Using coroutines for monadic composition

This repository shows how coroutines can be used to compose operations that return monadic types.

One example of a monadic type is one that either contain a result to be used in further computation or an indication of error, such as std::optional<T>.

The goal is to make such composition as easy as Haskell's do notation.

Requirements

The implementation requires an implementation of the Coroutines TS (https://isocpp.org/files/papers/N4680.pdf). As of March 10, 2018, the only such implementation is Clang 5 or later with a corresponding libc++.

In particular, the MSVC implementation converts the object returned by get_return_object to the return type of the coroutine immediately rather than waiting until the coroutine returns to its caller.

Limitations

Only certain kinds of monads can be made to work with the approach in this repository. Because coroutines can only move forwards, only monads that invoke their continuations at most once can be supported.

Monads that are like Maybe and Either are supported fully. These monads' bind operation invokes, or not, its continuation directly inside bind, so there is no possibility of it being invoked more than once.

Monads like List cannot be supported. These monads explicitly call their continuations multiple times inside bind.

Then there are monads like State that store their continuations, building up a combined continuation that is invoked later, after bind has returned, under the control of some other code. This kind of monad is supported with the caveat that the resulting continuation may only be invoked at most once.

Expected

See test_expected.cpp for three different ways to compose a sequence of calls to functions returning expected values.

The expected in question is that from viboes' std-make repository. This definition knows nothing about coroutines; all of the coroutine machinery is in monad_promise.h and test_expected.cpp.

Here's what one can write in Haskell:

data MyError = MyError Int

f1 :: Either MyError Int
f1 = Right 7

f2 :: Int -> Either MyError Float
f2 x = Right (fromIntegral x * 2.0)

f3 :: Int -> Float -> Either MyError Int
f3 x y = Left (MyError 42)

test :: Either MyError Int
test =
  do x <- f1
     y <- f2 x
     z <- f3 x y
     return z

With the code in the repository, you can write it like this in C++:

struct error {
  int code;
};

expected<int, error> f1() { return 7; }
expected<double, error> f2(int x) { return 2.0 * x; }
expected<int, error> f3(int x, double y) { return error{42}; }

auto test_expected_coroutine() {
  return []() -> expected<int, error> {
    auto x = co_await f1();
    auto y = co_await f2(x);
    auto z = co_await f3(x, y);
    co_return z;
  }();
}

State

An implementation of the State monad can be found in state.h. Examples of its usage, both with and without coroutines, are in test_state.cpp.