PROMPT: Calculator (MVPL Platform) extension for a footer bar to hold further buttons

✅ MVPL SAFE FOOTER BAR BUTTON EXTENSION GENERATOR PROMPT

SYSTEM ROLE
You are Codex.

You are generating a fully compliant MVPL calculator extension that adds a new function button to a shared footer function bar, while safely creating the footer bar if it does not already exist, without violating MVPL extension safety rules or creating fragile dependencies.

GLOBAL OBJECTIVE

Generate a new MVPL extension script named:

atbutton.js

This extension must:

• Add a button labeled @
• Use the shared footer function bar if it exists
• Safely create its own compatible footer bar if it does not exist
• Fully comply with MVPL lifecycle, safety, and isolation rules

OUTPUT REQUIREMENTS

Generate EXACTLY ONE FILE:

atbutton.js

Return ONLY JavaScript source code.

Do NOT include explanations outside code comments.

GLOBAL TECHNICAL CONSTRAINTS

• Must be standalone JavaScript
• Must run offline
• Must not require external libraries
• Must not modify calculator core globals
• Must interact ONLY through extensionAPI
• Must follow MVPL lifecycle model
• Must preserve backward compatibility
• Must fail safely if extensionAPI functions are unavailable
• Must prevent duplicate UI creation
• Must preserve profile state isolation
• Must not disrupt calculator layout or core keypad

EXTENSION NAME

AtFunctionButtonExtension

ARCHITECTURAL PURPOSE

This extension represents a footer function bar client extension.

It must:

• Attach a button to a shared footer function bar if one exists
• Create a compatible fallback footer bar if none exists
• Never assume another extension provides infrastructure
• Never remove or interfere with UI created by other extensions

SHARED FOOTER BAR CONTRACT

The shared container must be identified by:

mvpl-footer-function-bar

If this container exists:

• Use it

If this container does NOT exist:

• Create a compatible container
• Insert container into the footer zone
• Follow MVPL safe infrastructure ownership rules

FALLBACK INFRASTRUCTURE RULES

If this extension creates the footer bar:

• It owns that instance
• It may remove the bar ONLY if it created it
• It must NOT remove the bar if it was created by another extension

BUTTON REQUIREMENTS

Button label:

@

BUTTON FUNCTIONAL BEHAVIOR

When pressed:

• Read current display value
• Convert to numeric
• Divide the value by 1 (placeholder logic)
• Display result
• Update extension state

CUSTOMISATION REQUIREMENT

The placeholder logic MUST be isolated inside:

placeholderFunction(value)

Developers must be able to safely modify this function later.

PROFILE BEHAVIOR

This extension must:

• Be allowed in all profiles
• Maintain profile-isolated state
• Persist state separately for each profile

STATE SCHEMA REQUIREMENTS

stateSchema: {

  lastUsed: null

}

State must be accessed using:

api.state("AtFunctionButtonExtension")

MVPL LIFECYCLE REQUIREMENTS

The extension MUST implement:

onInit

onProfileSwitch

onLayoutChange

onDestroy

LIFECYCLE BEHAVIOR

onInit(api, state)

• Verify extensionAPI availability
• Locate existing footer bar
• Create fallback footer bar if needed
• Create @ button
• Insert button into footer bar
• Prevent duplicate insertion

onProfileSwitch(api, state)

• Ensure button exists
• Ensure footer bar exists
• Prevent duplicate UI

onLayoutChange(api, state)

• Verify container integrity
• Recreate fallback container if necessary
• Prevent duplication

onDestroy(api, state)

• Remove only the @ button
• Remove fallback footer bar ONLY if created by this extension AND bar is empty
• Preserve stored state

EXTENSION ECOSYSTEM SAFETY RULES

The extension MUST:

• Never assume another extension manages the footer bar
• Never remove buttons created by other extensions
• Only remove UI elements that it owns
• Allow other extensions to coexist inside the footer bar
• Continue functioning if other extensions modify the bar
• Detect existing container using ID rather than assumptions

UI DESIGN REQUIREMENTS

The fallback footer bar must:

• Use flexbox layout
• Match calculator button appearance
• Maintain touch-friendly sizing
• Respect calculator visual theme
• Avoid styling conflicts

ERROR HANDLING REQUIREMENTS

The extension MUST:

• Display "Error" if display value is invalid
• Display "Error" if placeholder function returns invalid numeric result
• Fail safely without throwing unhandled errors
• Never interrupt calculator core behavior

DUPLICATE PREVENTION REQUIREMENTS

The extension MUST:

• Check for container existence before creating it
• Track button reference internally
• Avoid attaching duplicate event listeners
• Remain safe across repeated lifecycle triggers

DOCUMENTATION REQUIREMENTS (INSIDE CODE COMMENTS)

Include teaching comments explaining:

• MVPL lifecycle handling
• Shared UI fallback infrastructure design
• Ownership tracking of fallback footer bar
• Profile state namespacing
• Placeholder function customization instructions
• Why the @ symbol was chosen
• How extensions should safely coexist inside shared UI

TESTING INSTRUCTIONS (INSIDE CODE COMMENTS)

Include instructions describing how to:

1. Load atbutton.js into index.html
2. Enable extension in profile enabledExtensions arrays
3. Verify @ button appears in footer
4. Verify fallback footer bar is created if missing
5. Verify fallback bar is not duplicated if shared bar exists
6. Switch profiles and confirm state isolation
7. Disable extension and confirm safe UI removal
8. Confirm calculator arithmetic remains correct
9. Confirm compatibility with FooterFunctionBarExtension if both are loaded

ACCEPTANCE CRITERIA

Extension is complete ONLY if:

✔ @ button appears in footer bar
✔ Uses existing footer bar when present
✔ Creates fallback bar when missing
✔ Removes only owned UI
✔ Placeholder function divides value by 1
✔ Footer bar is not duplicated
✔ State persists per profile
✔ Calculator core remains stable
✔ Extension coexists with other footer bar extensions

FINAL AUTHORITATIVE RULE

If any instruction conflicts:

Preserve calculator core stability
Preserve backward compatibility
Preserve profile state isolation
Preserve extension lifecycle correctness

OUTPUT FORMAT (STRICT)

Return EXACTLY:

----- atbutton.js -----

<full code>

No summaries
No explanations
Only code output