Rust Compiled Templates — ructe

This is my attempt at writing a HTML template system for Rust. Some inspiration comes from the scala template system used in play 2, as well as plain old jsp.

Design criteria

As many errors as possible should be caught in compile-time.
A compiled binary should include all the template code it needs, no need to read template files at runtime.
Compilation may take time, running should be fast.
Writing templates should be almost as easy as writing html.
The template language should be as expressive as possible.
It should be possible to write templates for any text-like format, not only html.
Any value that implements the Display trait should be outputable.
By default, all values should be html-escaped. There should be an easy but explicit way to output preformatted html.

Current status

This is currently more of a proof of concept that anyting ready for actual production use. That said, it actually does work; templates can be transpiled to rust functions, which are then compiled and can be called from rust code. The template syntax is not stable yet, but some examples in the current format can be seen below, and in examples/simple/templates.

Template format

A template consists of three basic parts: First a preamle of use statements, each prepended by an @ sign. Secondly a declaration of the parameters the template takes. And third, the template body.

@use any::rust::Type;

@(name: &str, items: Vec<Type>)

<html>
   ...
</html>

The curly brackets, { and }, is used for blocks (see Loops, Conditionals, and Calling other templates below). To use them in the template body, they must be escaped as @{ and @}.

Value expressions

A parameter can be used in an expression preceded by an @ sign.

<h1>@name</h1>

If a parameter is a struct or a trait object, its fields or methods can be used, and if it is a callable, it can be called.

<p>The user @user.name has email @user.email.</p>

Standard function and macros can also be used, e.g. for specific formatting needs:

<p>The value is @format!("{:.1}", float_value).</p>

Loops

Rust-like loops are supported like this:

<ul>@for item in items {
  <li>@item</li>
}</ul>

Note that the thing to loop over (items, in the example) is a rust expression, while the contents of the block is template code.

Conditionals

Rust-like conditionals are supported in a style similar to the loops:

@if items.is_empty() {
  <p>There are no items.</p>
}

Pattern matching let expressions are also supported, as well as an optional else part.

@if let Some(foo) = foo {
  <p>Foo is @foo.</p>
} else {
  <p>There is no foo.</p>
}

Calling other templates

While rust methods can be called as a simple expression, there is a special syntax for calling other templates: @:template_name(template_arguments). Also, before calling a template, it has to be imported by a use statement. Templates are declared in a templates module.

So, given something like this in header.rs.html:

@(title: &str)

<head>
  <title>@title</title>
  <link rel="stylesheet" href="/my/style.css" type="text/css">
</head>

It can be used like this:

@use templates::header;

@()

<html>
  @:header("Example")
  <body>
    <h1>Example</h1>
    <p>page content ...</p>
  </body>
</html>

It is also possible to send template blocks as parameters to templates. A structure similar to the above can breated by having something like this in base_page.rs.html:

@(title: &str, body: Content)

<html>
  <head>
    <title>@title</title>
    <link rel="stylesheet" href="/my/style.css" type="text/css">
  </head>
  <body>
    <h1>@title</h1>
    @:body()
  </body>
</html>

And use it like this:

@use templates::base_page;

@()

@:base_page("Example", {
    <p>page content ...</p>
})

How to use ructe

Ructe compiles your templates to rust code that should be compiled with your other rust code, so it needs to be called before compiling. Assuming you use cargo, it can be done like this: First, specify a build script and ructe as a build dependency in Cargo.toml:

build = "src/build.rs"

[build-dependencies]
ructe = "^0.2"

Then, in the build script, compile all templates found in the templates directory and put the output where cargo tells it to:

extern crate ructe;

use ructe::compile_templates;
use std::env;
use std::path::PathBuf;

fn main() {
    let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
    let in_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap())
        .join("templates");
    compile_templates(&in_dir, &out_dir).expect("foo");
}

And finally, include and use the generated code in your code. The file templates.rs will contain mod templates { ... }, so I just include it in my main.rs:

include!(concat!(env!("OUT_DIR"), "/templates.rs"));

When calling a template, the arguments declared in the template will be prepended by an argument that is the std::io::Write to write the output to. It can be a Vec<u8> as a buffer or for testing, or an actual output destination. The return value of a template is std::io::Result<()>, which should be Ok(()) unless writing to the destination fails.

#[test]
fn test_hello() {
    let mut buf = Vec::new();
    templates::hello(&mut buf, "World").unwrap();
    assert_eq!(from_utf8(&buf).unwrap(), "<h1>Hello World!</h1>\n");
}

When I use ructe with nickel, I use a rendering function that looks like this:

fn render<'mw, F>(res: Response<'mw>, do_render: F)
                  ->MiddlewareResult<'mw>
    where F: FnOnce(&mut Write) -> io::Result<()>
{
    let mut stream = try!(res.start());
    match do_render(&mut stream) {
        Ok(()) => Ok(Halt(stream)),
        Err(e) => stream.bail(format!("Problem rendering template: {:?}", e))
    }
}

Which I call like this:

render(res, |o| templates::foo(o, other, arguments))

ructe 0.2.2