Plan try, recover, and raise boundaries — Boundary detective

PART I

Warm-up

Reconnect with a useful idea before starting plan try, recover, and raise boundaries. 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

Distinguish timeout from confirmed rejection

Recover only when the workflow can preserve its promise. try, recover and raise describe error handling in the shared vocabulary, but host operation signatures and error payloads must be supplied. Recovery is a decision about state and responsibility: which failure can be handled locally, which must be reported upward, and which effects have already happened?

A store request S18 may commit before its response arrives. Case A returns RejectedBeforeWrite. Case B times out with no outcome confirmation. No idempotency guarantee is supplied.

Use a before-operation-after ledger. Mark the last durable change before a failure and identify the evidence available to the handler. Retrying without knowing whether an operation committed can duplicate effects; swallowing an error cannot turn incomplete work into success. Worked case: A is known not to have written, so a corrected retry may be possible. B is unknown: the record may exist even though the response was lost. Repeating B blindly can create a duplicate durable effect.

Compare the tempting alternative: Treating every missing response as “nothing happened” substitutes an assumption for the operation’s outcome evidence.

Worked reasoning and supplied artifact

// Paper trace / supplied exercise-host contract; no execution claimed.
// A store request S18 may commit before its response arrives. Case A returns RejectedBeforeWrite. Case B times out with no outcome confirmation. No idempotency guarantee is supplied.
// A is known not to have written, so a corrected retry may be possible. B is unknown: the record may exist even though the response was lost. Repeating B blindly can create a duplicate durable effect.

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: Safe extension design. Build a small, understandable change around plan try, recover, and raise boundaries.

Create separate state and retry rules for A and B. Propose the smallest host query or stable-identity guarantee that could reconcile B.

What to produce

A known-failure/unknown-outcome table plus a reconciliation requirement.

• 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;
const button = widget.Button({
    text: "Continue",
    events: {
        click: fnc() -> void {
            navigator.open("dashboard");
        }
    }
});

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.

• Distinguish timeout from confirmed rejection
• Create separate state and retry rules for A and B. Propose the smallest host query or stable-identity guarantee that could reconcile B.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Write a user message that accurately describes B.

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