Validate an input before using it — Change one requirement

PART I

Warm-up

Reconnect with a useful idea before starting validate an input before using it. 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

Choose a rejected-input message

A conditional first evaluates a Boolean decision, then selects its matching path. Read the explicit predicate and the function signature rather than guessing coercions. Returning a value communicates with the caller; it does not print, render or authorize host actions. These are paper traces of the source-grounded design, not executed tests.

Supplied worked case: Host receives count=-2 and the feature requires a positive quantity.

The predicate returns false. A useful supplied UI message says quantity must be greater than zero; saying system failure blames the wrong boundary. Keep user-facing input feedback distinct from an unavailable host-service error.

Worked reasoning and supplied artifact

// PAPER FIXTURE — Choose a rejected-input message
// Manual worked artifact; comments below are not executable source.
// INPUT / CONTRACT: Supplied worked case: Host receives count=-2 and the feature requires a positive
// quantity.
// DERIVATION / EXPECTED ARTIFACT: The predicate returns false. A useful supplied UI message says quantity
// must be greater than zero; saying system failure blames the wrong boundary. Keep user-facing input
// feedback distinct from an unavailable host-service error.

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

Guided task: reproduce the supplied worked artifact, then identify the assumption behind the rejected alternative “Claim all services are broken”.

• 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: Admission rules engine. Build a small, understandable change around validate an input before using it.

Write messages for count0 and a positive count with an unavailable service, explaining the difference. Supply your complete proposed source/configuration or explicit host contract, an intermediate trace, and a below/equal/above boundary or malformed-structure counterexample. Label predictions rather than claiming execution.

What to produce

Write messages for count0 and a positive count with an unavailable service, explaining the difference. Include the corrected sample artifact and one contrasting invalid case.

• 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.

fnc allowed(count: i32) -> bool {
    if count > 0 {
        return true;
    } else {
        return false;
    }
}

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.

• Choose a rejected-input message
• Host receives count=-2 and the feature requires a positive quantity
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Without copying the worked case, change one input or nested field and explain which branch/path changes and which contract stays fixed.

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