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Can't use `impl Fn` to return a closure that returns another closure #62223

eliaslevy opened this issue Jun 29, 2019 · 1 comment


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commented Jun 29, 2019

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() {

results in

error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
 --> src/
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.


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commented Jun 29, 2019

This is currently by design but it is a restriction we could sensibly lift imo.

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