[docs][fit::promise] Add v1 of user guide.
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+# `fit::promise<>` User Guide
+
+Welcome! You probably dislike writing code in C++ that describes multi-step asynchronous operations.
+
+`fit::promise<>` [[1](https://fuchsia.googlesource.com/zircon/+/master/system/ulib/fit/include/lib/fit/promise.h)] makes this a bit easier. This guide covers common problems in asynchronous control flow programming and offers common usage patterns which solve those problems in the `fit::promise<>` library.
+
+## What makes asynchronous code challenging?
+
+Within the `fit::promise<>` library an asynchronous task is defined as one that is made up of multiple *synchronous* blocks of code with explicit suspend points.
+
+When defining an asynchronous task, there must be solutions for the following problems:
+
+1) **Expressing the flow of control**: how is the *sequence* of synchronous blocks and how data flows between them expressed? How is this done in an understandable way?
+2) **Management of state & resources**: what intermediate state is needed to support task execution, and what external resources must be captured? How is this expressed and how is it done safely?
+
+
+## Terminology
+* `fit::promise<>` is a move-only object made up of a collection of lambdas or callbacks that describes an asynchronous task which eventually produces a value or an error.
+* a *handler function* is a callback provided at promise creation.
+* a *continuation function* is a callback provided to various *methods of continuation* on an existing promise.
+* a `fit::executor` is responsible for scheduling and executing promises. Promises do not run until their ownership has been transferred to a `fit::executor`. At this point the executor is responsible for its scheduling and execution.
+* `fit::context` is optionally passed to handler and continuation functions to gain access to the `fit::executor` and to low-level suspend and resume controls.
+## Building & executing your first `fit::promise<>`
+
+Let's write a simple promise.
+
+```cpp
+#include <lib/fit/promise.h>
+
+...
+fit::promise<> p = fit::make_promise([] {
+ // This is a handler function.
+
+ auto world_is_flat = AssessIfWorldIsFlat();
+ if (world_is_flat) {
+ return fit::error();
+ }
+ return fit::ok();
+});
+```
+
+`p` now contains a promise that describes a simple task.
+
+In order to run the promise, it must be scheduled it on an implementation of `fit::executor`. The most commonly used executor is an `async::Executor` [[2](https://fuchsia.googlesource.com/garnet/+/master/public/lib/async_promise/executor.h)] which schedules callbacks on an `async_dispatcher_t`. For the purposes of testing and exploration, there is also `fit::single_threaded_executor` and its associated method `fit::run_single_threaded()`[[3](https://fuchsia.googlesource.com/zircon/+/master/system/ulib/fit/include/lib/fit/single_threaded_executor.h#72)] which is used here.
+
+```cpp
+// When a promise is scheduled, the `fit::executor` takes ownership of it.
+fit::result<> result = fit::run_single_threaded(std::move(p));
+assert(result.is_ok());
+```
+
+## Building a more complex `fit::promise<>`
+
+### Return, error types & resolution states
+
+As mentioned above, the template arguments for `fit::promise<>` represent the return and error types:
+
+```cpp
+fit::promise<ReturnType, ErrorType>
+```
+
+The error type can be omitted and it will take the default error type of `void` (e.g. `fit::promise<MyReturnType>` is equivalent to `fit::promise<MyReturnType, void>`).
+
+During execution, a promise must eventually reach one of the following states:
+
+* Success: the handler function or the last continuation function (see below) has returned `fit::ok()`.
+* Error: the handler function or some continuation function has returned `fit::error()`, *and* no subsequent continuation function has intercepted it.
+* Abandoned: the promise was destroyed before resolving to either Success or Error.
+
+### `.then()`, `.and_then()`, `.or_else()`: Chaining asynchronous blocks
+
+Often complex tasks can be decomposed into smaller more granular tasks. Each of these tasks needs to be asynchronously executed, but if there is some dependency between the tasks, there is a need to preserve them. This can be achieved through different combinators, such as:
+
+* `fit::promise::then()` becomes useful for defining task dependency, as execute task 1 then task 2, regardless of task 1's status.
+```cpp
+auto execute_task_1_then_task_2 = fit::make_promise([] () -> fit::result<ReturnType, ErrorType> {
+ …..;
+}).then([] (fit::result<ReturnType, ErrorType>& result) {
+ if (result.is_ok()) {
+ ...
+ } else { // result.is_error()
+ ...
+ }
+});
+```
+
+* `fit::promise::and_then()` becomes useful for defining task dependency only in the case of task 1's success.
+```cpp
+auto execute_task_1_then_task_2 = fit::make_promise([] () {
+ ...
+}).and_then([] (ReturnType& success_value) {
+ ...
+});
+```
+
+* `fit::promise::or_else()` becomes useful for defining task dependency only in the case of task 1's failure.
