Better keywords for the Coroutines

“Our greatest weakness lies in giving up. The most certain

way to succeed is always to try just one more time.”

― Thomas A. Edison

I. Quick Introduction

At the moment Coroutines TS use keywords co_await, co_yield, and co_return. Those keywords were carefully chosen to be "ugly", so that nobody uses them in their code bases and no code break could happen with the Coroutines TS adoption.

This paper revises an approach that allows non "ugly" keywords usage without introducing any breaking change to existing code bases.

II. The idea

Introduce a new context sensitive keyword async. All the coroutine definitions must be marked with it. If a function is marked with async, then any usage of yield, await and return relate to the coroutines world.

struct y‌ield{};
struct a‌wait{};

template <class T>
future<int> some_coro() async {
    await something;     // Equivalent to co_await from Coroutines TS
    yield something2;    // Equivalent to co_yield from Coroutines TS
    return something3;   // Equivalent to co_return from Coroutines TS
}

template <class T>
T not_a_coro() {
    a‌wait something;     // `something` is an instance of `struct await`
    y‌ield something2;    // `something2` is an instance of `struct yield`
    return something3;   // returning a variable `something3`
}

III. Disadvantages

Users are forced to type async on the function definition. That gives at least 1 additional key press for simple cases when only one keyword is used in the coroutine body.
There are rare border cases when you can not identify what yield x; means without looking at the beginning of the function definition
co_await, co_yield, and co_return are unique. It makes simpler to search them in the Internet.
Using existing functions or classes with name yield while writing new code with coroutines cause problems.

IV. Advantages

No need in "ugly" co_await, co_yield, and co_return
Promise type could be partially validated by the compiler without looking into the coroutine body
return in a coroutine does not cause annoying compile time error
Users may put async on declarations to highlight that function is a coroutine
return is just a return. Less differences between functions and coroutines
co_await, co_yield, and co_return still have a chance to break existing code. There could be code bases where they are already used. For example co_y‌ield could be used for Carbon Monoxide Yield in chemistry, ecology and heavy industry.
await and yield are not unique to C++, but still searchable in the Internet. They and the async are familiar to people who come from other languages.
IDE could highlight the yield and await according to the context, which makes them distinguishable from classes and functions without looking at the beginning of the coroutine.

Coroutines TS	This proposal
template <class T> T function(string s) { if (s.empty()) { return {}; // ill formed } co_await query(s); co_yield s; co_return {"Done: " + s}; }	template <class T> T function(string s) async { if (s.empty()) { return {}; // OK } await query(s); yield s; return {"Done: " + s}; }
// 3rd paty code struct y‌ield{}; struct a‌wait{}; template <class T> T not_a_coro() { a‌wait something; // `something` is a `struct await` y‌ield something2; // `something2` is a `struct yield` return something3; // returning a variable `something3` }	// 3rd paty code struct y‌ield{}; struct a‌wait{}; template <class T> T not_a_coro() { // OK, no `async`, nothing is broken a‌wait something; y‌ield something2; return something3; }
future<int> f(stream str) { vector<char> buf = ...; int count = co_await str.read(512, buf); co_await str.write(512, buf); co_return count + 11; }	future<int> f(stream str) async { vector<char> buf = ...; int count = await str.read(512, buf); await str.write(512, buf); return count + 11; }
// Is it a coroutine? template <class T> T function(string s);	// This is a coroutine template <class T> T function(string s) async ;
y‌ield x{1, 2, 3}; // new variable of type `y‌ield` // ... y‌ield x; // new variable of type `y‌ield` // ... co_yield x;	y‌ield x{1, 2, 3}; // ... y‌ield x; // ... yield x;
auto x = []() noexcept -> future { co_await z; }; co_await x();	auto x = []() async noexcept -> future { await z; }; await x();

If the new async keyword is not acceptable, the following input could change it be used to preview the above table with other keywords (coro,nonlin,await,gap):

V. Generators and `async`

There is a concern that async keyword does not make sence for the generators.

According to the dictionaries asynchronous has the following meanings:

two or more objects or events not existing or happening at the same time.
(computing and telecommunications) a form of computer control timing protocol in which a specific operation begins upon receipt of an indication (signal) that the preceding operation has been completed.

Usage of `async` or `asynchronous` for generators matches both:

the caller execution is asynchronous to the generator execution.
`yield` indicates (signals) that the operation has been completed.

VI. History of the problem

Early versions of the coroutines (N3722 for example) were using a special keyword resumable to highlight that the function is a coroutine:

future<int> f(stream str) resumable
{
  shared_ptr<vector<char>> buf = ...;
  int count = await str.read(512, buf);
  return count + 11;
}

That keyword was dropped somewhere around the N4286 while keeping the await and yield:

std::future<void> tcp_reader(int total)
{
  char buf[64 * 1024];
  auto conn = await Tcp::Connect("127.0.0.1", 1337);
  do
  {
    auto bytesRead = await conn.read(buf, sizeof(buf));
    total -= bytesRead;
  }
  while (total > 0);
}

After N4402, the await and yield were changed to keyword-placeholders [discussion]: `the reason "yield" wasn't used was afraid about breaking code in finance and agriculture`. Later, the "ugly" versions of the keywords were introduced [discussion].

