Support customisable error policies

README.md

@@ -47,6 +47,8 @@ int main() {
 
 With this example, we notice that not all functions need to access the underlying `int` element and operate entirely on `even`. So we validate and convert the `int` element to `even` at the very beginning and only fall back to `int` at the very last moment, when we actually need it. Both operations should ideally at the edges of our applications.
 
+Although we reported errors via `std::optional` in the example, it's possible to customise it, e.g. to throw an exception.
+
 ## Requirements
 
 C++20

include/rvarago/refined.hpp

@@ -1,10 +1,60 @@
 #include <concepts>
+#include <exception>
 #include <functional>
 #include <optional>
 #include <utility>
 
 namespace rvarago::refined {
 
+// Error policies for the fallible refinements factory.
+namespace error {
+
+// Models the error reporting mechanism.
+template <typename Policy, typename Refinement>
+concept policy = requires(Policy p, Refinement refined) {
+  typename Refinement::value_type;
+  typename Refinement::predicate_type;
+
+  // On success.
+  { p.ok(refined) } -> std::same_as<typename Policy::return_type>;
+
+  // On error.
+  { p.err() } -> std::same_as<typename Policy::return_type>;
+};
+
+// Report errors as `std::optional<Refinement>`.
+template <typename Refinement> struct to_optional {
+  using return_type = std::optional<Refinement>;
+
+  // Returns an engaged optional.
+  constexpr auto ok(Refinement refinement) const -> return_type {
+    return std::optional{std::move(refinement)};
+  }
+
+  // Returns nullopt.
+  constexpr auto err() const -> return_type { return std::nullopt; }
+};
+
+// Report errors as exceptions.
+template <typename Refinement> struct to_exception {
+  using return_type = Refinement;
+
+  struct refinement_exception : std::exception {
+    const char *what() const noexcept override {
+      return "fail to refine argument due to unsastified predicate";
+    }
+  };
+
+  // Returns argument unchanged.
+  constexpr auto ok(Refinement refinement) const -> return_type {
+    return refinement;
+  }
+
+  // Throws a `refinement_exception`.
+  constexpr auto err() const -> return_type { throw refinement_exception{}; }
+};
+} // namespace error
+
 // `refinement<T, Pred>` constraints values `t: T` where `Pred{}(t)` holds.
 template <typename T, std::predicate<T const &> Pred> struct refinement {
   using value_type = T;
@@ -15,13 +65,16 @@ template <typename T, std::predicate<T const &> Pred> struct refinement {
 
   // `make(value, pred)` is the only factory to refinements.
   //
-  // It returns an engaged optional when `pred(value)` holds.
-  static constexpr auto make(T value, Pred pred = {})
-      -> std::optional<refinement<T, Pred>> {
+  // 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>>>
+  static constexpr auto make(T value, Pred pred = {}, Policy policy = {})
+      -> Policy::return_type {
     if (std::invoke(std::move(pred), value)) {
-      return std::optional{refinement<T, Pred>{std::move(value)}};
+      return policy.ok({refinement<T, Pred>{std::move(value)}});
     } else {
-      return std::nullopt;
+      return policy.err();
     }
   }
 

test/refined_test.cpp

@@ -2,16 +2,35 @@
 
 #include <rvarago/refined.hpp>
 
-using namespace rvarago::refined;
+namespace refined = rvarago::refined;
+
+using even =
+    refined::refinement<int, decltype([](auto const x) { return x % 2 == 0; })>;
 
 TEST_CASE(
     "A refinement type must admit only instance that satisfy its predicate",
-    "[make]") {
-  using even =
-      refinement<int, decltype([](auto const x) { return x % 2 == 0; })>;
+    "[make][default_policy]") {
+
+  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::optional<even>>);
 
   STATIC_REQUIRE(even::make(0)->value == 0);
   STATIC_REQUIRE(even::make(1) == std::nullopt);
   STATIC_REQUIRE(even::make(2)->value == 2);
   STATIC_REQUIRE(even::make(3) == std::nullopt);
 }
+
+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>);
+
+  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);
+}