How to make a trait into a generic on Vec<T>?

January 15, 2022

I have a working trait:

trait PopOrErr {
    fn pop_or_err(&mut self) -> Result<i8, String>;
}

impl PopOrErr for Vec<i8> {
    fn pop_or_err(&mut self) -> Result<i8, String> {
        self.pop().ok_or_else(|| "stack empty".to_owned())
    }
}

I’ve tried:

trait PopOrErr<T> {
    fn pop_or_err(&mut self) -> Result<T, String>;
}

impl PopOrErr<T> for Vec<T> {
    fn pop_or_err(&mut self) -> Result<T, String> {
        self.pop().ok_or_else(|| "stack empty".to_owned())
    }
}

but it gives me:

error[E0412]: cannot find type `T` in this scope
  --> src/main.rs:29:15
   |
29 | impl PopOrErr<T> for Vec<T> {
   |     -         ^ not found in this scope
   |     |
   |     help: you might be missing a type parameter: `<T>`

error[E0412]: cannot find type `T` in this scope
  --> src/main.rs:29:26
   |
29 | impl PopOrErr<T> for Vec<T> {
   |     -                    ^ not found in this scope
   |     |
   |     help: you might be missing a type parameter: `<T>`

error[E0412]: cannot find type `T` in this scope
  --> src/main.rs:30:40
   |
29 | impl PopOrErr<T> for Vec<T> {
   |     - help: you might be missing a type parameter: `<T>`
30 |     fn pop_or_err(&mut self) -> Result<T, String> {
   |                                        ^ not found in this scope

How can I make PopOrErr generic on the type contained within the vector?

>Solution :

"You might be missing a type parameter: <T>" the compiler is trying to help. Put <T> where it says and you are good to go.

trait PopOrErr<T> {
    fn pop_or_err(&mut self) -> Result<T, String>;
}

// Add it here: impl<T>
impl<T> PopOrErr<T> for Vec<T> {
    fn pop_or_err(&mut self) -> Result<T, String> {
        self.pop().ok_or("stack empty".to_string())
    }
}

You have an unrelated error with ok_or_else which I replaced with ok_or above.

If you’re curious as to why you need to put the generic with impl as well as fn check out this question