HAPI - Happy API

Zero-overhead modular composition engine for embedded C++.

HAPI lets you build clean, highly composable, type-safe stacks (output, menus, drivers, protocols) using static CRTP mixin composition with virtually no runtime cost.

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Philosophy

"Make the common case blazing fast and the advanced case still clean and possible."

No virtual calls on hot paths. Full compile-time safety and introspection. Designed for constrained devices (AVR, ESP, ARM Cortex-M).

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Current Status (May 2026)

• Version: 0.3.x (actively evolving)
• Core engine stable
• Strong integration with OneData and OneList

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Key Features

• Chain<> — Core linear CRTP mixin engine
• APIOf<Base, Features...> — Main composition template
• OutDef<Features...> — Convenient output stacks
• Rules system: Requires<>, Excludes<>
• Introspection: Has<>
• Reordering: Ins<>, App<>, Join<>

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Usage Examples

1. Simple Output Stack

using MyOut = OutDef<
    Gate<>,
    UTF8<>,
    TextWrap<>,
    Cursor<>,
    ANSIOut<>,
    ConsoleOut
>;

MyOut out;
out.put("Hello from HAPI!");

2. Advanced Menu Rendering Pipeline

using ScrollPrinter = Chain<
    ViewPrinter,
    MenuPrinter<
        TitlePrinter,
        ScrollBodyPrinter,
        ItemPrinter<IndexPrinter, NavCursorPrinter, ItemBodyPrinter>
    >
>;

using MainOutput = OutDef<
    ScrollPrinter,
    ANSIFmt<>,
    ClearFreeFmt<>,
    DataParser<>,
    UTF8<>,
    Clip<>,
    ANSIOut<>,
    ConsoleOut
>;

3. Data Components

using Volume = Chain<
    DataPrint,
    Data<int>,
    Watch<>,
    NumRange<int>
>;

Volume volume{75};
volume.up(10);
if (volume.changed()) { /* react */ }

4. Menu Item

auto ledItem = ItemDef<
    Id<"led">,
    Text,
    Action<toggleLed>
>("Toggle LED");

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Ecosystem

• HAPI — Core composition engine
• OneData — Data components
• OneList — Heterogeneous lists

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Links

• Repository: https://github.com/InternetOfPins/HAPI

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Author: Rui Azevedo (neu-rah)
Contributor: Grok (xAI) - architecture, cleanup & modern C++ patterns