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#40 in Template engine

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Apache-2.0 OR MIT

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Dead-simple string templating.

Check here for latest documentation.


Dead-simple string templating.

Leon parses a template string into a list of tokens, and then substitutes provided values in. Unlike other templating engines, it is extremely simple: it supports no logic, only replaces. It is even simpler than format!(), albeit with a similar syntax.


it is better to rule { group }
one can live {adverb} without power

A replacement is denoted by { and }. The contents of the braces, trimmed of any whitespace, are the key. Any text outside of braces is left as-is.

To escape a brace, use \{ or \}. To escape a backslash, use \\. Keys cannot contain escapes.

\{ leon \}

The above examples, given the values group = "no one" and adverb = "honourably", would render to:

it is better to rule no one
one can live honourably without power
{ leon }


A template is first parsed to a token list:

use leon::Template;

let template = Template::parse("hello {name}").unwrap();

The template can be inspected, for example to check if a key is present:


The template can be rendered to a string:

use leon::vals;
        &&vals(|_key| Some("marcus".into()))
    "hello marcus",

…or to a writer:

use std::io::Write;
use leon::vals;
let mut buf: Vec<u8> = Vec::new();
    &mut buf,
    &&vals(|key| if key == "name" {
    } else {
assert_eq!(buf.as_slice(), b"hello julius");

…with a map:

use std::collections::HashMap;
let mut values = HashMap::new();
values.insert("name", "brutus");
assert_eq!(template.render(&values).unwrap().as_str(), "hello brutus");

…or with your own type, if you implement the Values trait:

use std::borrow::Cow;
use leon::Values;

struct MyMap {
  name: &'static str,
impl Values for MyMap {
   fn get_value(&self, key: &str) -> Option<Cow<'_, str>> {
      if key == "name" {
     } else {
let values = MyMap { name: "pontifex" };
assert_eq!(template.render(&values).unwrap().as_str(), "hello pontifex");

Compile-time parsing

You can either use leon-macros's template!, a proc-macro, with the exact same syntax as the normal parser, or this crate's template! rules-macro, which requires a slightly different syntax but doesn't bring in additional dependencies. In either case, the leon library is required as a runtime dependency.


Leon will return a ParseError if the template fails to parse. This can happen if there are unbalanced braces, or if a key is empty.

Leon will return a RenderError::MissingKey if a key is missing from keyed values passed to [Template::render()], unless a default value is provided with Template.default.

It will also pass through I/O errors when using [Template::render_into()].


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