Add examples (#122)

* added a few examples (some incomplete)

* added an example using a timer and fixed an issue with sync_wait

* restored cmake presets

* clang-format

* another attempt at clang-format

Author: dietmarkuehl

docs/intro-examples.md

@@ -0,0 +1,130 @@
+<!-- SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception -->
+
+# Introduction by Example
+
+This page provides a series of examples showing how to use the
+`std::execution` components.
+
+<details>
+<summary>`"Hello, world"` - synchronous using asynchronous components</summary>
+
+Code: [`examples/intro-1-hello-world.cpp`]()
+
+The first example is a very complicated way to a version of `"Hello,
+world"`: it uses components for dealing with asynchronous work to
+synchronously produce the result. The intention is to show a basic
+use of some involved components to build up a feeling of how things
+work.
+
+The componentes for `std::execution` are declared in the header
+`<execution>`.  This particular implementation implements the
+cmponents in namespace `beman::execution26` declared in the header
+`<beman/execution26/execution.hpp>`:
+
+```c++
+#include <beman/execution26/execution.hpp>
+#include <iostream>
+#include <string>
+#include <tuple>
+
+namespace ex = ::beman::execution26;
+using namespace std::string_literals;
+```
+
+Most of these declarations should be familiar. The namespace alias
+`ex` is used to support easy migration to a different implementation,
+in particular the standard name `std::execution` once it becomes
+available with standard library implementations. The other examples
+will have a similar start which is only mentioned in the explanation
+to point out unusual parts like the use of custom components.
+
+All interesting work happens in the `main` function:
+
+```c++
+int main()
+{
+    auto[result] = ex::sync_wait(
+        ex::when_all(
+            ex::just("hello, "s),
+            ex::just("world"s)
+        )
+        | ex::then([](auto s1, auto s2){ return s1 + s2; })
+        ).value_or(std::tuple(""s));
+
+    std::cout << result << '\n';
+}
+```
+
+This code code be simpler even when using components from
+`std::execution`.  Showing a few more components is intended to
+better reflect how an asynchronous program might look like. This
+examples uses a _sender factory_ (`ex::just`), two _sender adaptors_
+(`ex::when_all` and `ex::then`), and finally a _sender consumer_
+(`ex::sync_wait`) to build up work and to execute it. The idea of
+a _sender_ is that it represents work which can be composed with
+algorithms into a unit of work which is eventually executed.
+
+Each work item can complete asynchronously at some later time, i.e.,
+calling it like a function and using a returned value isn't really
+an option. Instead, when the work is started it does whatever is
+needed to get the work completed and get a _completion signal_
+delivered. Delivering a completion signal consists of calling a
+function on a suitable objects. The important part is that once
+work is started it always delivers exactly one completion signal
+which can indicate success, failure, or cancellation. Later examples
+for creating senders will go into more details about the cancellation
+signals.
+
+The components used in this example do all of that synchronously:
+
+- `ex::just("string"s)` completes immediately when started with
+    successful completion which includes the string passed as
+    argument.
+- <code>ex::when_all(_sender1_, _sender2_)</code> starts the senders
+    passed as arguments. When all of the senders complete, it
+    produces its own completion. In the case of success all the
+    received values are passed to the completion signal. In case
+    of an error all outstanding work is cancelled and the first
+    error becomes `when_all`'s completion signal once all children
+    have completed.  Similarly, in case of cancellation all children
+    get cancelled and once all complete `when_all` produces a
+    cancellation signal. In the example the two children each produces
+    one string as completion signal and `when_all` produces these two
+    strings as its completion signal.
+- <code>_sender_ | ex::then(_fun_)</code> is equivalent to using
+    <code>ex::then(_sender_, _fun_)</code>. The `ex::then` calls
+    the function <code>_fun_</code> with its child sender completes
+    successful.  The arguments to <code>_fun_</code> are the values
+    received from the child completion signal.  In the example, the
+    child is `when_all(...)` and it produces two strings which are
+    passed to <code>_fun_</code>. The completion signal of `ex::then`
+    is successful with the value returned from the call to
+    <code>_fun_</code> (which may `void`) if the call returns
+    normally. If an exception is thrown `ex::then` completes with
+    an `std::exception_ptr` to the exception thrown. In the example
+    the completion is just a concatenation of the two strings.
+- <code>sync_wait(_sender_)</code> starts its argument and then
+    blocks until the work completes although the thread calling
+    `sync_wait` may contribute to the completion of the work. The
+    function returns a an
+    <code>std::optional&lt;std::tuple&lt;_results_...&gt;&gt;</code>>.
+    If the child sender completes successfully the values from the
+    child's completion signal become the elements of the tuple. If
+    the child completes with an error, the error is thrown as an
+    exception. Otherwise, if the work gets cancelled, an empty
+    `std::optional<...>` is returned. In the example, the child
+    sends a string which gets wrapped into a `std::tuple` which in
+    turn gets wrapped into an `std::optional`. Thus, the somewhat
+    round-about way to get the result: first using
+    `value_or(std::tuple(""s))` to get the value from the `std::optional`
+    which is then decomposed from the `std::tuple` using structured
+    bindings.
+
+</details>
+
+<details>
+<summary>`"Hello, async"` - a simple asynchronous example</summary>
+
+Code: [`examples/intro-2-hello-async.cpp`]()
+
+</details>

