package pure-html

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HTML generator eDSL


Dune Dependency






Write HTML directly in your OCaml source files with editor support.



Published: 09 Jul 2024


API Reference

dream-html - generate HTML markup from your Dream backend server

Copyright 2023 Yawar Amin

This file is part of dream-html.

dream-html is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

dream-html is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with dream-html. If not, see


An HTML, SVG, and MathML rendering library that is closely integrated with Dream. Most HTML elements and attributes from the Mozilla Developer Network references are included. Almost all non-standard or deprecated tags/attributes deliberately omitted. CSS support is out of scope. htmx attributes supported out of the box.


  • TyXML is a bit too complex.

  • Dream's built-in eml (Embedded ML) has some drawbacks like no editor support, quirky syntax that can be hard to debug and refactor, and manual dune rule setup for each view file

  • In general string-based HTML templating is suboptimal and mostly driven by familiarity.

First look

let page req =
  let open Dream_html in
  let open HTML in
  (* automatically injects <!doctype html> *)
  html [lang "en"] [
    head [] [
      title [] "Dream-html" ];
    body [] [
      h1 [] [txt "Dream-html"];
      p [] [txt "Is cool!"];
      form [method_ `POST; action "/feedback"] [
        (* Integrated with Dream's CSRF token generation *)
        csrf_tag req;

        label [for_ "what-you-think"] [txt "Tell us what you think!"];
        input [name "what-you-think"; id "what-you-think"];
        input [type_ "submit"; value "Send"] ] ] ]

(* Integrated with Dream response *)
let handler req = Dream_html.respond (page req)

Security (HTML escaping)

Attribute and text values are escaped using rules very similar to standards- compliant web browsers:

utop # open Dream_html;;
utop # open HTML;;
utop # #install_printer pp;;

utop # let user_input = "<script>alert('You have been pwned')</script>";;
val user_input : string = "<script>alert('You have been pwned')</script>"

utop # p [] [txt "%s" user_input];;
- : node = <p>&lt;script&gt;alert('You have been pwned')&lt;/script&gt;</p>

utop # div [title_ {|"%s|} user_input] [];;
- : node = <div title="&quot;<script>alert('You have been pwned')</script>"></div>

How to install

Make sure your local copy of the opam repository is up-to-date first:

opam update
opam install dream-html

Alternatively, to install the latest commit that may not have been released yet:

opam pin add dream-html git+


A convenience is provided to respond with an HTML node from a handler:

Dream_html.respond greeting

You can compose multiple HTML nodes together into a single node without an extra DOM node, like React fragments:

let view = null [p [] [txt "Hello"]; p [] [txt "World"]]

You can do string interpolation of text nodes using txt and any attribute which takes a string value:

let greet name = p [id "greet-%s" name] [txt "Hello, %s!" name]

You can conditionally render an attribute, and void elements are statically enforced as childless:

let entry =
    [ if should_focus then autofocus else null_;
      id "email";
      name "email";
      value "Email address" ]

You can also embed HTML comments in the generated document:

div [] [comment "TODO: xyz."; p [] [txt "Hello!"]]
(* <div><!-- TODO: xyz. -->Hello!</div> *)

You have precise control over whitespace in the rendered HTML; dream-html does not insert any whitespace by itself–all whitespace must be inserted inside text nodes explicitly:

p [] [txt "hello, "; txt "world!"];;
(* <p>hello, world!</p> *)

You can also conveniently hot-reload the webapp in the browser using the Dream_html.Livereload module. See the API reference for details.

Import HTML

One issue that you may come across is that the syntax of HTML is different from the syntax of dream-html markup. To ease this problem, you may use the bookmarklet import_html.js provided in this project. Simply create a new bookmark in your browser with any name, and set the URL to the content of that file (make sure it is exactly the given content).

Then, whenever you have a web page open, just click on the bookmarklet to copy its markup to the clipboard in dream-html format. From there you can simple paste it into your project.

Note that the dream-html version is not formatted nicely, because the expectation is that you will use ocamlformat to fix the formatting.

Also note that the translation done by this bookmarklet is on a best-effort basis. Many web pages don't strictly conform to the rules of correct HTML markup, so you will likely need to fix those issues for your build to work.


Run the test and print out diff if it fails:

dune runtest # Will also exit 1 on failure

Set the new version of the output as correct:

dune promote

Prior art/design notes

Surface design obviously lifted straight from elm-html.

Implementation inspired by both elm-html and ScalaTags.

Many languages and libraries have similar HTML embedded DSLs:

Dependencies (2)

  1. uri >= "4.4.0" & < "5.0.0"
  2. dune >= "2.7"

Dev Dependencies (1)

  1. odoc with-doc

Used by (1)

  1. dream-html >= "3.6.0"




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