From e6a401f804c7a88d6a6664d1500852082271b395 Mon Sep 17 00:00:00 2001
From: Rafael Varago <rafael.varago@gmail.com>
Date: Tue, 4 Mar 2025 16:25:15 +0100
Subject: [PATCH] Move templates from error policy into its methods

This avoids repeating the refinement type when parameterising a
refinement factory with a custom policy.

E.g. from: `error::to_optional<even>` to: `error::to_optional`
(no more `<even>` is required)
---
 include/rvarago/refined.hpp | 43 ++++++++++++++++++++++++-------------
 test/refined_test.cpp       | 12 +++++------
 2 files changed, 34 insertions(+), 21 deletions(-)

diff --git a/include/rvarago/refined.hpp b/include/rvarago/refined.hpp
index 4f7b66b..5e14658 100644
--- a/include/rvarago/refined.hpp
+++ b/include/rvarago/refined.hpp
@@ -16,28 +16,36 @@ concept policy = requires(Policy p, Refinement refined) {
   typename Refinement::predicate_type;
 
   // On success.
-  { p.ok(refined) } -> std::same_as<typename Policy::return_type>;
+  {
+    p.template ok<Refinement>(refined)
+  } -> std::same_as<typename Policy::template wrapper_type<Refinement>>;
 
   // On error.
-  { p.err() } -> std::same_as<typename Policy::return_type>;
+  {
+    p.template err<Refinement>()
+  } -> std::same_as<typename Policy::template wrapper_type<Refinement>>;
 };
 
 // Report errors as `std::optional<Refinement>`.
-template <typename Refinement> struct to_optional {
-  using return_type = std::optional<Refinement>;
+struct to_optional {
+  template <typename Refinement> using wrapper_type = std::optional<Refinement>;
 
   // Returns an engaged optional.
-  constexpr auto ok(Refinement refinement) const -> return_type {
+  template <typename Refinement>
+  constexpr auto ok(Refinement refinement) const -> wrapper_type<Refinement> {
     return std::optional{std::move(refinement)};
   }
 
   // Returns nullopt.
-  constexpr auto err() const -> return_type { return std::nullopt; }
+  template <typename Refinement>
+  constexpr auto err() const -> wrapper_type<Refinement> {
+    return std::nullopt;
+  }
 };
 
 // Report errors as exceptions.
-template <typename Refinement> struct to_exception {
-  using return_type = Refinement;
+struct to_exception {
+  template <typename Refinement> using wrapper_type = Refinement;
 
   struct refinement_exception : std::exception {
     const char *what() const noexcept override {
@@ -46,13 +54,18 @@ template <typename Refinement> struct to_exception {
   };
 
   // Returns argument unchanged.
-  constexpr auto ok(Refinement refinement) const -> return_type {
+  template <typename Refinement>
+  constexpr auto ok(Refinement refinement) const -> wrapper_type<Refinement> {
     return refinement;
   }
 
   // Throws a `refinement_exception`.
-  constexpr auto err() const -> return_type { throw refinement_exception{}; }
+  template <typename Refinement>
+  constexpr auto err() const -> wrapper_type<Refinement> {
+    throw refinement_exception{};
+  }
 };
+
 } // namespace error
 
 // `refinement<T, Pred>` constraints values `t: T` where `Pred{}(t)` holds.
@@ -67,14 +80,14 @@ template <typename T, std::predicate<T const &> Pred> struct refinement {
   //
   // If `pred(value)` holds, then produces a valid instance by delegating to
   // `policy.ok`. Else reports error via `policy.err`.
-  template <error::policy<refinement<T, Pred>> Policy =
-                error::to_optional<refinement<T, Pred>>>
+  template <error::policy<refinement<T, Pred>> Policy = error::to_optional>
   static constexpr auto make(T value, Pred pred = {}, Policy policy = {})
-      -> Policy::return_type {
+      -> Policy::template wrapper_type<refinement<T, Pred>> {
     if (std::invoke(std::move(pred), value)) {
-      return policy.ok({refinement<T, Pred>{std::move(value)}});
+      return policy.template ok<refinement<T, Pred>>(
+          {refinement<T, Pred>{std::move(value)}});
     } else {
-      return policy.err();
+      return policy.template err<refinement<T, Pred>>();
     }
   }
 
diff --git a/test/refined_test.cpp b/test/refined_test.cpp
index 047cb8c..4a517e6 100644
--- a/test/refined_test.cpp
+++ b/test/refined_test.cpp
@@ -14,7 +14,7 @@ TEST_CASE(
   STATIC_REQUIRE(std::is_same_v<decltype(even::make(0)), std::optional<even>>);
 
   STATIC_REQUIRE(
-      std::is_same_v<decltype(even::make<refined::error::to_optional<even>>(0)),
+      std::is_same_v<decltype(even::make<refined::error::to_optional>(0)),
                      std::optional<even>>);
 
   STATIC_REQUIRE(even::make(0)->value == 0);
@@ -27,10 +27,10 @@ TEST_CASE("A to_exception policy should throw on invalid argument",
           "[error_policy][to_exception]") {
 
   STATIC_REQUIRE(
-      std::is_same_v<
-          decltype(even::make<refined::error::to_exception<even>>(10)), even>);
+      std::is_same_v<decltype(even::make<refined::error::to_exception>(10)),
+                     even>);
 
-  STATIC_REQUIRE(even::make<refined::error::to_exception<even>>(0).value == 0);
-  REQUIRE_THROWS_AS(even::make<refined::error::to_exception<even>>(1),
-                    refined::error::to_exception<even>::refinement_exception);
+  STATIC_REQUIRE(even::make<refined::error::to_exception>(0).value == 0);
+  REQUIRE_THROWS_AS(even::make<refined::error::to_exception>(1),
+                    refined::error::to_exception::refinement_exception);
 }