#compile #evaluation #evaluate #variables #function #output


A template engine to compile text and evaluate code from template files

6 releases

0.2.1 Jun 16, 2021
0.2.0 Jun 14, 2021
0.1.3 Jun 10, 2021
0.1.2 May 6, 2021
0.1.0 Apr 23, 2021

#216 in Template engine

MIT license

990 lines


A template engine in Rust. Allows compiling text templates into output text using variable substitution or code evaluation.

What does it do?

It allows to use a template with some placeholders that will be replaced. Consider the following example:

Hello there, {{ name }}!

Depending on what value will be supplied for name, the output might be different. For example, if one used name with value of Joe, it will generate the following output:

Hello there, Joe!

This library evaluates such templates and compiles them into output text like in the example above.


To simply compile a template, you can use compile_template_from_file(file: PathBuf, variables: HashMap<String, String>, functions: HashMap<String, Box<dyn Function>>).

let file = PathBuf::from("template-file.txt");
let functions: HashMap<String, Box<dyn Function>> = std_functions();

let mut variables: HashMap<String, String> = HashMap::new();
variables.insert("name".to_string(), "Joe".to_string());

let result = compile_template_from_file(file, variables, functions);

assert_eq!(result.ok(), Some("Hello there, Joe!".to_string()));


By default, rubble-templates while parsing a template looks for all blocks starting {{ and ending with }} those are marked as an evaluation spots with code that can be evaluated by an Evaluator.

Given the following example:

Hello there, {{ name }}!

This template contains three parts:

  • Hello there, - raw text
  • {{ name }} - code
  • ! - raw text

The second part will be passed to a given Evaluator by the Compiler in order to get a String output. The code fragment will be substituted with the output.

The rubble-templates library can also evaluate more sophisticated code. You can pass your own functions that can enrich the template.

This library uses Lisp-like syntax during evaluation. All function calls look like the following:

function_name arg0 arg1 arg2

To call a plus function to calculate the result of 2 + 2 you will need to write

The result is: {{ plus 2 2 }}

The parameters can also be grouped using parenthesis. This can be helpful in certain cases. For example, given plus and multiply functions, you will need to use the following code to calculate (1 + 2) * 3:

The result is: {{ multiply (plus 1 2) 3 }}

The above functions are the example functions and are not actual functions that are available out-of-the-box. They might be added in the future releases however.

Note: The code blocks are matched by finding {{ and terminated by the first occurrence of }}. To use }} or " in strings please refer to standard functions (ie. std_function).

Standard functions

A set of standard, built-in functions is still being expanded. To see current list of standard functions, refer to the rubble-templates documentation.

The std_fun module contains basic math and string functions. Each function has usage examples in the documentation.


Compilation contains three phases:

  • parsing - done by the template iterator, eg. EvaluableMixedContentIterator<'a, T>, which can extract template parts,
  • evaluation - done by the Evaluator, which evaluates all code found by the iterator,
  • compiling - done by the Compiler, which uses iterator to parse the content, feeds the Evaluator and then joins everything into output text.

You can implement your own iterators, evaluators or compilers. To modify the compilation process, you just need to use your own trait implementations instead of the default ones.

To show you how it works, here is what compile_template_from_string does in practice:

// parameters
let raw_input = "Hello there, {{ name }}!".to_string();
let variables: HashMap<String, String> = HashMap::new();
let functions: HashMap<String, Box<dyn Function>> = HashMap::new();

// prepare compilation evironment
let template = Template::from(raw_input);
let engine = SimpleEvaluationEngine::from(functions);
let compiler = TemplateCompiler::new(engine);

// compile template
compiler.compile(&template, Context::with_variables(variables))

Custom functions

Evaluator can be extended with functions by using custom Function trait implementations. To make this process easier, the following structs are available in rubble-templates-core, that can be used with static functions and lambdas:

  • SimpleFunction - for Fn(&[String]) -> String. Use this when you want to implement a simple function without any side effects.
  • FunctionWithContext - for Fn(&[String], &mut Context) -> Result<String, SyntaxError>. Use this when you want to use pre-evaluated parameters, but you still need variables. Side effects can cause errors indicated by SyntaxError.
  • FunctionWithAst - for Fn(&dyn Evaluator, &[SyntaxNode], &mut Context) -> Result<String, SyntaxError>. Gives full access to SyntaxNodes of parameters and Evaluator. Allows evaluating additional expressions, manipulating the AST or introducing DSL (domain-specific language).

Mind you, Context is a struct that holds variables and states that can be shared between function invocations. You can use it to store some properties.

For more info about functions, refer to the rubble-templates-core documentation.


If you see any error or feel like suggesting a feature, create an issue.

Please attach the following when creating an issue:

  • a brief description of what happened/what should be improved,
  • an actual output (if possible),
  • an expected output,
  • a minimal reproducible example (a code fragment, a test, a repository, anything that proves that the problem exists).