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Can't use impl Fn to return a closure that returns another closure #62223
Description
While trying to write some currying code using stable (1.35.0), writing a fn that returns impl Fn(T1) -> impl Fn(T2) -> T3 results in a compile error. E.g.
fn foo(a: i64, b: bool, c: f64) {
println!("{:?} {:?} {:?}", a, b, c)
}
fn foo2(a: i64, b: bool) -> impl Fn(f64) -> () { move |c: f64| foo(a,b,c) }
fn foo1(a: i64) -> impl Fn(bool) -> impl Fn(f64) -> () { move |b: bool| foo2(a,b) }
fn main() {
foo1(1)(true)(2.5);
}results in
error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
--> src/main.rs:7:37
|
7 | fn foo1(a: i64) -> impl Fn(bool) -> impl Fn(f64) -> () { move |b: bool| foo2(a,b) }
| ^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
The expectation was that this would be allowed as the impl Fn(f64) -> () is part of the function return type of foo1.
Activity
Centril commentedon Jun 29, 2019
This is currently by design but it is a restriction we could sensibly lift imo.
dcormier commentedon Jul 22, 2021
I'm being bit by this as well. It would be nice to see this changed.
douglas-raillard-arm commentedon Aug 9, 2022
I'm affected by this as well. The function is a combinator that returns a closure returning a Future, so I need the closure to return an
impl Future. I worked around with a Box but it's a bit annoying to have to pay for a Box price when it's actually not really required.EDIT: It looks like it's possible to avoid the Box in nightly, but the pattern can become cumbersome very quickly for complicated types (e.g.
impl Foo<T>with constraints on T. The constraints will need to be copy pasted, possibly multiple times over if you have "impl nesting", e.g. one layer for an Fn() and another for a impl Future returned by the Fn())https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d092d001154d56789a9ba77155401885