Functions as reusable contracts — Build the smallest useful slice

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

Build a single-purpose price helper

A reusable function preserves one explicit promise. Read a function as inputs, result and allowed effects. Parameter names belong to a call’s local reasoning; callers supply values, not a new copy of the function definition. Keep pure calculations separate from host operations so a paper trace can substantiate the numeric rule without pretending to execute a host.

A shop combines item subtotal 60 and delivery fee 8. The reusable helper must return the sum; selecting a customer, reading stock and showing a receipt are caller responsibilities.

For every call, substitute actual values for parameters, evaluate intermediate expressions, and return the result to the caller. Then inspect whether the caller uses that result. A function returning a value does not automatically print, store or display it. Worked case: The helper contract accepts two i32 values and predicts 68. A caller may later display 68 through a separately supplied host contract. Restricting the helper to arithmetic makes the rule reusable in both a preview and an invoice workflow.

Compare the tempting alternative: Reading the current customer inside the helper adds a hidden dependency and makes identical explicit inputs capable of different unexplained outcomes.

Worked reasoning and supplied artifact

// Paper trace / supplied exercise-host contract; no execution claimed.
// A shop combines item subtotal 60 and delivery fee 8. The reusable helper must return the sum; selecting a customer, reading stock and showing a receipt are caller responsibilities.
// The helper contract accepts two i32 values and predicts 68. A caller may later display 68 through a separately supplied host contract. Restricting the helper to arithmetic makes the rule reusable in both a preview and an invoice workflow.

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

Write a helper with source-backed fnc/return syntax and two callers supplying different values. Give a result table and explicitly exclude host effects.

What to produce

One pure helper contract, two call sites and their predicted results.

• 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.

• Build a single-purpose price helper
• Write a helper with source-backed fnc/return syntax and two callers supplying different values. Give a result table and explicitly exclude host effects.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

State the helper’s one-sentence promise.

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