examples/CMakeLists.txt

@@ -12,6 +12,9 @@ list(
   stop_token
   stopping
   allocator
+  intro-1-hello-world
+  intro-2-hello-async
+  intro-5-consumer
   doc-just
   doc-just_error
   doc-just_stopped

examples/intro-1-hello-world.cpp

@@ -0,0 +1,27 @@
+// examples/intro-1-hello-world.cpp                                   -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <beman/execution26/execution.hpp>
+#include <iostream>
+#include <string>
+#include <tuple>
+
+namespace ex = ::beman::execution26;
+using namespace std::string_literals;
+
+// ----------------------------------------------------------------------------
+// Please see the explanation in docs/intro-examples.md for an explanation.
+
+int main() {
+    // clang-format off
+     auto [result] = ex::sync_wait(
+         ex::when_all(
+             ex::just("hello, "s),
+             ex::just("world"s)
+         ) | ex::then([](auto s1, auto s2) { return s1 + s2; })
+         ).value_or(std::tuple(""s)
+     );
+    // clang-format on
+
+    std::cout << result << '\n';
+}

examples/intro-2-hello-async.cpp

@@ -0,0 +1,39 @@
+// examples/intro-2-hello-async.cpp                                   -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <beman/execution26/execution.hpp>
+#include "intro-timer.hpp"
+#include <chrono>
+#include <iostream>
+#include <string>
+#include <tuple>
+
+namespace ex = ::beman::execution26;
+using namespace std::string_literals;
+using namespace std::chrono_literals;
+
+// ----------------------------------------------------------------------------
+// Please see the explanation in docs/intro-examples.md for an explanation.
+
+int main() {
+    std::cout << std::unitbuf;
+    intro::timer timer;
+
+    // clang-format off
+     auto [result] = ex::sync_wait(
+         ex::when_all(
+             timer.run(),
+             ex::when_all(
+                 timer.resume_after(3s)
+                     | ex::then([] { std::cout << "h\n"; return std::string("hello"); }),
+                 timer.resume_after(1s)
+                     | ex::then([] { std::cout << ",\n"; return std::string(", "); }),
+                 timer.resume_after(2s)
+                     | ex::then([] { std::cout << "w\n"; return std::string("world"); })
+             ) | ex::then([](auto s1, auto s2, auto s3) { return s1 + s2 + s3; })
+         )
+     ).value_or(std::tuple(std::string("")));
+    // clang-format on
+
+    std::cout << result << "\n";
+}

