How do I implement a trait on a curried function?

Question

If I try to implement the trait Frob for functions like foo as follows:

fn foo<'b>(state: &'b mut i32) -> impl FnMut(&str) -> i32 + 'b {
    move |i| *state
}

trait Frob<S, I, O> {
    fn frob(self, state: &mut S, input: I) -> O;
}

impl<S, I, O, F, G> Frob<S, I, O> for F
where
    F: FnMut(&mut S) -> G,
    G: FnMut(I) -> O,
{
    fn frob(mut self, state: &mut S, input: I) -> O {
        self(state)(input)
    }
}

fn bar() {
    foo.frob(&mut 1, "hi");
}

I get the error

error[E0599]: the method `frob` exists for fn item `for<'b> fn(&'b mut i32) -> impl for<'b, 'r> FnMut<(&'r str,)> {foo}`,
but its trait bounds were not satisfied
...
   = note: the following trait bounds were not satisfied:
           `<for<'b> fn(&'b mut i32) -> impl for<'b, 'r> FnMut<(&'r str,)> {foo} as FnOnce<(&mut _,)>>::Output = _`
           which is required by `for<'b> fn(&'b mut i32) -> impl for<'b, 'r> FnMut<(&'r str,)> {foo}: Frob<_, _, _>`
           `<&for<'b> fn(&'b mut i32) -> impl for<'b, 'r> FnMut<(&'r str,)> {foo} as FnOnce<(&mut _,)>>::Output = _`
           which is required by `&for<'b> fn(&'b mut i32) -> impl for<'b, 'r> FnMut<(&'r str,)> {foo}: Frob<_, _, _>`
           `<&mut for<'b> fn(&'b mut i32) -> impl for<'b, 'r> FnMut<(&'r str,)> {foo} as FnOnce<(&mut _,)>>::Output = _`
           which is required by `&mut for<'b> fn(&'b mut i32) -> impl for<'b, 'r> FnMut<(&'r str,)> {foo}: Frob<_, _, _>`

First of all, how do I interpret this error message? Second, how do I write the trait bound correctly? Presumably the problem has something to do with the returned closure hanging on to state , but I can't find a place to specify a lifetime for G .

Answer 1

1. First of all, how do I interpret this error message?

   Yes, it is a tad cryptic isn't it? Two things to recognise:

   • <for<'b> fn(&'b mut i32) -> impl for<'b, 'r> FnMut<(&'r str,)> {foo} is the compiler's very wordy way of expressing the type of function foo ; and

   • the same note is repeated for that function type, a shared reference to that function type, and a mutable reference to that function type—this happens when the compiler attempts automatic referencing in method call syntax such as you have in foo.frob(...) .

   So we can quickly distill the error message down to:

    error[E0599]: the method `frob` exists for fn item `{foo}`, but its trait bounds were not satisfied ... = note: the following trait bounds were not satisfied: `<{foo} as FnOnce<(&mut _,)>>::Output = _` which is required by `{foo}: Frob<_, _, _>`

   The compiler is telling us that it found a potential frob method on {foo} but in order for it to be applicable, {foo} 's return type must match the constraints of the Frob trait (but it doesn't).

2. Second, how do I write the trait bound correctly? Presumably the problem has something to do with the returned closure hanging on to state , but I can't find a place to specify a lifetime for G .

   You need to add the lifetime constraint to the trait ( playground ):

    trait Frob<'b, S, I, O> { fn frob(self, state: &'b mut S, input: I) -> O; } impl<'b, S: 'b, I, O, F, G> Frob<'b, S, I, O> for F where F: FnMut(&'b mut S) -> G, G: 'b + FnMut(I) -> O, { fn frob(mut self, state: &'b mut S, input: I) -> O { self(state)(input) } }