Traits as a return value from a function
rust, traits
Solution
The answer is trait objects. This means that you will work with `Box<Colour>` as your type; bare `Colour` is not an instantiable type. You can cast `Box<T>` objects to `Box<Colour>` with the `as` operator: `Box::new(NormalColour::White) as Box<Colour>`. In many places this is not necessary (just write `Box::new(NormalColour::White)` and it can be automatically coerced to `Box<Colour>`), but sometimes it will still be necessary.
Still, if you can do it as an enum, that will probably be a nicer solution.
Problem
I have two enums, `NormalColour` and `BoldColour`, both of which implement the `Colour` trait. They contain `Blue`, `BoldGreen`, and so on. I'd like to return values of both of these types from the same function, treating them as though they're just a `Colour` value, calling the `paint` function on the result, but I can't find a way to coerce the Rust complier into doing this for me. I'd like to be able to write something like this: ``` pub trait Colour { fn paint(&self, input: &str) -> String; } fn file_colour(stat: &io::FileStat) -> Colour { if stat.kind == io::TypeDirectory { Blue } else if stat.perm & io::UserExecute == io::UserExecute { BoldGreen } else { White } } ``` What type do I have to make the function return for it to work? I'll eventually like to make more types implement `Colour`, which is why I'm not interested in just turning the two enums into one big enum.