Wire a typed callback — Discover and trace

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

Read a zero-argument callback precisely

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.

The host contract installs app.ready as fnc() -> void. A reviewer asks the response to read event.windowId, but no Event argument or ambient event binding is 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 parameter list is empty, so there is no event parameter to inspect. The response can perform only operations supported by its separately supplied context. The void result promises no returned event record.

Compare the tempting alternative: Copying an Event parameter from a different host is not a harmless annotation: it changes the callback contract and assumes a payload this host never supplies.

Worked reasoning and supplied artifact

// Paper trace / supplied exercise-host contract; no execution claimed.
// The host contract installs app.ready as fnc() -> void. A reviewer asks the response to read event.windowId, but no Event argument or ambient event binding is supplied.
// The parameter list is empty, so there is no event parameter to inspect. The response can perform only operations supported by its separately supplied context. The void result promises no returned event record.

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.

Make an input/result contract card for this callback and list the additional information needed to route readiness by window.

What to produce

A contract card distinguishing parameters, external context and result.

• 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.

• Read a zero-argument callback precisely
• Make an input/result contract card for this callback and list the additional information needed to route readiness by window.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Circle the syntax that establishes the absence of parameters.

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