Higher function taking async reference-taking closures

Struggled when designing libraries for async/await.

We can do this in the sync world:

// `f` is a reference-taking closure
pub fn call<F>(f: F) -> i32
where
    F: Fn(&i32) -> i32,
{
    f(&42)
}

#[test]
fn test_call() {
    fn double(x: &i32) -> i32 { *x * 2 }
    assert_eq!(call(double), 84);
}

Naive async equivalent doesn't compile.

#![feature(async_await)]

use std::future::Future;

// `f` is a reference-taking closure
pub async fn call<F, Fut>(f: F) -> i32
where
    F: Fn(&i32) -> Fut,
    Fut: Future<Output = i32>,
{
    f(&42).await
}

// #[runtime::test]
async fn test_call() {
    async fn double(x: &i32) -> i32 { *x * 2 }
    assert_eq!(call(double).await, 84);
}

This is legit because Fut is dependent on the HRTB lifetime parameter for F. This means we need either Fn internals or lifetime-dependent type constructor parameters.

However, the code below doesn't compile either:

#![feature(async_await)]
#![feature(unboxed_closures)]

use std::future::Future;

pub async fn call<F>(f: F) -> i32
where
    F: for<'a> Fn<(&'a i32,)>,
    for<'a> <F as FnOnce<(&'a i32)>>::Output: Future<Output = i32>,
{
    f(&42).await
}

// #[runtime::test]
async fn test_call() {
    async fn double(x: &i32) -> i32 { *x * 2 }
    assert_eq!(call(double).await, 84);
}

Am I missing something?

As far as I know there is currently no workaround that will allow writing a function like this, see this issue for more details:

1 Like

Thanks for the useful link! That thread discusses the same problem. Still I'd like to know why the last snippet in my post doesn't work.

I've found a workaround:

#![feature(async_await)]

use std::future::Future;

macro_rules! impl_async_fn {
    ($(($FnOnce:ident, $FnMut:ident, $Fn:ident, ($($arg:ident: $arg_ty:ident,)*)),)*) => {
        $(
            pub trait $FnOnce<$($arg_ty,)*> {
                type Output;
                type Future: Future<Output = Self::Output> + Send;
                fn call_once(self, $($arg: $arg_ty,)*) -> Self::Future;
            }
            pub trait $FnMut<$($arg_ty,)*>: $FnOnce<$($arg_ty,)*> {
                fn call_mut(&mut self, $($arg: $arg_ty,)*) -> Self::Future;
            }
            pub trait $Fn<$($arg_ty,)*>: $FnMut<$($arg_ty,)*> {
                fn call(&self, $($arg: $arg_ty,)*) -> Self::Future;
            }
            impl<$($arg_ty,)* F, Fut> $FnOnce<$($arg_ty,)*> for F
            where
                F: FnOnce($($arg_ty,)*) -> Fut,
                Fut: Future + Send,
            {
                type Output = Fut::Output;
                type Future = Fut;
                fn call_once(self, $($arg: $arg_ty,)*) -> Self::Future {
                    self($($arg,)*)
                }
            }
            impl<$($arg_ty,)* F, Fut> $FnMut<$($arg_ty,)*> for F
            where
                F: FnMut($($arg_ty,)*) -> Fut,
                Fut: Future + Send,
            {
                fn call_mut(&mut self, $($arg: $arg_ty,)*) -> Self::Future {
                    self($($arg,)*)
                }
            }
            impl<$($arg_ty,)* F, Fut> $Fn<$($arg_ty,)*> for F
            where
                F: Fn($($arg_ty,)*) -> Fut,
                Fut: Future + Send,
            {
                fn call(&self, $($arg: $arg_ty,)*) -> Self::Future {
                    self($($arg,)*)
                }
            }
        )*
    }
}
impl_async_fn! {
    (AsyncFnOnce0, AsyncFnMut0, AsyncFn0, ()),
    (AsyncFnOnce1, AsyncFnMut1, AsyncFn1, (a0:A0, )),
    (AsyncFnOnce2, AsyncFnMut2, AsyncFn2, (a0:A0, a1:A1, )),
    (AsyncFnOnce3, AsyncFnMut3, AsyncFn3, (a0:A0, a1:A1, a2:A2, )),
    (AsyncFnOnce4, AsyncFnMut4, AsyncFn4, (a0:A0, a1:A1, a2:A2, a3:A3, )),
    (AsyncFnOnce5, AsyncFnMut5, AsyncFn5, (a0:A0, a1:A1, a2:A2, a3:A3, a4:A4, )),
    (AsyncFnOnce6, AsyncFnMut6, AsyncFn6, (a0:A0, a1:A1, a2:A2, a3:A3, a4:A4, a5:A5, )),
    (AsyncFnOnce7, AsyncFnMut7, AsyncFn7, (a0:A0, a1:A1, a2:A2, a3:A3, a4:A4, a5:A5, a6:A6, )),
    (AsyncFnOnce8, AsyncFnMut8, AsyncFn8, (a0:A0, a1:A1, a2:A2, a3:A3, a4:A4, a5:A5, a6:A6, a7:A7, )),
    (AsyncFnOnce9, AsyncFnMut9, AsyncFn9, (a0:A0, a1:A1, a2:A2, a3:A3, a4:A4, a5:A5, a6:A6, a7:A7, a8:A8, )),
    (AsyncFnOnce10, AsyncFnMut10, AsyncFn10, (a0:A0, a1:A1, a2:A2, a3:A3, a4:A4, a5:A5, a6:A6, a7:A7, a8:A8, a9:A9, )),
    (AsyncFnOnce11, AsyncFnMut11, AsyncFn11, (a0:A0, a1:A1, a2:A2, a3:A3, a4:A4, a5:A5, a6:A6, a7:A7, a8:A8, a9:A9, a10:A10, )),
    (AsyncFnOnce12, AsyncFnMut12, AsyncFn12, (a0:A0, a1:A1, a2:A2, a3:A3, a4:A4, a5:A5, a6:A6, a7:A7, a8:A8, a9:A9, a10:A10, a11:A11, )),
}

pub async fn call<F>(f: F) -> i32
where
    F: for<'a> AsyncFn1<&'a i32, Output = i32>,
{
    f.call_once(&42).await
}

// #[runtime::test]
async fn test_call() {
    async fn double(x: &i32) -> i32 { *x * 2 }
    assert_eq!(call(double).await, 84);
}
1 Like

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