. These are explicit exercise requirements, not asserted standard WillowScript cancellation semantics.

Supplied case: J is ready; cancel and publish are concurrently requested. Contract requires one serialized terminal decision.

Worked reasoning: If cancel wins, no publication. If publish wins, the result is already accepted and cancel reports that boundary. The test must record which decision committed first rather than demand both incompatible outcomes.

Contrast and diagnosis: A vague always-cancel guarantee ignores the irreversible boundary the exercise explicitly preserves.

Worked reasoning and supplied artifact

// Disposable exercise fixture / host-contract notes; not executable API syntax.
// Supplied input: J is ready; cancel and publish are concurrently requested. Contract requires one serialized terminal decision.
// Worked prediction: If cancel wins, no publication. If publish wins, the result is already accepted and cancel reports that boundary. The test must record which decision committed first rather than demand both incompatible outcomes.
// Rejected alternative: A vague always-cancel guarantee ignores the irreversible boundary the exercise explicitly preserves.

Illustrative example from the proposed WillowScript design. The language and WUIX/WMATH/WDB/WAUDIO/WTMP services are not implemented in the supplied source. This site has no interpreter, VM or native host and does not execute or certify the snippet.

Try the guided change

Reconstruct the supplied worked case first: list its inputs, intermediate decisions and final artifact. Compare each row with the explanation and identify exactly where the rejected alternative fails. Then use the changed inputs in Part IV for independent work.

• Write the supplied inputs and initial state before following the example.
• Reconstruct each intermediate calculation or host decision, then compare with the worked artifact.
• Explain why the contrasting alternative fails before attempting the changed inputs in Part IV.

PART III

Take a breather

Step back after the guided example. Look away from the screen, stretch, or reread your prediction. This is an optional learning pause, not a timer or an assessment gate.

When you return, try to name the lesson’s key idea without copying its wording. If that feels difficult, return to Part II and trace one line rather than restarting the entire lesson.

PART IV

Independent practice

Project context: Cancelable workflow design. Build a small, understandable change around write a cancellation contract.

Write two legal traces and one illegal trace showing both canceled and published terminal states. Explain the contradiction.

What to produce

Write two legal traces and one illegal trace showing both canceled and published terminal states. Explain the contradiction. Include input → intermediate decision/state → predicted outcome, with any required host rule stated explicitly.

• State the intended behavior in a sentence before editing.
• Use only source-backed syntax and the host services whose contract you have been given.
• Record a normal case and a boundary or unavailable-resource case.
• Label hand predictions separately from anything actually measured in a supplied toolchain.

Syntax colors are a reading aid, not a compiler. The line you are editing stays uncolored until you move to a different line; capitalization is never changed. Drafts stay on this browser and code is not executed.

import wuix.widget;
import app;
domain Workspace {
    const application = Application({
        name: "Workspace",
        window: { title: "Dashboard", size: [1280, 800] },
        events: {
            "app.ready": fnc() -> void { console.log("Ready"); }
        }
    });
}

PART V

Self-check

Use this check to notice what to revisit. It does not award a private course grade, block another page or claim to verify compiled code. Every answer has a learning explanation.

You may keep exploring even if you are unsure. Use the hint, compare the trace, and revise your explanation.

PART VI

Recap & small task

Bring the idea back to the mod feature you are building. A useful recap connects the mechanism to a decision you can make in your own code.

• Check cancellation at the publication race
• Explain the decision for the supplied case, then independently work the changed fixture.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

In one or two sentences, name the decisive rule in “Check cancellation at the publication race” and predict the outcome of one changed input from your independent fixture. State which host behavior still needs real verification.

This marks your personal practice record only. It is not a certification or an official workspace assessment.