Defend version compatibility, capability policy, and migration decisions — Build the smallest useful slice

PART I

Warm-up

Reconnect with a useful idea before starting defend version compatibility, capability policy, and migration decisions. 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

Build a compatibility adapter

An unchanged client is a separate compatibility target. An adapter must preserve the meaning of inputs and results promised by the old contract, even when the new implementation has a richer internal model. Define the old behavior before reviewing the new feature; otherwise the new implementation can quietly redefine what passing means.

Exercise contract, not a supplied host implementation: reward v 1 accepts a nonnegative whole-number base and returns base + 2. Reward v 2 may accept an optional difficulty bonus, but an omitted bonus must retain v 1 behavior. Invalid inputs are refused before reward state changes. No host API or execution is implied by these fixtures.

Supplied case: The new internal calculator takes base and explicit totalBonus. Old requests provide only base 5.

Worked reasoning: At the boundary translate the old request to base 5, totalBonus 2; the internal result is 7. The adapter owns defaulting once. The calculator should not add a second hidden +2.

Contrast and diagnosis: If both adapter and calculator supply the default, the result becomes 9. Correct types alone would not detect that semantic regression.

Worked reasoning and supplied artifact

// Disposable exercise fixture / host-contract notes; not executable API syntax.
// Supplied input: The new internal calculator takes base and explicit totalBonus. Old requests provide only base 5.
// Worked prediction: At the boundary translate the old request to base 5, totalBonus 2; the internal result is 7. The adapter owns defaulting once. The calculator should not add a second hidden +2.
// Rejected alternative: If both adapter and calculator supply the default, the result becomes 9. Correct types alone would not detect that semantic regression.

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: Extension compatibility portfolio. Build a small, understandable change around defend version compatibility, capability policy, and migration decisions.

Write a source-neutral adapter table for base 0 and base 8, then sketch a small fnc whose arithmetic matches the table. Label decoding as a host requirement.

What to produce

Write a source-neutral adapter table for base 0 and base 8, then sketch a small fnc whose arithmetic matches the table. Label decoding as a host requirement. Include input → intermediate decision/state → predicted outcome, with any required host rule stated explicitly.

• 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.

• Build a compatibility adapter
• Explain the decision for the supplied case, then independently work the changed fixture.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

In one or two sentences, name the decisive rule in “Build a compatibility adapter” and predict the outcome of one changed input from your independent fixture. State which host behavior still needs real verification.

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