Functions as reusable contracts — Boundary detective

PART I

Warm-up

Reconnect with a useful idea before starting functions as reusable contracts. 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

Distinguish arithmetic boundaries from price validation

The shared fnc/parameter/return grammar can make a Willow learner feel immediately at home, but familiar arithmetic does not establish that a proposed .ws program has a runtime. Treat each helper as a typed input/result contract and keep the caller’s units and role assumptions explicit. This bridge separates portable reasoning from unverified execution.

The worked total helper predicts (0,15)→15, (120,0)→120 and (-120,15)→-105. All supplied values are integers, but a shop policy may disallow a negative price. Sample case artifact records the arithmetic result, value kind and a separate feature-validity field rather than calling every unsuitable price a type error.

Do not transfer an unstated .w range or overflow rule to .ws. Ordinary paper sums are clear here; extreme i32 behavior and real currency constraints require their own specified semantics. A negative price question is not solved by the helper’s name.

Worked reasoning and supplied artifact

// Worked contract artifact:
// The worked total helper predicts (0,15)→15, (120,0)→120 and (-120,15)→-105. All supplied values are integers, but a shop policy may disallow a negative price. Sample case artifact records the arithmetic result, value kind and a separate feature-validity field rather than calling every unsuitable price a type 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

Classify the three cases by arithmetic prediction and additional price-policy concern.

• 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: Shared pricing rules. Build a small, understandable change around functions as reusable contracts.

Design normal, zero-fee, zero-price and negative-price cards for a 70-cent fee-bearing service. State which policy needs clarification before real use.

What to produce

Four result cards, a separate validation column and no guessed runtime rejection or overflow claim.

• 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 add(a: i32, b: i32) -> i32 {
    return a + b;
}
fnc main() -> i32 {
    return add(2, 3);
}

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.

• Distinguish arithmetic boundaries from price validation
• Classify the three cases by arithmetic prediction and additional price-policy concern.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Classify the three cases by arithmetic prediction and additional price-policy concern.

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