Functions as reusable contracts — Connect the previous skills

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

Compose a subtotal with a fee helper

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 caller computes quantity*unitPrice before supplying a fee: subtotal 3*80=240 cents, then total(240,15)=255 cents. Sample data-route artifact is items/count and cents/item → subtotal cents → fee addition → returned total cents. This preserves meaning through a helper boundary, rather than passing the quantity where a monetary subtotal belongs.

Both quantity 3 and subtotal 240 may be i32, so type compatibility alone cannot catch total(3,15). That wrong route predicts 18 with mixed meanings. Keep the semantic unit visible, especially when bridging native numeric habits into script contracts.

Worked reasoning and supplied artifact

// Worked contract artifact:
// The worked caller computes quantity*unitPrice before supplying a fee: subtotal 3*80=240 cents, then total(240,15)=255 cents. Sample data-route artifact is items/count and cents/item → subtotal cents → fee addition → returned total cents. This preserves meaning through a helper boundary, rather than passing the quantity where a monetary subtotal belongs.

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 both the correct subtotal route and the incorrect direct-quantity route.

• 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.

Calculate four items at 65 cents plus a 25-cent fee through the reusable helper. Include a countertrace that passes the item count instead of subtotal.

What to produce

Correct trace 260→285 cents, wrong-route 4+25=29 and an explicit unit-mismatch diagnosis.

• 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.

• Compose a subtotal with a fee helper
• Trace both the correct subtotal route and the incorrect direct-quantity route.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Trace both the correct subtotal route and the incorrect direct-quantity route.

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