Reuse one price rule

v3 · Activity 11 of 12 · composition · reuse · 10 min Runs here

Chapter outline

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Mini-mission

A helper contains one named rule. A larger function calls it rather than repeating its implementation.

Composition means using the result of one function in another calculation. Keep inputs and returned results visible so the larger behavior is easy to trace.

Your task

Repair pairPrice so the packaging fee applies to both first and second. Keep the withFee helper available as the shared rule. Keep the final return shown in the starter so the checker can call or inspect your result.

Interface: pairPrice(first, second) -> number

Input	Expected
Two small items`3, 4`	`11`
Free base items`0, 0`	`4`

See another example

local function withFee(base)
    return base + 2
end
local function pairPrice(first, second)
    return withFee(first) + withFee(second)
end
return pairPrice

Two helper calls apply the same fee consistently. Changing the fee in one place updates both items.

Syntax in this activity

New syntax: calling a function from another function

Earlier ideas used here

functions
parameters
return-values
addition

Explain more

A helper contains one named rule. A larger function calls it rather than repeating its implementation.

Composition means using the result of one function in another calculation. Keep inputs and returned results visible so the larger behavior is easy to trace.

Here each item costs its base amount plus a fixed packaging charge of 2. Calculate both items with the same helper before adding the results.

Explain this code

Select a word or symbol in this authored example. Your editor draft stays unchanged.

  withFeebase
     base  2

  pairPricefirst, second
     withFeefirst  second

 pairPrice

Choose a highlighted part to see what it means.

Free experiment

This is a private experiment copy. It does not replace your answer or complete the activity.

These inputs apply when your scratchpad returns the function described by the activity interface.

Test different inputs

Font size Plain text

local function withFee(base)
    return base + 2
end
local function pairPrice(first, second)
    return withFee(first) + withFee(second)
end
return pairPrice

Luau keyword highlighting · Ctrl/⌘ + Enter: Run · Esc: leave editorTab inserts indentation

Experiment Output

Your output will appear here.

Reset starter

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Code

Luau

Font size Plain text

local function withFee(base)
    return base + 2
end
local function pairPrice(first, second)
    return withFee(first) + second
end
return pairPrice

Luau keyword highlighting · Ctrl/⌘ + Enter: Run · Esc: leave editorTab inserts indentation

Your output will appear here.

Ready

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Your task

Interface: pairPrice(first, second) -> number

Input	Expected
Two small items`3, 4`	`11`
Free base items`0, 0`	`4`

See another example

local function withFee(base)
    return base + 2
end
local function pairPrice(first, second)
    return withFee(first) + withFee(second)
end
return pairPrice

Two helper calls apply the same fee consistently. Changing the fee in one place updates both items.

Syntax in this activity

New syntax: calling a function from another function

Earlier ideas used here

functions
parameters
return-values
addition

Explain more

A helper contains one named rule. A larger function calls it rather than repeating its implementation.

Composition means using the result of one function in another calculation. Keep inputs and returned results visible so the larger behavior is easy to trace.

Here each item costs its base amount plus a fixed packaging charge of 2. Calculate both items with the same helper before adding the results.

Explain this code

Select a word or symbol in this authored example. Your editor draft stays unchanged.

  withFeebase
     base  2

  pairPricefirst, second
     withFeefirst  second

 pairPrice

Choose a highlighted part to see what it means.

Free experiment

This is a private experiment copy. It does not replace your answer or complete the activity.

These inputs apply when your scratchpad returns the function described by the activity interface.

Test different inputs

Font size Plain text

local function withFee(base)
    return base + 2
end
local function pairPrice(first, second)
    return withFee(first) + withFee(second)
end
return pairPrice

Luau keyword highlighting · Ctrl/⌘ + Enter: Run · Esc: leave editorTab inserts indentation

Experiment Output

Your output will appear here.