First attempt

include/kpn/traits.hpp

@@ -0,0 +1,86 @@
+#pragma once
+#include <tuple>
+#include <type_traits>
+
+namespace kpn {
+
+// ── Primary template — not defined; only specialisations match ────────────────
+template<typename F>
+struct function_traits;
+
+// Free function
+template<typename R, typename... Args>
+struct function_traits<R(Args...)> {
+    using return_t = R;
+    using args     = std::tuple<Args...>;
+    static constexpr std::size_t arity = sizeof...(Args);
+};
+
+// Function pointer
+template<typename R, typename... Args>
+struct function_traits<R(*)(Args...)> : function_traits<R(Args...)> {};
+
+// Member function pointer (const)
+template<typename C, typename R, typename... Args>
+struct function_traits<R(C::*)(Args...) const> : function_traits<R(Args...)> {};
+
+// Member function pointer (non-const)
+template<typename C, typename R, typename... Args>
+struct function_traits<R(C::*)(Args...)> : function_traits<R(Args...)> {};
+
+// Callable (lambda / std::function) — delegate to operator()
+template<typename F>
+struct function_traits : function_traits<decltype(&F::operator())> {};
+
+// ── Helpers ───────────────────────────────────────────────────────────────────
+
+template<typename F>
+using return_t = typename function_traits<std::remove_cvref_t<F>>::return_t;
+
+template<typename F>
+using args_t = typename function_traits<std::remove_cvref_t<F>>::args;
+
+template<typename F>
+inline constexpr std::size_t arity_v = function_traits<std::remove_cvref_t<F>>::arity;
+
+// ── Tuple detection ───────────────────────────────────────────────────────────
+
+template<typename T>
+struct is_tuple : std::false_type {};
+
+template<typename... Ts>
+struct is_tuple<std::tuple<Ts...>> : std::true_type {};
+
+template<typename T>
+inline constexpr bool is_tuple_v = is_tuple<T>::value;
+
+// ── Normalise return type to always be a tuple ────────────────────────────────
+// void        → std::tuple<>
+// T (non-tup) → std::tuple<T>
+// tuple<...>  → tuple<...>  (unchanged)
+
+template<typename T>
+struct normalise_return {
+    using type = std::tuple<T>;
+};
+
+template<>
+struct normalise_return<void> {
+    using type = std::tuple<>;
+};
+
+template<typename... Ts>
+struct normalise_return<std::tuple<Ts...>> {
+    using type = std::tuple<Ts...>;
+};
+
+template<typename T>
+using normalised_return_t = typename normalise_return<T>::type;
+
+// ── Output count from a function type ────────────────────────────────────────
+
+template<typename F>
+inline constexpr std::size_t output_count_v =
+    std::tuple_size_v<normalised_return_t<return_t<F>>>;
+
+} // namespace kpn