beman.monadics: A Beman Library Monadics

beman.monadics is a minimal C++ library conforming to The Beman Standard.

Implements: Monadics operations as free functions

Status: Under development and not yet ready for production use.

This library generalizes the monadic vocabulary to any type that models a “box”.

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Features

• Free-function monadic operations
• Pipe (|) syntax for composition
• Extensible via box_traits
• Value-category preserving
• Concept-constrained APIs
• Semantics aligned with standard monadic operations

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Provided Operations (Initial Scope)

The initial operation set mirrors the C++23/C++26 monadic vocabulary:

• and_then
• transform
• or_else
• transform_error

Semantics follow std::expected / std::optional.

The goal is to validate the unifying abstraction first.

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Quick Example

// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include <beman/monadics/monadics.hpp>
#include <optional>
#include <cstdlib>

int main() {
    namespace bms = beman::monadics;

    const auto result =
        std::optional{10}
        | bms::and_then([](auto&& v) {
              return std::optional{v * 2.0};
          })
        | bms::transform([](double v) {
              return static_cast<int>(v);
          })
        | bms::and_then([](int) {
              return std::optional<char>{};  // failure
          })
        | bms::or_else([] {
              return std::optional{EXIT_SUCCESS};
          });

    return result.value_or(EXIT_FAILURE);
}

std::shared_ptr

std::shared_ptr can act as a box: it may contain a value or be empty. Unlike std::optional, it has no error type, so transform_error is disabled. We can still use and_then, transform, and or_else.

#include <beman/monadics/monadics.hpp>
#include <memory>

template <typename T>
struct beman::monadics::box_traits<std::shared_ptr<T>> {
    [[nodiscard]] static constexpr bool has_value(const auto& box) noexcept {
        return static_cast<bool>(box);
    }

    [[nodiscard]] static constexpr decltype(auto) value(auto&& box) noexcept {
        return *std::forward<decltype(box)>(box);
    }

    [[nodiscard]] static constexpr auto error() noexcept { return nullptr; }

    [[nodiscard]] static constexpr decltype(auto) make(auto&& value) noexcept {
        return std::make_shared<T>(std::forward<decltype(value)>(value));
    }
};

int main() {
    namespace bms = beman::monadics;

    auto ptr = std::make_shared<int>(10);

    auto result =
        ptr
        | bms::and_then([](int v) { return std::make_shared<int>(v * 2); })
        | bms::transform([](int v) { return v + 3; })
        | bms::or_else([]() { return std::make_shared<int>(0); });

    if (result) {
        std::cout << "Result: " << *result << "\n"; // prints 23
    }

    return 0;
}

Build Requirements

Supported Platforms

Compiler	Version	C++ Standards	Standard Library
GCC	15-13	C++26-C++20	libstdc++
GCC	12-11	C++23, C++20	libstdc++
Clang	22-19	C++26-C++20	libstdc++, libc++
Clang	18-17	C++26-C++20	libc++
Clang	18-17	C++20	libstdc++
AppleClang	latest	C++26-C++20	libc++
MSVC	latest	C++23	MSVC STL