hatter 0.1.4

positively mad, zero dependency html templating language


It is practically impossible to teach good programming to students that have had a prior exposure to Hatter: as potential programmers they are mentally mutilated beyond hope of regeneration.

-– Edsger W. Dijkstra (allegedly)

Hatter is a small, whitespace sensitive templating language with HTML support built right in. Its HTML features and syntax are a cheap knock off of Imba, except Hatter produces raw, static HTML - no JavaScript in sight.

Hatter can be used to generate static web sites or to render server side content in a good ol' fashioned web application - either with Vial or your Rust web framework of choice.

If you're feeling adventerous, or mad as a hatter, you can use the standalone binary to turn templates into HTML files, or include the zero-dependency Rust library in your (web/cli/?) application.

Hello Hatter

Here are a few basic examples of what Hatter looks like and the HTML it generates:

<!-- Hatter -->
<#main> Hi there!

<!-- Generated HTML -->
<div id='main'>Hi there!</div>
<span.big.bold> Welcome!

<span class='big bold'>Welcome!</span>
<.links> for link in nav-links
  <a href={link.href}> link.text

<div class='links'>
  <a href='/link1'>First Link</a>
  <a href='/link2'>2nd Link</a>
  <a href='/link3'>Final Link</a>
<form GET="/search">
  <input@query:text placeholder="Search..." /> <input:submit />

<form method='GET' action='/search'>
  <input name='query' type='text' placeholder='Search...' />
  <input type='submit' />


  • Auto-closing HTML tags and code blocks based on indentation:
    • <h1> Welcome, <i> Rob becomes <h1> Welcome, <i> Rob </i></h1>
  • Shorthand for id, class, type, and name attributes:
    • <div#id>
    • <div.class1.class2>
    • <input@form-field-name>
    • <input:text>
  • Dynamic values for regular attributes:
    • <div page-num={page.id}>
  • Conditionally set attributes or enable shorthand:
    • <div .logged-in=logged-in?>
    • <div data-map=is-map?>
  • String interpolation:
    • <span.greeting> "Hey there {name}. 2 + 2 is {2 + 2}"
  • Shorthand interpolation:
    • <span #page-{page.id} .is-{page.type}> page.title
  • Implicit divs:
    • <#main> becomes <div id='main'>
  • Implicit closing tags:
    • <i>delicious</> becomes <i>delicious</i>
  • Easy inline JavaScript:
    • <li> <a onclick=(alert("Oink!"))> "🐷"
  • Basic types:
    • bool, int, float, string, list, map, fn
  • Loop over list and map:
    • <ul> for page in pages do <li id=page-{page.id}> page.name
    • for k, v in some-map do <td> k </> <td> v
  • if/else statements
    • if logged_in? then <h2> Welcome back!
  • Error-checked assignment with := and =:
    • name := 'Bob' will error if name is already set.
    • name = 'Bob' will error if name isn't already set.
  • Call functions defined in Rust:
    • <div.name> to-uppercase(name)
  • Define your own Hatter functions with strict arity and implicit return values:
    • def greet(name) do print("Hey there, {name}!")
    • greet("Lydia") prints Hey there, Lydia!
  • Define your own Hatter operators:
    • def ++(a, b) do concat(to-uppercase(a), ' ', to-uppercase(b))
    • "one" ++ "two" returns ONE TWO
  • Closures and function literals:
    • adder := fn(x) fn(y) x + y then add1 := adder(1)
    • add1(200) returns 201
  • Call functions with keyword arguments:
    • def greet(title, name) do print("Hiya, {title}. {name}!")
    • greet(name: "Marley", title: "Dr") prints Hiya, Dr. Marley!
  • do keyword for one-line blocks:
    • if 2 > 1 do print("Obviously")
    • for x in list do print(x)
  • then keyword for one-line if statements:
    • if 2 > 1 then print("Yup!") else if 2 < 1 then print("Impossible.")
  • Hatter will add a <!DOCTYPE> and wrap everything in <html> if the first tag in your template is <head>.

Getting Started

There are two ways to use Hatter:

1. In Your Rust Application

Hatter can (primarily) be used as a templating language from within your Rust applications.

Simply add Hatter to Cargo.toml:

hatter = "0.1"

Then create a hatter::Env, which represents the top-level Hatter scope for your template, to set variables and render your template:

use hatter::{Args, Env, Value};

let mut env = Env::new();
env.set("name", "Bobby Boucher");
env.set("age", 31);
<p> <b>Name:</> name
<p> <b>Age:</> age

You can also write functions in Rust and make them available to your HTML templates:

use hatter::prelude::*;

fn quote(args: Args) -> Result<Value> {
  let file = std::fs::read_to_string("quotes.txt")?;
  let list_of_quotes: Vec<_> = file.split('\n').collect();
  let line = match args.need_number(0)? as usize {
    n if n > list_of_quotes.len() => 0,
    n => n,


fn main() {
    let mut env = Env::new();
    env.set("quote", quote);
    println!("{}", env.render("<div> quote(1)").unwrap());

For more infomation see the API Documentation.

2. As A Standalone Executable

Hatter can be used as a regular command line program to turn .hat files into HTML.

Just install it using cargo:

cargo install hatter

Then point it at any .hat file:

$ cat test.hat
<b.test> "Testing 1 2 3 {2 + 2}"

$ hatter test.hat
<b class='test'>Testing 1 2 3 4 </b>

You can also install Hatter with a readline-powered REPL:

cargo install hatter --features repl

To launch it, start hatter with no arguments:

$ hatter
Hatter v0.0.1 REPL
>> 1 + 2


  • HTMLized error page
  • show error location in source text on runtime errors

Future Features

  • Define your own tags:
    • def <item(item)> do <li.item data-id={item.id}> item.text.
  • Optional type checking for functions
  • stdlib
  • VSCode Extension
  • luacheck-style tool
  • LSPc


Hatter is licensed under either of the following, at your pleasure:

Imba is licensed under the MIT License.