hui.liu 2023-11-09

When I was in the midst of coding my little rust program, I encountered a perplexing compiler error: expected a closure that implements the Fn trait, but this closure only implements FnMut. I can understand the root cause of the error, but I didn't know how to overcome it.

In my quest for a solution, I turned to an AI, Google Bard, which I found both intriguing and impressive. The following conversation captures the essence of my experience:

(Please note: For brevity, I'll use ... to omit overly lengthy or unhelpful responses.)

Me:

You are a rust expert, please answer my rust-related questions

Bard:

I understand. I am a Rust expert, and I will try my best to answer your Rust-related questions in a comprehensive and informative way. (...disclaimer)

Me:

Here is the code:

  use std::time::SystemTime;
  use rdev::{grab, Button, Event, EventType};

  fn main() {
     let mut prev_click_time: SystemTime = SystemTime::now();
     let mut curr_click_time: SystemTime = SystemTime::now();

     let callback = |event: Event| -> Option<Event> {
         if let EventType::ButtonPress(Button::Left) = event.event_type {
             // println!("Left click {:?}", event);
             prev_click_time = curr_click_time;
             curr_click_time = event.time;
             if let Ok(duration) = curr_click_time.duration_since(prev_click_time) {
                 println!("Duration {:?}", duration);
                 if duration.as_millis() < 500 {
                     println!("Double click");
                 }
             }
             // None // CapsLock is now effectively disabled
             Some(event)
         } else {
             Some(event)
         }
     };

     if let Err(error) = grab(callback) {
         println!("Error: {:?}", error)
     }
  }

Bard:

...(provides a line-by-line explanation of the code, which isn't helpful for the specific issue.)

Me:

What's the problem with above code?

Bard:

...(offers improvement suggestions but overlooks the compiler error.)

Me:

I got an error: expected a closure that implements the Fn trait, but this closure only implements FnMut. How can I fix this?

Bard:

...(advises against mutating variables in the callback function, missing the root issue.)

Me:

But I do need to capture click time between clicks, how to do that?

Bard:

To capture the click time between clicks, you can use a mutable variable to store the time of the previous click. Then, when a new click event is received, you can calculate the duration between the current click and the previous click.

Here is an example of how to capture the click time between clicks:

...(still not addressing the core issue.)

Me:

The problem is the callback passed to grab only implemented Fn trait, so I can't mutate variable inside the callback function.

Bard:

You are correct. The callback passed to grab must implement the Fn trait, which prohibits mutating variables in the environment.

However, there are a few ways to work around this problem. One way is to use a thread-safe data structure to store the click time between clicks. For example, you could use a Mutex or a RwLock to protect the variable.

Another way to work around the problem is to use a message queue to communicate between the callback function and the main thread. The callback function could send a message to the main thread with the click time, and the main thread could then update the variable.

Here is an example of how to use a message queue to work around the problem:

...(thread is not what I want, but I got Mutex.)

Me:

Use Mutex or a RwLock please.

Bard:

Sure. Here is an example of how to use a Mutex to store the click time between clicks:

...(finally help me figure it out.)

AI is particularly useful in this kind of scenario when we don't know how to ask the right question. That's when we need a rubber duck!