Is it possible to write Haskell's flip function in Rust?

Question

Haskell's flip function is defined as follows:

flip :: (a -> b -> c) -> b -> a -> c
flip f x y =  f y x

All it does is take a function and return another function that has its two parameters reversed. If you called a function f as f a b (f(a, b) in Rust-like syntax), you would call flip f as (flip f) b a.

My unsuccessful attempt at writing this in Rust:

fn flip<A, B, C, F: Fn(A, B) -> C>(f: F) -> impl Fn(B, A) -> C {
    |a, b| f(b, a)
}

Is it possible to write this in Rust?

Answer 1

I'm not sure what error you're getting, but I was able to implement it properly with this code (playground link for testing):

fn flip<A, B, C, F>(f: F) -> impl Fn(B, A) -> C where F: Fn(A, B) ->C {
    move |a,b| f(b, a)
}

fn main() {
    let sub = |a, b| a - b;
    let flipped = flip(sub);
    println!("Result: {}", flipped(5, 10));
}

Note the move keyword here, which is required to force the closure returned by flip to take ownership of f.