Gor Nishanov explained the resumable keyword removal:

    1) compiler knows that a function is a coroutine
    2) it forces you to write some kind of tag on a function anyway

Gor also noted, that during the keywords discussion in Lexena 2015 he proposed to bring some tag back, but that idea was shouted down at that moment.

Nowadays, 4 years later, people on the reflector and probably outside the WG21 find the solution with async like keywords tempting. Because of that and because the "ugly" keywords and the resumable keyword did not met, we propose to revise the idea based on the lessons learned on ~4 years of experience with co_*.

VII. Using `y‌ield` and `a‌wait` functions/classes in coroutines

For an already written code with functions/classes named y‌ield or a‌wait nothing gets broken with this paper.

Writing a new code with coroutines and with functions/classes named y‌ield or a‌wait may require some workarounds.

void y‌ield();
struct a‌wait{};

template <class T>
future<int> some_coro() async {
    await something;     // co_await, not a class. Compile time error
    yield();             // co_yield, not a function call. Compile time error
}

The workarounds for the above code are quite simple, and require either renaming the function and class, or adding a type alias and a function with other name:

void y‌ield();
struct a‌wait{};

// workarounds
void corn_y‌ield() { y‌ield(); }
using corn_a‌wait = a‌wait;

template <class T>
future<int> some_coro() async {
    corn_a‌wait something;     // OK
    corn_y‌ield();             // OK
}

Note that such workarounds do not prevent ADL or templates usage:

struct a‌wait{};
struct foo{};
struct tst{ void y‌ield() };

void y‌ield(a‌wait aw);
void y‌ield(foo f);

// workarounds
template <class T> void adl_y‌ield(T v) { y‌ield(v); }
void adl_y‌ield(tst v) { v.y‌ield(); }

template <class T>
future<int> some_coro(T val) async {
    adl_y‌ield(val);             // OK
}

auto res = some_coro(a‌wait{});  // OK

VIII. Distinguishability

A concern of distinguishability was raised on reflector during the proposal discussion. Here's a few examples on border cases when we see "only a single line of code in function":

Coroutines TS	This proposal
y‌ield x{1, 2, 3};	y‌ield x{1, 2, 3};
y‌ield x;	y‌ield x;
co_yield x;	yield x;
co_yield (x == 0);	yield (x == 0);
y‌ield(x == 0);	y‌ield(x == 0);

Note that the above problem only arises if we do not see the beginning of the function. Otherwise it's obvious:

Coroutines TS	This proposal
type foo() { y‌ield x{1, 2, 3};	type foo() { y‌ield x{1, 2, 3};
type foo() { y‌ield x;	type foo() { y‌ield x;
type foo() { co_yield x;	type foo() async { yield x;
type foo() { co_yield (x == 0);	type foo() async { yield (x == 0);
type foo() { y‌ield(x == 0);	type foo() { y‌ield(x == 0);

VIII. Wording, relative to N4810 (related paper: N4775 C++ Extensions for Coroutines)

Remove the keywords co_await, co_yield, and co_return from Table 5 "Keywords".
Change [dcl.fct.def.coroutine] p1:
A function is a coroutine if ~~it contains a coroutine-return-statement (9.6.3.1), an await-expression (8.3.8), a yield-expression (8.21), or a range-based for (9.5.4) with co_await~~its definition marked with async.
Change examples [dcl.fct.def.coroutine] in p2. and p10. by adding async to each of the coroutines (f() async, g1() async, g2() async, g3() async).
Change [expr.await] p1:
The co_await expression appears only in coroutines and is used to suspend evaluation of a coroutine (11.4.4) while awaiting completion of the computation represented by the operand expression.
Change examples [expr.await] in p6. by adding async to each of the coroutines (h() async, g() async).
Change [stmt.return.coroutine]:
9.6.3.1 The co_routine return statement [stmt.return.coroutine]

...

A coroutine returns to its caller or resumer (11.4.4) by the co_return statement or when suspended (8.3.8). ~~A coroutine shall not return to its caller or resumer by a return statement(9.6.3).~~
Remove all the co_ prefixes from all the occurrences of co_await, co_yield, and co_return.

Add to the [dcl.fct] p1 and p2 respectively:

D1 ( parameter-declaration-clause ) cv-qualifier-seqopt
	ref-qualifieropt asyncopt noexcept-specifieropt attribute-specifier-seqopt
	...
D1 ( parameter-declaration-clause ) cv-qualifier-seqopt
	ref-qualifieropt asyncopt noexcept-specifieropt attribute-specifier-seqopt trailing-return-type

Add to the [dcl.decl] p5 grammar:

parameters-and-qualifiers:
	( parameter-declaration-clause ) cv-qualifier-seqopt
		ref-qualifieropt asyncopt noexcept-specifieropt attribute-specifier-seqopt

IX. Acknowledgements

Many thanks to Gor Nishanov, for exploring the coroutines design space, for TS implementations, for teaching people about the coroutines, and for an insane amount of interesting coroutines related measurements/talks/presentations.

Thanks to Ville Voutilainen and Bjarne Stroustrup for pointing me to the previous discussions of the problem and to some concerns.