+```cpp
+auto execute_task_1_then_task_2 = fit::make_promise([] () {
+ ...;
+}).or_else([] (ErrorType& failure_value) {
+ ...;
+});
+```
+
+### `fit::join_promises()` & `fit::join_promise_vector()`: Executing in parallel
+
+Sometimes, multiple promises can be executed with no dependencies between them, but the aggregate result is a dependency of the next asynchronous step. In this case, `fit::join_promises()` and `fit::join_promise_vector()` are used to join on the results of multiple promises.
+
+`fit::join_promises()` is used when each promise is referenced by a variable. `fit::join_promises()` supports heterogeneous promise types.
+```cpp
+auto DoImportantThingsInParallel() {
+ auto promise1 = FetchStringFromDbAsync("foo");
+ auto promise2 = InitializeFrobinatorAsync();
+ return fit::join_promises(std::move(promise1), std::move(promise2))
+ .and_then([](std::tuple<fit::result<std::string>, fit::result<Frobinator>> results) {
+ return fit::ok(results.get<0>.value() + results.get<1>.value().GetFrobinatorSummary());
+ });
+}
+```
+
+`fit::join_promise_vector()` is used when the promises are stored in `std::vector<>`. This has the added constraint that all promises must be homogeneous (be of the same type).
+
+### `return fit::make_promise()`: Chaining or branching by returning new promises
+
+It may become useful to defer the decision of which promises to chain together until runtime. This method is in contrast with chaining that is performed syntactically (through the use of consecutive `.then()`, `.and_then()` and `.or_else()` calls).
+
+```cpp
+fit::make_promise(...)
+ .then(fit::result<> result) {
+ if (result.is_ok()) {
+ return fit::make_promise(...); // Do work in success case.
+ } else {
+ return fit::make_promise(...); // Error case.
+ }
+ });
+```
+
+This pattern is also useful to decompose what could be a long promise into smaller readable chunks, such as by having a continuation function return the result of `DoImportantThingsInParallel()` from the example above.
+
+> NOTE: See the gotcha "Handlers / continuation functions can return ..." below.
+
+### Declaring and keeping intermediate state alive
+
+Some tasks require state be kept alive only so long as the promise itself is either pending or executing. This state is not suited to be moved into any given lambda due to its need to be shared, nor is it appropriate to transfer ownership to a longer-lived container due to a desire for its lifecycle to be coupled to the promise.
+
+Although not the only solution, usage of both `std::unique_ptr<>` and `std::shared_ptr<>` are common patterns:
+
+#### `std::unique_ptr<>`
+
+```cpp
+fit::promise<> MakePromise() {
+ struct State {
+ int i;
+ };
+ // Create a single std::unique_ptr<> container for an instance of State and capture
+ // raw pointers to the state in the handler and continuations.
+ //
+ // Ownership of the underlying memory is transferred to a lambda passed to
+ // `.inspect()`. |state| will die when the returned promise is resolved or is abandoned.
+ auto state = std::make_unique<State>();
+ state->i = 0;
+ return fit::make_promise([state = state.get()] {
+ state->i++;
+ }).and_then([state = state.get()] {
+ state->i--;
+ }).inspect([state = std::move(state)] {});
+}
+```
+
+#### `std::shared_ptr<>`
+
+fit::promise<> MakePromise() {
+ struct State {
+ int i;
+ };
+ // Rely on shared_ptr's reference counting to destroy |state| when it is safe to do so.
+ auto state = std::make_shared<State>();
+ state->i = 0;
+ return fit::make_promise([state] {
+ state->i++;
+ }).and_then([state] {
+ state->i--;
+ });
+}
+### `fit::scope`: Abandoning promises to avoid memory safety violations
+
+`fit::scope` becomes useful to tie the lifecycle of a `fit::promise<>` to a resource in memory. For example:
+
+```cpp
+#include <lib/fit/scope.h>
+
+class A {
+ public:
+ fit::promise<> MakePromise() {
+ // Capturing |this| is dangerous: the returned promise will be scheduled
+ // and executed in an unknown context. Use |scope_| to protect against
+ // possible memory safety violations.
+ //
+ // The call to `.wrap_with(scope_)` abandons the promise if |scope_| is destroyed. Since
+ // |scope_| and |this| share the same lifecycle, it is safe to capture |this|.
+ return fit::make_promise([this] {
+ // |foo_| is critical to the operation!
+ return fit::ok(foo_.Frobinate());
+ }).wrap_with(scope_);
+ }
+
+ private:
+ Frobinator foo_;
+ fit::scope scope_;
+};
+
+void main() {
+ auto a = std::make_unique<A>();
+ auto promise = a->MakePromise();
+ a.reset();
+ // |promise| will not run any more, even if scheduled, protected access to the out-of-scope
+ // resources.
