Patternia is a header-only pattern matching library for modern C++. It keeps matching expression-oriented, explicit, and zero-overhead.

Syntax

#include <ptn/patternia.hpp>

int classify(int x) {
  using namespace ptn;

  return match(x) | on(
    lit(0) >> 0,
    lit(1) >> 1,
    _ >> -1
  );
}

match(subject) creates the evaluation context. on(...) provides the ordered case list. pattern >> handler defines one case. _ is the required fallback case.

Highlights

• Literal, structural, and std::variant matching in one DSL.
• Explicit binding through $ and $(...).
• Declarative guards via PTN_LET, PTN_WHERE, _0, arg<N>, rng(...), and callables.
• No RTTI, no virtual dispatch, no heap allocation.
• Static literal and variant dispatch lowering for hot paths.

Quick Examples

Guarded value match

using namespace ptn;

const char *bucket(int x) {
  return match(x) | on(
    $[PTN_LET(value, value < 0)] >> "negative",
    $[PTN_LET(value, value < 10)] >> "small",
    _ >> "large"
  );
}

Structural match

using namespace ptn;

struct Point { int x; int y; };

int magnitude2(const Point &p) {
  return match(p) | on(
    $(has<&Point::x, &Point::y>()) >> [](int x, int y) {
      return x * x + y * y;
    },
    _ >> 0
  );
}

Variant match

using namespace ptn;

using Value = std::variant<int, std::string>;

std::string describe(const Value &v) {
  return match(v) | on(
    is<int>() >> "int",
    $(is<std::string>()) >> [](const std::string &s) {
      return "str:" + s;
    },
    _ >> [] { return std::string("other"); }
  );
}

Installation

Patternia is header-only with no external dependencies.

FetchContent (recommended):

include(FetchContent)
FetchContent_Declare(patternia
  GIT_REPOSITORY https://github.com/sentomk/patternia.git
  GIT_TAG v0.9.0
)
FetchContent_MakeAvailable(patternia)

target_link_libraries(your_target PRIVATE patternia::patternia)

Direct clone:

git clone https://github.com/SentoMK/patternia.git
cd patternia
cmake -S . -B build
cmake --build build

See Installation Guide for find_package, submodule, and header-copy options.

Tests

cmake -S . -B build -DPTN_BUILD_TESTS=ON
cmake --build build --target ptn_tests
ctest --test-dir build --output-on-failure

Performance-Oriented Usage

Cache the case pack for repeated hot paths:

using namespace ptn;

int fast_classify(int x) {
  return match(x) | PTN_ON(
    lit<1>() >> 1,
    lit<2>() >> 2,
    _ >> 0
  );
}

PTN_ON(...) is a convenience wrapper over static_on(...). It avoids rebuilding the matcher object on every call.

Documentation

• Getting Started
• API Reference
• Design Overview
• Tutorials
• Performance Notes

Contributing

Please read CONTRIBUTING.md before sending changes. This project is governed by CODE_OF_CONDUCT.md.