Runtime values

It is possible to define variables at runtime and to use them in the templates using the values filter or the askama::get_value function and to call the _with_values variants of the render methods. It expects an extra argument implementing the Values trait. This trait is implemented on a few types provided by the std, like HashMap:

use std::collections::HashMap;

let mut values: HashMap<&str, Box<dyn Any>> = HashMap::new();
// We add a new value named "name" with the value "Bibop".
values.insert("name", Box::new("Bibop"));
values.insert("age", Box::new(12u32));

The Values trait is expecting types storing data with the Any trait, allowing to store any type.

Then to render with these values:

template_struct.render_with_values(&values).unwrap();

There are two ways to get the values from the template, either by using the value filter or by calling directly the askama::get_value function:

{% if let Ok(name) = "name"|value::<&str> %}
  name is {{ name }}
{% endif %}
{% if let Ok(age) = askama::get_value::<u32>("age") %}
  age is {{ age }}
{% endif %}

If you try to retrieve a value with the wrong type or that you didn’t set, you will get an Err(askama::Error::ValueType) or a Err(askama::Error::ValueMissing).

Another example with a key-value tuple:

let value = "value".to_string();
let tuple: (&str, &dyn Any) = ("a", &value);

template_struct.render_with_values(&tuple).unwrap();

With this setup, only the "a" key will return a value:

{% if let Ok(name) = "a"|value::<String> %}
  a is {{ a }}
{% endif %}