Add expected caster to Union (#3135)

libmambapy/src/libmambapy/bindings/expected_caster.hpp

@@ -0,0 +1,93 @@
+// Copyright (c) 2023, QuantStack and Mamba Contributors
+//
+// Distributed under the terms of the BSD 3-Clause License.
+//
+// The full license is in the file LICENSE, distributed with this software.
+
+#include <type_traits>
+#include <utility>
+#include <variant>
+
+#include <pybind11/cast.h>
+#include <pybind11/pybind11.h>
+#include <pybind11/stl.h>
+#include <tl/expected.hpp>
+
+#ifndef MAMBA_PY_EXPECTED_CASTER
+#define MAMBA_PY_EXPECTED_CASTER
+
+namespace PYBIND11_NAMESPACE
+{
+    namespace detail
+    {
+        namespace
+        {
+            template <
+                typename Expected,
+                typename T = typename Expected::value_type,
+                typename E = typename Expected::error_type>
+            auto expected_to_variant(Expected&& expected) -> std::variant<T, E>
+            {
+                if (expected)
+                {
+                    return { std::forward<Expected>(expected).value() };
+                }
+                return { std::forward<Expected>(expected).error() };
+            }
+
+            template <
+                typename Variant,
+                typename T = std::decay_t<decltype(std::get<0>(std::declval<Variant>()))>,
+                typename E = std::decay_t<decltype(std::get<1>(std::declval<Variant>()))>>
+            auto expected_to_variant(Variant&& var) -> tl::expected<T, E>
+            {
+                static_assert(std::variant_size_v<Variant> == 2);
+                return std::visit(
+                    [](auto&& v) -> tl::expected<T, E> { return { std::forward<deltype(v)>(v) }; },
+                    var
+                );
+            }
+        }
+
+        /**
+         * A caster for tl::expected that converts to a union.
+         *
+         * The caster works by converting to a the expected to a variant and then calls the
+         * variant caster.
+         *
+         * A future direction could be considered to wrap the union into a Python "Expected",
+         * with methods such as ``and_then``, ``or_else``, and thowing method like ``value``
+         * and ``error``.
+         */
+        template <typename T, typename E>
+        struct type_caster<tl::expected<T, E>>
+        {
+            using value_type = tl::expected<T, E>;
+            using variant_type = std::variant<T, E>;
+            using caster_type = std::decay_t<decltype(make_caster<variant_type>())>;
+
+            auto load(handle src, bool convert) -> bool
+            {
+                auto caster = make_caster<variant_type>();
+                if (caster.load(src, convert))
+                {
+                    value = variant_to_expected(cast_op<variant_type>(std::move(caster)));
+                }
+                return false;
+            }
+
+            template <typename Expected>
+            static auto cast(Expected&& src, return_value_policy policy, handle parent) -> handle
+            {
+                return caster_type::cast(expected_to_variant(std::forward<Expected>(src)), policy, parent);
+            }
+
+            PYBIND11_TYPE_CASTER(
+                value_type,
+                const_name(R"(Union[)") + detail::concat(make_caster<T>::name, make_caster<E>::name)
+                    + const_name(R"(])")
+            );
+        };
+    }
+}
+#endif

libmambapy/src/libmambapy/bindings/legacy.cpp

@@ -39,6 +39,7 @@
 #include "mamba/validation/update_framework_v0_6.hpp"
 
 #include "bindings.hpp"
+#include "expected_caster.hpp"
 #include "flat_set_caster.hpp"
 
 namespace py = pybind11;