Improve the documentation of the fail handler

* State that the exception of type exception_t can be passed
  with an initial state.
* Ref #32

doc/Doxyfile

@@ -38,7 +38,7 @@ PROJECT_NAME           = Continuable
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = 4.0.0
+PROJECT_NUMBER         = 4.1.0
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a

include/continuable/continuable-base.hpp

@@ -109,13 +109,13 @@ class continuable_base {
 
   /// Constructor accepting the data object while erasing the annotation
   explicit continuable_base(Data data, ownership ownership)
-      : data_(std::move(data)), ownership_(std::move(ownership)) {
-  }
+    : data_(std::move(data))
+    , ownership_(std::move(ownership)) {}
 
 public:
   /// Constructor accepting the data object while erasing the annotation
-  explicit continuable_base(Data data) : data_(std::move(data)) {
-  }
+  explicit continuable_base(Data data)
+    : data_(std::move(data)) {}
 
   /// Constructor accepting any object convertible to the data object,
   /// while erasing the annotation
@@ -124,10 +124,10 @@ public:
                 Data, Annotation,
                 detail::traits::unrefcv_t<OtherData>>::value>* = nullptr>
   /* implicit */ continuable_base(OtherData&& data)
-      : data_(detail::base::proxy_continuable<
-              Annotation, detail::traits::unrefcv_t<OtherData>>(
-            std::forward<OtherData>(data))) {
-  }
+    : data_(
+          detail::base::proxy_continuable<Annotation,
+                                          detail::traits::unrefcv_t<OtherData>>(
+              std::forward<OtherData>(data))) {}
 
   /// Constructor taking the data of other continuable_base objects
   /// while erasing the hint.
@@ -138,8 +138,7 @@ public:
             std::enable_if_t<std::is_convertible<
                 detail::traits::unrefcv_t<OData>, Data>::value>* = nullptr>
   /* implicit */ continuable_base(continuable_base<OData, Annotation>&& other)
-      : data_(std::move(other).consume()) {
-  }
+    : data_(std::move(other).consume()) {}
 
   /// Constructor taking the data of other continuable_base objects
   /// while erasing the hint.
@@ -148,8 +147,7 @@ public:
   /// the continuable by any object which is useful for type-erasure.
   template <typename OData, typename OAnnotation>
   /* implicit */ continuable_base(continuable_base<OData, OAnnotation>&& other)
-      : continuable_base(std::move(other).finish().consume()) {
-  }
+    : continuable_base(std::move(other).finish().consume()) {}
 
   /// \cond false
   continuable_base(continuable_base&&) = default;
@@ -320,13 +318,19 @@ public:
   /// ```cpp
   /// http_request("github.com")
   ///   .then([](std::string github) { })
-  ///   .fail([](std::exception_ptr ptr) {
+  ///   .fail([](std::exception_ptr ep) {
+  ///     // Check whether the exception_ptr is valid (not default constructed)
+  ///     // if bool(ep) == false this means that the operation was cancelled
+  ///     // by the user or application (promise.set_canceled() or
+  ///     // make_cancelling_continuable()).
+  ///     if (ep) {
   ///       // Handle the error here
   ///       try {
-  ///       std::rethrow_exception(ptr);
+  ///         std::rethrow_exception(ep);
   ///       } catch (std::exception& e) {
   ///         e.what(); // Handle the exception
   ///       }
+  ///     }
   ///   });
   /// ```
   ///        In case exceptions are disabled, `std::error_condition` is
@@ -345,6 +349,18 @@ public:
   /// \returns Returns a continuable_base with an asynchronous return type
   ///          depending on the previous result type.
   ///
+  /// \attention The given exception type exception_t can be passed to the
+  ///            handler in a default constructed state <br>`bool(e) == false`.
+  ///            This always means that the operation was cancelled by the user,
+  ///            possibly through:
+  ///              - \ref promise_base::set_canceled
+  ///              - \ref make_cancelling_continuable
+  ///              - \ref result::set_canceled
+  ///              - \ref cancel<br>
+  ///            In that case the exception can be ignored safely (but it is
+  ///            recommended not to proceed, although it is possible to
+  ///            recover from the cancellation).
+  ///
   /// \since 2.0.0
   template <typename T, typename E = detail::types::this_thread_executor_tag>
   auto fail(T&& callback,
@@ -907,8 +923,8 @@ constexpr auto make_exceptional_continuable(Exception&& exception) {
                 "Requires at least one type for the fake signature!");
 
   using hint_t = typename detail::hints::from_args<Args...>::type;
-  using ready_continuation_t =
-      typename detail::base::ready_continuation_from_hint<hint_t>::type;
+  using ready_continuation_t = typename detail::base::
+      ready_continuation_from_hint<hint_t>::type;
   using result_t = typename detail::base::result_from_hint<hint_t>::type;
   return detail::base::attorney::create_from_raw(
       ready_continuation_t(result_t::from(exception_arg_t{},

include/continuable/external/asio.hpp

@@ -45,10 +45,12 @@ using asio_error_code_t = detail::asio::error_code_t;
 /// \since 4.1.0
 using asio_basic_errors_t = detail::asio::basic_errors_t;
 
+#if defined(CONTINUABLE_HAS_EXCEPTIONS)
 /// The system error type used by your asio distribution
 ///
 /// \since 4.1.0
 using asio_system_error_t = detail::asio::system_error_t;
+#endif // CONTINUABLE_HAS_EXCEPTIONS
 
 /// Type used as an ASIO completion token to specify an asynchronous operation
 /// that should return a continuable_base.