Smalto

A general-purpose syntax highlighting library for Gleam, with regex-based grammars for 36+ languages.

Smalto tokenizes source code using regex-based grammar definitions inspired by Prism.js, with output to structured tokens, ANSI terminal colors, or HTML with CSS classes.

Features

• Works on both Erlang and JavaScript targets
• Regex-based tokenizer engine with greedy matching and nested grammar support
• 36 built-in languages including Gleam, Rust, Python, JavaScript, C#, F#, HTML, CSS, and more
• Three output formats: tokens, ANSI terminal colors, HTML with CSS classes
• Language inheritance (e.g., TypeScript extends JavaScript)
• Nested tokenization for embedded languages (e.g., CSS inside HTML)
• Hybrid token types: well-known semantic variants plus a Custom escape hatch
• Lustre integration via the smalto_lustre package, with 45 pre-built themes

Installation

gleam add smalto@2

Quick start

import smalto
import smalto/languages/python

pub fn main() {
  let code = "
def greet(name: str) -> str:
    \"\"\"Return a greeting.\"\"\"
    return f'Hello, {name}!'
"

  // Highlight to HTML
  let html = smalto.to_html(code, python.grammar())

  // Highlight to ANSI terminal colors
  let ansi = smalto.to_ansi(code, python.grammar())

  // Get raw tokens for custom processing
  let tokens = smalto.to_tokens(code, python.grammar())
}

HTML output

The HTML renderer wraps each token in a <span> with a CSS class. Place the output within <pre><code>...</code></pre> and style the classes:

pre code .smalto-keyword  { color: #ffd596; }
pre code .smalto-string   { color: #c8ffa7; }
pre code .smalto-number   { color: #c8ffa7; }
pre code .smalto-comment  { color: #d4d4d4; font-style: italic; }
pre code .smalto-function { color: #9ce7ff; }
pre code .smalto-operator { color: #ffaff3; }
pre code .smalto-type     { color: #ffddfa; }
pre code .smalto-module   { color: #ffddfa; }
pre code .smalto-tag      { color: #ff6b6b; }
pre code .smalto-attribute { color: #ffd596; }
pre code .smalto-selector { color: #9ce7ff; }
pre code .smalto-property { color: #ffd596; }

Supported languages

Lustre integration

The smalto_lustre package renders tokens as LustreElement nodes:

gleam add smalto smalto_lustre

import smalto
import smalto/languages/python
import smalto/lustre as smalto_lustre

let tokens = smalto.to_tokens("print('hello')", python.grammar())
let elements = smalto_lustre.to_lustre(tokens, smalto_lustre.default_config())

The default config uses inline-styled <span> elements matching smalto’s ANSI color scheme. Customize individual token renderers with the builder functions. See the smalto_lustre README for full details.

Pre-built themes

The smalto_lustre_themes package provides 45 ready-to-use inline-styled theme configs ported from Prism.js — no CSS files needed:

gleam add smalto smalto_lustre smalto_lustre_themes

import smalto
import smalto/languages/python
import smalto/lustre as smalto_lustre
import smalto/lustre/themes

let tokens = smalto.to_tokens("print('hello')", python.grammar())
let elements = smalto_lustre.to_lustre(tokens, themes.dracula())

See the smalto_lustre_themes README for the full list of available themes.

Try it out

Smalto ships with example tools in the dev/ directory:

# Print syntax-highlighted source to the terminal using ANSI colors
gleam run -m cat -- path/to/file.py

# Render a standalone HTML file with an embedded CSS theme
gleam run -m html_render -- path/to/file.py dracula output.html

The cat tool detects the language from the file extension and prints highlighted output to stdout. The html_render tool does the same but writes a self-contained HTML file using one of the 45 bundled CSS themes (see themes/ directory).

Documentation

API reference is available on HexDocs.

Development

gleam build                        # Compile the project
gleam test                         # Run the tests (Erlang)
gleam test --target javascript     # Run the tests (JavaScript)
gleam format src test              # Format code

Prism.js converter

Language grammars are generated from Prism.js definitions using a Node.js converter tool:

cd tools/prism_converter
npm install
node convert.js python rust javascript  # Convert specific languages
node convert.js --all                    # Convert all target languages

License

Smalto is licensed under the MIT License. See LICENSE for details.