Wire a typed callback — Change one requirement

PART I

Warm-up

Reconnect with a useful idea before starting wire a typed callback. 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

Adapt a payload version safely

A callback has an input and result contract. A callback is a callable value installed for a later invocation. Read its parameter list and return type before its body. A no-argument fnc() -> void does not receive an Event merely because its registration sits inside events. If the host supplies an event payload, it must state the payload shape and callback signature.

Version 1 selection events carry id:string. Version 2 carries selectedId:string and origin:string. The documented adapter receives an explicit version tag. Input is version=2, selectedId=Birch, origin=keyboard.

Separate configuration-time values from delivery-time inputs. For each invocation, list the actual input fields, the values read, and the promised result. An undeclared field or wrong type is a contract mismatch, not an invitation to guess a host conversion. Worked case: Choose the v2 shape from the tag, read selectedId=Birch, and retain origin only if the workflow uses it. The v1 path instead reads id. A version tag selects interpretation; it is not permission to guess missing fields.

Compare the tempting alternative: Trying id first and silently falling back hides malformed v2 payloads and can route to stale data when both fields appear.

Worked reasoning and supplied artifact

// Paper trace / supplied exercise-host contract; no execution claimed.
// Version 1 selection events carry id:string. Version 2 carries selectedId:string and origin:string. The documented adapter receives an explicit version tag. Input is version=2, selectedId=Birch, origin=keyboard.
// Choose the v2 shape from the tag, read selectedId=Birch, and retain origin only if the workflow uses it. The v1 path instead reads id. A version tag selects interpretation; it is not permission to guess missing fields.

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: Support request workflow. Build a small, understandable change around wire a typed callback.

Build a version-to-field table and trace valid v1, valid v2, and v2 missing selectedId. Define malformed-input handling explicitly.

What to produce

A migration adapter contract with three payload cases.

• 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.

• Adapt a payload version safely
• Build a version-to-field table and trace valid v1, valid v2, and v2 missing selectedId. Define malformed-input handling explicitly.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Name the field required by the v2 contract.

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