Happy API (HAPI)

Build modular, zero-overhead, highly composable output and UI stacks in C++.

A powerful static composition engine for embedded systems, Arduino, and modern C++.

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Why Happy API?

Most embedded UI/output libraries force rigid hierarchies or expensive virtual calls.
HAPI lets you compose features like Lego blocks — at compile time — with full type safety and virtually zero overhead.

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Features

• Fully static CRTP mixin composition
• Powerful chaining + reordering (Ins<>, App<>, Join<>)
• Strong dependency validation (Requires<>, Excludes<>)
• Rich compile-time introspection (Has<>)
• Optional virtual facade (IOutDef<>)
• Designed from the ground up for tiny devices (AVR, ESP, etc.)

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Quick Start

using MyOutput = OutDef<
    Gate<>,           // locking, measuring, partial updates
    UTF8<>, 
    TextWrap<>,
    Cursor<>,
    MyRenderer<>      // your hardware driver
>;

MyOutput out;
out.put("Hello from Happy API! 🎉");

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Core Concepts

• Chain<> — Linear CRTP mixin chain (the engine heart)
• APIOf<Base, Features...> — Main composition template
• OutDef<Features...> — Convenient alias for output stacks
• Part<O> — Every feature provides a template<typename O> struct Part : O

Feature Example

struct Gate {
    template<typename O>
    struct Part : O {
        using Base = O;
        using HasGate = std::true_type;

        void put(auto o) {
            if (unlocked()) Base::put(o);
        }
    };
};

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Key Mechanisms

Mechanism	Purpose	Example
Requires<>	Dependencies	Cursor needs DataParser
Excludes<>	Incompatibilities	Mutual exclusions
Has<Tag>	Introspection	Has<HasGate>
Ins<>/App<>	Reordering	Insert features anywhere

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Performance

• Zero runtime overhead on hot path (pure static OutDef)
• Excellent inlining
• Minimal code size (no vtables unless using virtual facade)
• Scales well up to ~10-12 features on embedded compilers

Pro Tip: Use static OutDef<...> for performance-critical code. Use IOutDef<...> only when you need polymorphism.

Comparison:

• Much faster and smaller than traditional virtual inheritance or runtime plugin systems.
• Comparable (or better) than hand-written monolithic classes, while being far more flexible.

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Philosophy

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

HAPI is the next evolution of the ArduinoMenu lineage.

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Current Status

Early but functional. Focused on correctness, performance, and tiny-device compatibility.

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Contributing

Highly welcome! Especially:

• New hardware drivers / renderers
• Useful features (input, protocols, menus...)
• Better compile-time diagnostics

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Made with obsession in the Azores 🇵🇹

By Rui Azevedo • @ruihfazevedo