Welcome to the Threepenny library for graphical user interfaces.

A program written with Threepenny is essentially a small web server that displays the user interface as a web page to any browser that connects to it.

For an introduction, see the example below. The module Graphics.UI.Threepenny.Core contains the main functions.

The following example should help to get you started with Threepenny. (The lines of code below are meant to be concatenated into a single file.)

module Main where

First, we have to import the library. It is a good idea to import the core module verbatim and import all other functions with a mandatory UI prefix.

import qualified Graphics.UI.Threepenny       as UI
import           Graphics.UI.Threepenny.Core

We begin by starting a server on port 8023 using the startGUI function. Additional static content is served from the ../wwwroot directory.

main :: IO ()
main = do
    startGUI defaultConfig
        { jsPort       = Just 8023
        , jsStatic     = Just "../wwwroot"
        } setup

Whenever a browser connects to the server, the following function will be executed to start the GUI interaction. It builds the initial HTML page.

setup :: Window -> UI ()
setup window = do

First, set the title of the HTML document

    return window # set UI.title "Hello World!"

Then create a button element

    button <- UI.button # set UI.text "Click me!"

DOM elements can be accessed much in the same way they are accessed from JavaScript; they can be searched, updated, moved and inspected. In the line above, we set the text contents.

To actually display the button, we have to attach it to the body of the HTML element. The #+ combinator allows you to nest elements quickly in the style of a HTML combinator library.

    getBody window #+ [element button]

Finally, we register an event handler for the click event, which occurs whenever the user clicks on the button. When that happens, we change the text of the button.

    on UI.click button $ const $ do
        element button # set UI.text "I have been clicked!"

That's it! Now, run the program and visit the URL http://localhost:8023/ in your browser to interact with the user interface.

The library comes with a plethora of additional example code.