From a6ebfcb236a1754cec6d10212b4848f3f6ca52ef Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Tue, 4 Mar 2025 15:51:11 -0700
Subject: [PATCH] :sparkles: Add useful things for environment handling

Problem:
- There is no way to override one environment with another.
- There is no concept to identify an environment type.

Solution:
- Add `append_env_t`.
- Add `envlike`.
---
 include/stdx/env.hpp | 23 +++++++++++++++--------
 test/env.cpp         | 18 +++++++++++++-----
 2 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/include/stdx/env.hpp b/include/stdx/env.hpp
index 8c7b0a5..aae4e3d 100644
--- a/include/stdx/env.hpp
+++ b/include/stdx/env.hpp
@@ -4,18 +4,19 @@
 
 #include <stdx/compiler.hpp>
 #include <stdx/ct_string.hpp>
+#include <stdx/type_traits.hpp>
 
 #include <boost/mp11/algorithm.hpp>
 
 namespace stdx {
 inline namespace v1 {
+namespace _env {
 template <auto Query, auto Value> struct ct_prop {
     [[nodiscard]] CONSTEVAL static auto query(decltype(Query)) noexcept {
         return Value;
     }
 };
 
-namespace _env {
 template <typename Q, typename Env>
 concept valid_query_for = requires(Env const &e) { e.query(Q{}); };
 
@@ -38,6 +39,9 @@ template <typename... Envs> struct env {
     }
 };
 
+template <typename T>
+concept envlike = is_specialization_of_v<T, env>;
+
 namespace _env {
 template <typename T> struct autowrap {
     // NOLINTNEXTLINE(google-explicit-constructor)
@@ -64,14 +68,14 @@ template <auto V> struct wrap {
 template <typename> struct for_each_pair;
 template <std::size_t... Is> struct for_each_pair<std::index_sequence<Is...>> {
     template <auto... Args>
-    using type =
-        env<ct_prop<boost::mp11::mp_at_c<boost::mp11::mp_list<wrap<Args>...>,
-                                         2 * Is>::value.value,
-                    boost::mp11::mp_at_c<boost::mp11::mp_list<wrap<Args>...>,
-                                         (2 * Is) + 1>::value.value>...>;
+    using type = env<
+        _env::ct_prop<boost::mp11::mp_at_c<boost::mp11::mp_list<wrap<Args>...>,
+                                           2 * Is>::value.value,
+                      boost::mp11::mp_at_c<boost::mp11::mp_list<wrap<Args>...>,
+                                           (2 * Is) + 1>::value.value>...>;
 };
 
-template <typename Env = env<>>
+template <envlike Env = env<>>
 constexpr auto make_env = []<autowrap... Args> {
     using new_env_t = typename for_each_pair<
         std::make_index_sequence<sizeof...(Args) / 2>>::template type<Args...>;
@@ -79,10 +83,13 @@ constexpr auto make_env = []<autowrap... Args> {
 };
 } // namespace _env
 
-template <typename Env, _env::autowrap... Args>
+template <envlike Env, _env::autowrap... Args>
 using extend_env_t =
     decltype(_env::make_env<Env>.template operator()<Args...>());
 
+template <envlike... Envs>
+using append_env_t = boost::mp11::mp_reverse<boost::mp11::mp_append<Envs...>>;
+
 template <_env::autowrap... Args>
 using make_env_t = extend_env_t<env<>, Args...>;
 } // namespace v1
diff --git a/test/env.cpp b/test/env.cpp
index 61700fa..2a72559 100644
--- a/test/env.cpp
+++ b/test/env.cpp
@@ -21,11 +21,6 @@ TEST_CASE("lookup query with default", "[env]") {
     static_assert(custom(stdx::env<>{}) == 42);
 }
 
-TEST_CASE("lookup with single-value prop", "[env]") {
-    using E = stdx::ct_prop<custom, 17>;
-    static_assert(custom(E{}) == 17);
-}
-
 TEST_CASE("make an environment", "[env]") {
     using E = stdx::make_env_t<custom, 17>;
     static_assert(custom(E{}) == 17);
@@ -37,8 +32,21 @@ TEST_CASE("extend an environment", "[env]") {
     static_assert(custom(E2{}) == 18);
 }
 
+TEST_CASE("append an environment", "[env]") {
+    using E1 = stdx::make_env_t<custom, 17>;
+    using E2 = stdx::make_env_t<custom, 18>;
+    using E3 = stdx::make_env_t<custom, 19>;
+    using E = stdx::append_env_t<E1, E2, E3>;
+    static_assert(custom(E{}) == 19);
+}
+
 TEST_CASE("environment converts string literals to ct_string", "[env]") {
     using namespace stdx::literals;
     using E = stdx::make_env_t<custom, "hello">;
     static_assert(custom(E{}) == "hello"_cts);
 }
+
+TEST_CASE("envlike concept", "[env]") {
+    static_assert(stdx::envlike<stdx::env<>>);
+    static_assert(stdx::envlike<stdx::make_env_t<custom, 17>>);
+}