display-vector-of-strings.md

title	timestamp	author	published	description	tags
Add the Display trait to a vector of strings in Rust	2023-12-21 01:50:01 -0800	szabgab	true	How to implement a trait for a built-in type or for a type-alias for a built-in type?	Rust type struct Display error code

The problem

Let's say we have a vector of strings, how can we print them easily?

let animals: Vec<String> = vec![String::from("snake"), String::from("camel"), String::from("crab")];

Trying to use the simplest way won't work:

println!("{}", animals);

We get the following error:

error[E0277]: `Vec<String>` doesn't implement `std::fmt::Display`
  --> src/main.rs:24:20
   |
24 |     println!("{}", animals);
   |                    ^^^^^^^ `Vec<String>` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `Vec<String>`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

further referring us to E0277

We can do what was suggested:

println!("{:?}", animals);

will yield

["snake", "camel", "crab"]

and

println!("{:#?}", animals);

will yield:

[
    "snake",
    "camel",
    "crab",
]

Those are actually not bad, but I wanted to show them in a different way.

Primarily as an experiment to see how to implement Display in general for a vector.

Implement Display for Vec

The implementation should look something like this:

impl fmt::Display for Vec<String> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", "Something comes here")
    }
}

However, this will give us a compiler error:

error[E0117]: only traits defined in the current crate can be implemented for types defined outside of the crate
  --> src/main.rs:23:1
   |
23 | impl fmt::Display for Vec<String> {
   | ^^^^^^^^^^^^^^^^^^^^^^-----------
   | |                     |
   | |                     `Vec` is not defined in the current crate
   | impl doesn't use only types from inside the current crate
   |
   = note: define and implement a trait or new type instead

Referencing us to E0117.

How can we implement the Display trait for a vector in Rust?

We cannot as we can only implement traits to our own data types.

What about creating an alias for this type? Would crating a type-alias work?

How to implement traits for type aliases in Rust?

type Words = Vec<String>;

impl fmt::Display for Words {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", "Something comes here")
    }
}

let animals: Words = vec![String::from("snake"), String::from("camel"), String::from("crab")];
println!("{:?}", animals);

We can define the type-alias and we can even use it in the declaration of variables, but we cannot implement the Display trait on a type-alias to a standard type. Type-aliases are just that. Aliases. They are not separate types.

Using a one-element tuple-based struct

A working solution is to create a one-element tuple-based struct. Because it is a totally new type we can implement the Display trait for that.

struct Words(Vec<String>);

impl fmt::Display for Words {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.0.join(","))
    }
}

This is then how to use it:

let animals: Words = Words(vec![String::from("snake"), String::from("camel"), String::from("crab")]);
println!("{}", animals);

It will print:

snake,camel,crab

The drawback is that this means we need to reference to the 0th element in the tuple so we must write animals.0 in order to access the real vector:

println!("{}", animals.0[0]);
println!("{}", animals.0[1]);

to print:

snake
camel

I don't think I like this.

The full example

{% include file="examples/display-vector-of-strings/src/main.rs" %}

Conclusion

We could not implement Display for a vector of strings in a nice way. If we frequently need to print out a whole vector of strings other than for debugging, then we might want to write a separate function or a macro for that.