Wire a typed callback — Build the smallest useful slice

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

Design a typed event adapter

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.

A supplied conceptual event contract says selectionChanged carries oldId:string and newId:string. Example delivery is oldId=Cedar, newId=Maple. This is a host contract exercise; exact event-registration syntax is not supplied.

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: The adapter reads newId=Maple, checks the destination is permitted, and requests the Maple view. oldId=Cedar is historical context, not the new target. The result is void; a host request is an effect rather than a returned string.

Compare the tempting alternative: Using oldId opens the previous selection and can look like a delayed interface. Returning Maple without requesting a view does not perform the specified host effect.

Worked reasoning and supplied artifact

// Paper trace / supplied exercise-host contract; no execution claimed.
// A supplied conceptual event contract says selectionChanged carries oldId:string and newId:string. Example delivery is oldId=Cedar, newId=Maple. This is a host contract exercise; exact event-registration syntax is not supplied.
// The adapter reads newId=Maple, checks the destination is permitted, and requests the Maple view. oldId=Cedar is historical context, not the new target. The result is void; a host request is an effect rather than a returned string.

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.

Describe the callback signature and adapter steps without inventing a registration API. Trace Cedar→Maple and Maple→Maple.

What to produce

A typed contract card and two input-to-effect rows.

• 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.

• Design a typed event adapter
• Describe the callback signature and adapter steps without inventing a registration API. Trace Cedar→Maple and Maple→Maple.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Explain how an input field differs from a callback result.

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