examples/intro-5-consumer.cpp

@@ -0,0 +1,93 @@
+// examples/intro-1-hello-world.cpp                                   -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <beman/execution26/execution.hpp>
+#include <beman/execution26/detail/suppress_push.hpp>
+#include <expected>
+#include <iostream>
+#include <string>
+#include <tuple>
+
+namespace ex = ::beman::execution26;
+using namespace std::string_literals;
+
+enum class success { one };
+enum class failure { fail_one };
+
+struct expected_to_channel_t {
+    template <typename Receiver>
+    struct own_receiver {
+        using receiver_concept = ex::receiver_t;
+        Receiver* receiver;
+        template <typename Value, typename Error>
+        auto set_value(std::expected<Value, Error>&& exp) noexcept -> void {
+            if (exp) {
+                std::cout << "received an expected with value from child/upstream\n" << std::flush;
+                ex::set_value(std::move(*receiver), exp.value_or(Value{}));
+                std::cout << "set_value\n" << std::flush;
+            } else {
+                std::cout << "received an expected with error from child/upstream\n";
+                ex::set_error(std::move(*receiver), exp.error_or(Error{}));
+            }
+        }
+        template <typename Error>
+        auto set_error(Error&& error) noexcept -> void {
+            std::cout << "received an error from child/upstream";
+            ex::set_error(std::move(*receiver), std::forward<Error>(error));
+        }
+        auto set_stopped() noexcept -> void {
+            std::cout << "received an cancelletion from child/upstream";
+            ex::set_stopped(std::move(*receiver));
+        }
+    };
+    template <ex::sender CSender, ex::receiver Receiver>
+    struct state {
+        using operation_state_concept = ex::operation_state_t;
+        using child_state_t = decltype(ex::connect(std::declval<CSender>(), std::declval<own_receiver<Receiver>>()));
+        Receiver      parent_receiver;
+        child_state_t child_state;
+        template <ex::sender S, ex::receiver R>
+        state(S&& child_sender, R&& parent_receiver)
+            : parent_receiver(std::forward<R>(parent_receiver)),
+              child_state(ex::connect(std::forward<S>(child_sender), own_receiver<Receiver>{&this->parent_receiver})) {
+        }
+        void start() & noexcept {
+            std::cout << "starting execpted_to_channel\n";
+            ex::start(child_state);
+        }
+    };
+
+    template <ex::sender CSender>
+    struct sender {
+        using sender_concept = ex::sender_t;
+        // value_types_of<CSender....> -> set_value_t(std::expected<T, E>)
+        // -> completion_signatures<set_value_t(T), set_error_t(E)>
+        // -> + error_type_of<CSender...>
+        // -> + sends_stopped<CSender...> -> set_stopped_t()
+        // -> unique
+        using completion_signatures = ex::completion_signatures<ex::set_value_t(success), ex::set_error_t(failure)>;
+        CSender child_sender;
+
+        template <ex::receiver Receiver>
+        state<CSender, std::remove_cvref_t<Receiver>> connect(Receiver&& receiver) && {
+            return {std::move(this->child_sender), std::forward<Receiver>(receiver)};
+        }
+        template <ex::receiver Receiver>
+        state<CSender, std::remove_cvref_t<Receiver>> connect(Receiver&& receiver) const& {
+            return {this->child_sender, std::forward<Receiver>(receiver)};
+        }
+    };
+
+    template <ex::sender CSender>
+    sender<std::remove_cvref_t<CSender>> operator()(CSender&& child_sender) const {
+        return {std::forward<CSender>(child_sender)};
+    }
+    auto operator()() const { return ex::detail::sender_adaptor{*this}; }
+};
+inline constexpr expected_to_channel_t expected_to_channel{};
+
+int main() {
+    ex::sync_wait(ex::just(std::expected<success, failure>(success::one)) | expected_to_channel() |
+                  ex::then([](success) noexcept { std::cout << "success\n"; }) |
+                  ex::upon_error([](failure) noexcept { std::cout << "fail\n"; }));
+}