Module rocket_dyn_templates::minijinja::functions

Global functions and abstractions.

This module provides the abstractions for functions that can registered as global functions to the environment via add_function.

Using Functions

Functions can be called in any place where an expression is valid. They are useful to retrieve data. Some functions are special and provided by the engine (like super) within certain context, others are global.

The following is a motivating example:

<pre>{{ debug() }}</pre>

Custom Functions

A custom global function is just a simple rust function which accepts optional arguments and then returns a result. Global functions are typically used to perform a data loading operation. For instance these functions can be used to expose data to the template that hasn’t been provided by the individual render invocation.

use minijinja::{Error, ErrorKind};

fn include_file(name: String) -> Result<String, Error> {
    std::fs::read_to_string(&name)
        .map_err(|e| Error::new(
            ErrorKind::InvalidOperation,
            "cannot load file"
        ).with_source(e))
}

env.add_function("include_file", include_file);

Arguments in Custom Functions

All arguments in custom functions must implement the ArgType trait. Standard types, such as String, i32, bool, f64, etc, already implement this trait. There are also helper types that will make it easier to extract an arguments with custom types. The ViaDeserialize<T> type, for instance, can accept any type T that implements the Deserialize trait from serde.

use minijinja::value::ViaDeserialize;

#[derive(Deserialize)]
struct Person {
    name: String,
    age: i32,
}

fn is_adult(person: ViaDeserialize<Person>) -> bool {
    person.age >= 18
}

env.add_function("is_adult", is_adult);

Note on Keyword Arguments

MiniJinja inherits a lot of the runtime model from Jinja2. That includes support for keyword arguments. These however are a concept not native to Rust which makes them somewhat uncomfortable to work with. In MiniJinja keyword arguments are implemented by converting them into an extra parameter represented by a map. That means if you call a function as foo(1, 2, three=3, four=4) the function gets three arguments:

[1, 2, {"three": 3, "four": 4}]

If a function wants to disambiguate between a value passed as keyword argument or not, the Value::is_kwargs can be used which returns true if a value represents keyword arguments as opposed to just a map. A more convenient way to work with keyword arguments is the Kwargs type.

Built-in Functions

When the builtins feature is enabled a range of built-in functions are automatically added to the environment. These are also all provided in this module. Note though that these functions are not to be called from Rust code as their exact interface (arguments and return types) might change from one MiniJinja version to another.

Traits

• Function
  A utility trait that represents global functions.

Functions

• debug
  Outputs the current context or the arguments stringified.
• dict
  Creates a dictionary.
• namespace
  Creates a new container that allows attribute assignment using the {% set %} tag.
• range
  Returns a range.