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 one pricing requirement without mutating callers

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.

Old rule adds subtotal and fee. New helper adds a fixed handling amount of 2 after that sum. Example subtotal=20, fee=3. Unchanged callers still supply only subtotal and fee.

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: Compute 20+3=23, then 23+2=25. The handling amount belongs to the new rule, not to caller-side fee edits. If a caller also adds 2, its result becomes 27 and charges handling twice.

Compare the tempting alternative: Updating every caller to increase fee while also updating the shared helper duplicates the requirement and obscures which layer owns it.

Worked reasoning and supplied artifact

// Paper trace / supplied exercise-host contract; no execution claimed.
// Old rule adds subtotal and fee. New helper adds a fixed handling amount of 2 after that sum. Example subtotal=20, fee=3. Unchanged callers still supply only subtotal and fee.
// Compute 20+3=23, then 23+2=25. The handling amount belongs to the new rule, not to caller-side fee edits. If a caller also adds 2, its result becomes 27 and charges handling twice.

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.

Trace the original helper, revised helper and accidentally double-adjusted caller for inputs (20,3) and (0,0). State whether the new rule is backward compatible in meaning.

What to produce

A before/after pricing table and an ownership note for the handling amount.

• 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 one pricing requirement without mutating callers
• Trace the original helper, revised helper and accidentally double-adjusted caller for inputs (20,3) and (0,0). State whether the new rule is backward compatible in meaning.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Name the layer that owns the new handling charge.

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