Template literals

If you've been paying attention on previous sections you might have seen odd looking strings like `Hello ${name}`. These are called template literals, and allow you to conveniently build strings using variables from the environment.

Template literals are a concept borrowed from EcmaScript.

let age = 30;
dbg!(`I am ${age} years old!`);

$> cargo run -- run scripts/book/template_literals/basic_template.rn
"I am 30 years old!"

Template strings are accelerated by the Vm, each argument uses a display protocol and it can be very efficient to build complex strings out of it.

The DISPLAY_FMT protocol

The DISPLAY_FMT protocol is a function that can be implemented by any external type which allows it to be used in a template string.

It expects a function with the signature fn(&self, buf: &mut String) -> fmt::Result.

use rune::{ContextError, Module};
use rune::runtime::{Protocol, Formatter};
use std::fmt::Write as _;
use std::fmt;

#[derive(Debug)]
pub struct StatusCode {
    inner: u32,
}

impl StatusCode {
    #[rune::function(protocol = DISPLAY_FMT)]
    fn display_fmt(&self, f: &mut Formatter) -> fmt::Result {
        write!(f, "{}", self.inner)
    }
}

pub fn module() -> Result<Module, ContextError> {
    let mut module = Module::new(["http"]);
    module.function_meta(StatusCode::display_fmt)?;
    Ok(module)
}

This is what allows status codes to be formatted into template strings, any types which do not implement this protocol will fail to run.

let vec = [1, 2, 3];
dbg!(`${vec}`);

$> cargo run -- run scripts/book/template_literals/not_a_template.rn
== ! (`Vec` does not implement the `display_fmt` protocol (at 5)) (77.7µs)
error: virtual machine error
  ┌─ scripts/book/template_literals/not_a_template.rn:3:9
  │
3 │     dbg!(`${vec}`);
  │          ^^^^^^^^ `Vec` does not implement the `display_fmt` protocol