1 Introduction

Execution environments are a fundamental part of the std::execution proposal, [P2300R9], but that paper provides no utility for creating or manipulating environments. Such utilities are increasingly necessary considering that some proposed APIs (e.g., write_env from [P3284R0]) require passing an environment.

This paper proposes two simple utilities for creating execution environments: one to create an environment out of a query/value pair and another to join several environments into one.

2 Executive Summary

This paper proposes the following changes to P2300R9:

• Add a class template provisionally called prop to the std::execution namespace. prop associates a query Q with a value V, yielding a queryable object E such that E.query(Q) is equal to V.

• Add a class template provisionally called env to the std::execution namespace. env aggregates several environments into one, giving precedence to the environments in lexical order.

• Optionally, replace empty_env with env<>.

3 Discussion

3.1 Motivation

In [P2300R9], execution environments are an internal implementation detail of the sender algorithms. There are no public-facing APIs that accept environment objects as a parameter. One may wonder why a utility for constructing environments is even necessary.

There are several reasons why the Standard Library would benefit from a utility to construct an environment:

1. The set of sender algorithms is openly extensible. For those who decide to implement their own sender algorithms, the manipulation of execution environments is part of the job. Standard utilities will make this easier.

2. [P3284R0] proposes a write_env sender adaptor that merges a user-specified environment with the environment of the receiver to which the sender is eventually connected. Using this adaptor requires the user to construct an environment. Although implementing an environment is not hard, an out-of-the-box solution will make using write_env simpler.

3. [P3149R3] proposes two new algorithms, spawn and spawn_future, both of which can be parameterized by passing an environment as an optional argument.

4. Currently, there is no way to parameterize algorithms like sync_wait and start_detached. The author intends to bring a paper proposing overloads of those functions that accept environments as a way to inject things like stop tokens, allocators, and schedulers into the asynchronous operations that those algorithms launch.

In short, environments are how users will inject dependencies into async computations. We can expect to see more APIs that will require (or will optionally allow) the user to pass an environment. Thus, a standard utility for making environments is desirable.

3.2 Design Considerations

3.2.1 Singleton Environments

Now that tag_invoke has been removed from P2300, defining an execution environment is quite simple. To build an environment with a single query/value pair, the following suffices:

template <class Query, class Value>
struct prop
{
  [[no_unique_address]] Query query_;
  Value value_;

  auto query(Query) const noexcept -> const Value &
  {
    return value_;
  }
};

// Example usage:
constexpr auto my_env = prop(get_allocator, std::allocator{});

Although simple, this template has some nice properties:

1. It is an aggregate so that members can be direct initialized from temporaries without so much as a move. A function that constructs and returns an environment using prop will benefit from RVO:

   // The frobnicator object will be constructed directly in
   // the callers stack frame:
   return prop(get_frobnicator, make_a_frobnicator());

2. An object constructed like prop(get_allocator, my_allocator{}) will have the simple and unsurprising type prop<get_allocator_t, my_allocator>.

The prop utility proposed in this paper is only slightly more elaborate than the prop class template shown above, and it shares these properties.

3.2.2 Environment Composition

What if you need to construct an environment with more than one query/value pair, say, an allocator and a scheduler? A utility to join multiple environments would work together with prop to make a general environment-building utility. This paper calls that utility env.

The following code demonstrates:

template <class Env, class Query>
concept has-query = requires (const Env& env) { env.query(Query()); };

template <class... Envs>
struct env : Envs...
{
  template <class Query>
  static constexpr size_t _index_of() {
    constexpr bool flags[] = {has-query<Envs, Query>...};
    return ranges::find(flags, true) - flags;
  }

  template <class Query>
    requires (has-query<Envs, Query> ||...)
  decltype(auto) query(Query q) const noexcept(...)
  {
    auto tup = tie(static_cast<const Envs&>(*this)...);
    return get<_index_of<Query>()>(tup).query(q);
  }
};

template <class... Envs>
env(Envs...) -> env<Envs...>;

// Example usage:
constexpr auto my_env = env(prop(get_allocator, my_alloc{}),
                            prop(get_scheduler, my_sched{}));

This env class template shares the desirable properties of prop: aggregate initialization and unsprising naming. A query is handled by the “leftmost” child environment that can handle it.

Note that the above code has the issue that two child environments cannot have the same type. That is not a limitation of the facility proposed below.

3.2.3 By Value vs. By Reference

There are times when you would like an environment to respond to a query with a reference to a particular object rather than a copy. Capturing a reference is dangerous, so the opt-in to reference semantics should be explicit.

std::reference_wrapper is how the Standard Library deals with such problems. It should be possible to construct an environment using std::ref to specify that the “value” should be stored by reference:

std::mutex mtx;
const auto my_env = prop(get_mutex, std::ref(mtx));
std::mutex & ref = get_mutex(my_env);
assert(&ref == &mtx);

Similarly, there are times when you would like to store the child environment by reference. std::ref should work for that case as well:

// At namespace scope, construct a reusable environment:
constexpr auto global_env = prop(get_frobnicator, make_frobnicator());

// Construct an environment with a particular value for the get_allocator
// query, and a reference to the constexpr global_env:
auto env_with_allocator(auto alloc)
{
  return env(prop(get_allocator, alloc), std::ref(global_env));
}

The utility proposed in this paper satisfies all of the above use cases.

3.3 Naming

For the prop utility, a couple of other plausible names come to mind:

• attr
• with
• property
• key_value
• query_value

Other possibile names for env are:

• attributes, or attrs
• properties, or props
• dictionary, or dict

4 Implementation Experience

Types with a very similar designs have seen heavy use in stdexec for over two years. The design proposed below has been prototyped and can be found on Compiler Explorer¹.

A note on implementability: The implementation of env is simple if we only want to support braced list initialization with class template argument deduction, like env{e1, e2, e2}. Initialization from a parenthesized expression list with CTAD for an arbitrary number of arguments (e.g., env(e1, e2, e3, ...)) is not implementable in standard C++ to the author’s knowledge.

In the proposed wording, env is specified such that it will be able to benefit from member packs ([P3115R0]) if and when they become available or should aggregate initialization with a parenthesized expression list ever be extended to support brace elision for subobjects.

5 Future Extensions

There are several ways this utility might be extended in the future:

• Add a way to remove a query from an environment.
• Add a wrapper for a reference to an environment.
• Add a wrapper that only accepts queries that are forwarding (see [exec.fwd.env]).

6 Proposed Wording

[ Editor's note: Replace all occurances of empty_env with env<>. Change [exec.syn] as follows: ]

Header <execution> synopsis [exec.syn]

namespace std::execution {
  …as before…

  struct empty_env {};
  struct get_env_t { see below };
  inline constexpr get_env_t get_env{};

  template<class T>
    using env_of_t = decltype(get_env(declval<T>()));

  // [exec.prop] class template prop
  template<class Query, class Value>
    struct prop;

  // [exec.env] class template env
  template<queryable... Envs>
    struct env;

  // [exec.domain.default], execution domains
  struct default_domain;

  // [exec.sched], schedulers
  struct scheduler_t {};

  …as before…
}

[ Editor's note: After [exec.utils], add a new subsection [exec.envs] as follows: ]

7 Acknowledgements

I would like to thank Lewis Baker for many enlightening conversations about the design of execution environments. The design presented here owes much to Lewis’s insights.

8 References