fn foo(f: fn(u32) -> u32) -> fn(u32, u32) -> u32 {
unimplemented!()
}
A function that takes a function pointer as an argument, and returns a function pointer that calls the original function, ignoring the extra parameter.
Something similar in concept to the following, ignoring that it doesn't compile:
fn foo(f: fn(u32) -> u32) -> fn(u32, u32) -> u32 {
|x, _| f(x)
}
Unfortunately, this is not possible. Functions do not have states, so each time you call foo, a brand new function has to be generated that calls f internally, which is apparently not possible.
If you are willing to relax the return type, you can return an impl Fn(u32, u32) -> u32:
fn foo(f: fn(u32) -> u32) -> impl Fn(u32, u32) -> u32 {
move |x, _| f(x)
}
fn main() {
let new_f = foo(|x| x);
assert_eq!(new_f(1, 2), 1);
}