From 27adcb9f5dcf4f67ae5b48e12b025ba1e12881c8 Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Fri, 1 May 2026 16:24:25 -0600
Subject: [PATCH] :art: Allow conversion when reading a single value

Problem:
- When reading a register value, it's common to want to get (say) the
  `std::uint32_t` out directly, rather than having to write:

```c++
  read(grp / "reg"_r)
| then([] (auto spec) { return spec["reg"_r]; })
```

Solution:
- Allow a template argument to `read` to do the conversion inside.

```c++
read<std::uint32_t>(grp / "reg"_r);
```
---
 include/groov/read.hpp                   | 30 ++++++++++++++++-----
 test/fail/read/CMakeLists.txt            |  5 ++--
 test/fail/read/read_multi_conversion.cpp | 34 ++++++++++++++++++++++++
 test/read.cpp                            |  8 ++++++
 4 files changed, 68 insertions(+), 9 deletions(-)
 create mode 100644 test/fail/read/read_multi_conversion.cpp

diff --git a/include/groov/read.hpp b/include/groov/read.hpp
index d2589a0..71fd08f 100644
--- a/include/groov/read.hpp
+++ b/include/groov/read.hpp
@@ -21,6 +21,7 @@
 #include <boost/mp11/algorithm.hpp>
 #include <boost/mp11/list.hpp>
 
+#include <concepts>
 #include <optional>
 #include <type_traits>
 #include <utility>
@@ -84,9 +85,16 @@ consteval auto check_write_only() -> void {
         L{}, Masks{});
 }
 
+template <typename T, typename Spec> consteval auto check_read_conversion() {
+    STATIC_ASSERT(
+        (std::convertible_to<Spec, T>),
+        "Cannot convert the result of read ({}) to {} -- are you reading "
+        "multiple paths?",
+        Spec, T);
+}
 } // namespace detail
 
-template <typename Group, typename Paths>
+template <typename T = void, typename Group, typename Paths>
 constexpr auto read(read_spec<Group, Paths> const &s) -> async::sender auto {
     using Spec = decltype(to_write_spec(s));
 
@@ -104,35 +112,43 @@ constexpr auto read(read_spec<Group, Paths> const &s) -> async::sender auto {
     detail::check_write_only<typename Spec::bus_t, read_fields_per_reg_t,
                              field_masks_t>();
 
+    using R = stdx::conditional_t<std::is_void_v<T>, Spec, T>;
+    detail::check_read_conversion<R, Spec>();
+
     return []<typename... Rs, typename... Ms>(stdx::tuple<Rs...>,
                                               stdx::tuple<Ms...>) {
         return async::when_all(detail::read<Rs, Group, Ms>()...) |
                async::then(stdx::overload{
-                   [](typename Rs::type_t... values) {
+                   [](typename Rs::type_t... values) -> R {
                        return Spec{{}, {Rs{{}, values}...}};
                    },
-                   [](std::optional<typename Rs::type_t>... values) {
+                   [](std::optional<typename Rs::type_t>... values)
+                       -> std::optional<R> {
                        return stdx::transform(
-                           [](auto... vs) { return Spec{{}, {Rs{{}, vs}...}}; },
+                           [](auto... vs) -> R {
+                               return Spec{{}, {Rs{{}, vs}...}};
+                           },
                            values...);
                    }});
     }(typename Spec::value_t{}, field_masks_t{});
 }
 
 namespace _read {
-struct pipeable {
+template <typename T> struct pipeable {
   private:
     template <async::sender S>
     friend constexpr auto operator|(S &&s, pipeable) -> async::sender auto {
         return std::forward<S>(s) |
                async::let_value([]<typename Spec>(Spec &&spec) {
-                   return read(std::forward<Spec>(spec));
+                   return read<T>(std::forward<Spec>(spec));
                });
     }
 };
 } // namespace _read
 
-constexpr auto read() { return async::compose(_read::pipeable{}); }
+template <typename T = void> constexpr auto read() {
+    return async::compose(_read::pipeable<T>{});
+}
 
 namespace _sync_read {
 template <typename Behavior, async::sender S> [[nodiscard]] auto wait(S &&s) {
diff --git a/test/fail/read/CMakeLists.txt b/test/fail/read/CMakeLists.txt
index 06e81e1..b72f8fd 100644
--- a/test/fail/read/CMakeLists.txt
+++ b/test/fail/read/CMakeLists.txt
@@ -10,5 +10,6 @@ if(${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU" AND ${CMAKE_CXX_COMPILER_VERSION}
     return()
 endif()
 
-add_fail_tests(read_from_wo_field_indirect_by_register
-               read_from_wo_field_direct read_from_wo_register_direct)
+add_fail_tests(
+    read_from_wo_field_indirect_by_register read_from_wo_field_direct
+    read_from_wo_register_direct read_multi_conversion)
diff --git a/test/fail/read/read_multi_conversion.cpp b/test/fail/read/read_multi_conversion.cpp
new file mode 100644
index 0000000..65aaae3
--- /dev/null
+++ b/test/fail/read/read_multi_conversion.cpp
@@ -0,0 +1,34 @@
+#include "../dummy_bus.hpp"
+
+#include <groov/config.hpp>
+#include <groov/identity.hpp>
+#include <groov/path.hpp>
+#include <groov/read.hpp>
+
+#include <async/concepts.hpp>
+#include <async/just.hpp>
+
+#include <cstdint>
+
+// EXPECT: to int -- are you reading multiple paths
+
+namespace {
+struct read_bus : dummy_bus {
+    template <stdx::ct_string, auto>
+    static auto read(auto...) -> async::sender auto {
+        return async::just(42);
+    }
+};
+
+using F0 = groov::field<"f0", std::uint8_t, 0, 0>;
+using F1 = groov::field<"f1", std::uint8_t, 1, 1>;
+
+std::uint32_t data{};
+using R = groov::reg<"reg", std::uint32_t, &data, groov::w::replace, F0, F1>;
+using G = groov::group<"group", read_bus, R>;
+} // namespace
+
+auto main() -> int {
+    using namespace groov::literals;
+    [[maybe_unused]] auto x = read<int>(G{}("reg.f0"_f, "reg.f1"_f));
+}
diff --git a/test/read.cpp b/test/read.cpp
index 40d3b95..053c829 100644
--- a/test/read.cpp
+++ b/test/read.cpp
@@ -58,6 +58,14 @@ TEST_CASE("read a register", "[read]") {
     CHECK(r["reg0"_r] == data0);
 }
 
+TEST_CASE("read a register and convert", "[read]") {
+    using namespace groov::literals;
+    data0 = 0xa5a5'a5a5u;
+    auto s = read<std::uint32_t>(grp / "reg0"_r);
+    auto r = get<0>(*(s | async::sync_wait()));
+    CHECK(r == data0);
+}
+
 TEST_CASE("sync_read a register", "[read]") {
     using namespace groov::literals;
     data0 = 0xa5a5'a5a5u;