system/utest/fit/examples/promise_example1.cpp

// Copyright 2018 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "promise_example1.h"

#include <string>

#include <lib/fit/promise.h>
#include <lib/fit/single_threaded_executor.h>

#include "utils.h"

// This example demonstrates sequencing of tasks using combinators.
namespace promise_example1 {

fit::promise<int, std::string> pick_bananas(int hours) {
    return fit::make_promise([hours, time = 0, harvest = 0](fit::context& context) mutable
                             -> fit::result<int, std::string> {
        if (time == 0) {
            printf("Starting the day picking bananas for %d hours...\n", hours);
        } else {
            printf("... %d hour elapsed...\n", time);
        }
        if (random() % 7 == 0) {
            return fit::error("A wild animal ate all the bananas we picked today!");
        }
        if (time < hours) {
            // Simulate time passing.
            // Here we call |suspend_task()| to obtain a |fit::suspended_task|
            // which acts as a handle which will later be used by
            // |resume_in_a_little_while()| to resume the task.  In the
            // meantime, we unwind the call stack by returning |fit::pending()|.
            // Once the task is resumed, the promise's handler will restart
            // execution from the top again, however it will have retained
            // state (in |time| and |harvest|) from its prior execution.
            utils::resume_in_a_little_while(context.suspend_task());
            time++;
            harvest += random() % 31;
            return fit::pending();
        }
        return fit::ok(harvest);
    });
}

fit::promise<void, std::string> eat_bananas(int appetite) {
    return fit::make_promise([appetite](fit::context& context) mutable
                             -> fit::result<void, std::string> {
        if (appetite > 0) {
            printf("... eating a yummy banana....\n");
            utils::resume_in_a_little_while(context.suspend_task());
            appetite--;
            if (random() % 11 == 0) {
                return fit::error("I ate too many bananas.  Urp.");
            }
            return fit::pending();
        }
        puts("Ahh.  So satisfying.");
        return fit::ok();
    });
}

fit::promise<> prepare_simulation() {
    int hours = random() % 8;
    return pick_bananas(hours)
        .and_then([](int harvest) -> fit::result<int, std::string> {
            printf("We picked %d bananas today!\n", harvest);
            if (harvest == 0)
                return fit::error("What will we eat now?");
            return fit::ok(harvest);
        })
        .and_then([](int harvest) {
            int appetite = random() % 7;
            if (appetite > harvest)
                appetite = harvest;
            return eat_bananas(appetite);
        })
        .or_else([](std::string error) {
            printf("Oh no!  %s\n", error.c_str());
            return fit::error();
        })
        .and_then([] {
            puts("*** Simulation finished ***");
        })
        .or_else([] {
            puts("*** Restarting simulation ***");
            return prepare_simulation();
        });
}

void run() {
    auto simulation = prepare_simulation();
    fit::run_single_threaded(std::move(simulation));
}

} // namespace promise_example1