Functions as reusable contracts — Change one requirement

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

Change fee policy without hiding a compatibility change

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 original total returns price+fee; a new policy returns price+fee*2. The worked unchanged call (200,10) now predicts 220 rather than 210. Sample compatibility card: caller unchanged; types unchanged; semantic expectation changed; migration question = which callers require the old fee rule? This catches a change that a signature-only review misses.

Keeping the same name and i32 signature does not guarantee behavioral compatibility. Do not claim old clients are preserved just because the call still has two arguments. Record the new fee policy explicitly and compare known expectations.

Worked reasoning and supplied artifact

// Worked contract artifact:
// The original total returns price+fee; a new policy returns price+fee*2. The worked unchanged call (200,10) now predicts 220 rather than 210. Sample compatibility card: caller unchanged; types unchanged; semantic expectation changed; migration question = which callers require the old fee rule? This catches a change that a signature-only review misses.

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

Trace old/new bodies at (200,10), then use fee zero to show a case that conceals the change.

• 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.

Compare old and doubled-fee rules for a 350-cent order with fee 30 and with fee zero. Propose a documented rollout choice that keeps caller expectations explicit.

What to produce

Old/new results 380/410, zero-fee agreement, and a behavior-compatibility decision.

• 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.

• Change fee policy without hiding a compatibility change
• Trace old/new bodies at (200,10), then use fee zero to show a case that conceals the change.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Trace old/new bodies at (200,10), then use fee zero to show a case that conceals the change.

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