Plan try, recover, and raise boundaries — Discover and trace

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

Trace failure before any durable effect

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 draft import first validates the supplied text and only then asks the host to store a record. Input text lacks the required title. Contract: validation failure performs no storage request.

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: Read text → discover missing title → report validation problem. There is no storage transition and the previous stored record remains unchanged. A recovery prompt can request a title because the operation is known not to have started.

Compare the tempting alternative: Deleting the previous record before validation introduces a durable effect into what should be a harmless input error.

Worked reasoning and supplied artifact

// Paper trace / supplied exercise-host contract; no execution claimed.
// A draft import first validates the supplied text and only then asks the host to store a record. Input text lacks the required title. Contract: validation failure performs no storage request.
// Read text → discover missing title → report validation problem. There is no storage transition and the previous stored record remains unchanged. A recovery prompt can request a title because the operation is known not to have started.

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.

Draw the effect boundary and trace missing title versus valid title Birch. State the unchanged-data postcondition for invalid input.

What to produce

A before/after ledger with a no-storage-call row.

• 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.

• Trace failure before any durable effect
• Draw the effect boundary and trace missing title versus valid title Birch. State the unchanged-data postcondition for invalid input.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Name the precondition checked before storage.

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