+}
+```
+### `fit::sequencer`: Blocking a promise on a separate promise's completion
+
+TODO: you can .wrap_with(sequencer) to block this promise on the completion of the last promise wrapped with the same sequencer object
+
+```cpp
+#include <lib/fit/sequencer.h>
+// TODO
+```
+### `fit::bridge`: integrating with callback-based async functions
+
+TODO: fit::bridge is useful to chain continuation off a callback-based async function
+
+```cpp
+#include <lib/fit/bridge.h>
+// TODO
+```
+### `fit::bridge`: decoupling execution of a single chain of continuation
+
+TODO: fit::bridge is also useful to decouple one chain of continuation into two promises that can be executed on different `fit::executor` instances
+
+## Common gotchas
+
+### It is not possible to change ErrorType mid-promise
+
+A promise's handler and all of its continuations may have different *ResultTypes*, but they must all share the same *ErrorType*.
+
+TODO: write more and how to get around this problem
+
+### Handlers / continuation functions can return fit::result<> or a new fit::promise<>, not both
+
+You may wish to write a handler which return a `fit::promise<>` in one conditional branch and a `fit::ok()` or `fit::error()` in another. This is illegal because there is no way for the compiler to cast a `fit::result<>` to a `fit::promise<>`.
+
+The workaround is to return a `fit::promise<>` that resolves to the result you want:
+
+```cpp
+auto a = fit::make_promise([] {
+ if (condition) {
+ return MakeComplexPromise();
+ }
+ return fit::make_promise([] { return fit::ok(); });
+});
+```
+### Continuation signatures
+
+Have you seen an error message like this?
+
+```
+../../zircon/system/ulib/fit/include/lib/fit/promise_internal.h:342:5: error: static_assert failed "The provided handler's last argument was expected to be of type V&, const V&, or V (if the value is copy-constructible). Please refer to the combinator's documentation for
+ a list of supported handler function signatures."
+```
+
+or:
+
+```
+../../zircon/system/ulib/fit/include/lib/fit/promise.h:288:5: error: static_assert failed due to requirement '::fit::internal::is_continuation<fit::internal::and_then_continuation<fit::promise_impl<fit::function_impl<16, false, fit::result<fuchsia::modular::storymodel::St
+oryModel, void> (fit::context &)> >, (lambda at ../../peridot/bin/sessionmgr/story/model/ledger_story_model_storage.cc:222:17)>, void>::value' "Continuation type is invalid. A continuation is a callable object with this signature: fit::result<V, E>(fit::context&)."
+```
+
+This most likely means that one of the continuation functions has a signature that isn't valid. The valid signatures for different continuation functions are shown below:
+
+For `.then()`:
+
+```cpp
+.then([] (fit::result<V, E>& result) {});
+.then([] (const fit::result<V, E>& result) {});
+.then([] (fit::result<V, E> result) {}); // Only if V and E are copyable
+.then([] (fit::context& c, fit::result<V, E>& result) {});
+.then([] (fit::context& c, const fit::result<V, E>& result) {});
+.then([] (fit::context& c, fit::result<V, E> result) {}); // Only if V and E are copyable
+```
+
+For `.and_then()`:
+
+```cpp
+.and_then([] (V& success_value) {});
+.and_then([] (const V& success_value) {});
+.and_then([] (V success_value) {}); // Only if V is copyable
+.and_then([] (fit::context& c, V& success_value) {});
+.and_then([] (fit::context& c, const V& success_value) {});
+.and_then([] (fit::context& c, V& success_value) {}); // Only if V is copyable
+```
+
+For `.or_else()`:
+
+```cpp
+.or_else([] (E& error_value) {});
+.or_else([] (const E& error_value) {});
+.or_else([] (E error_value) {}); // Only if E is copyable
+.or_else([] (fit::context& c, E& error_value) {});
+.or_else([] (fit::context& c, const E& error_value) {});
+.or_else([] (fit::context& c, E& error_value) {}); // Only if E is copyable
+```
+
+For `.inspect()`:
+
+```cpp
+// TODO
+```
+### Captures and Argument Lifecycle
+
+Promises are composed of handler and continuation functions that are usually
+lambdas. Care must be taken when constructing lambda capture lists to avoid
+capturing memory that is will not be valid when the handler or continuation in
+question executes.
+
+For example, this promise captures memory that is guaranteed to be invalid
+by the time Foo() returns (and thus, when the returned promise is scheduled and
+executed).
+
+```cpp
+fit::promise<> Foo() {
+ int i;
+ return fit::make_promise([&i] {
+ i++; // |i| is only valid within the scope of Foo().
+ });
+}
+```
+
+Instances in real code are more nuanced. A slightly less obvious example:
+
+```cpp
+fit::promise<> Foo() {
+ return fit::make_promise([i = 0] {
+ return fit::make_promise([&i] {
+ i++;
+ });
+ });
+}
+```
+
+`fit::promise` eagerly destroys handler and continuation functions: the outer-most
+handler will be destroyed once it returns the inner-most handler. See
+"Declaring and keeping intermediate state alive" above for the correct pattern
+to use in this case.
+
+## >>> sections to write
+
+* converting from one error type to another
+* fit::bridge
+* common gotchas:
+captured state lifecycle
+
