Haskell Blog

GUI - Release of the threepenny-gui library, version

Sun, 03 May 2015 17:33:45 +0200

I am pleased to announce release of threepenny-gui version 0.6, a cheap and simple library to satisfy your immediate GUI needs in Haskell.

Want to write a small GUI thing but forgot to sacrifice to the giant rubber duck in the sky before trying to install wxHaskell or Gtk2Hs? Then this library is for you! Threepenny is easy to install because it uses the web browser as a display.

The library also has functional reactive programming (FRP) built-in, which makes it a lot easier to write GUI application without getting caught in spaghetti code. For an introduction to FRP, see for example my slides from a tutorial I gave in 2012. (The API is slightly different in Reactive.Threepenny.)

In version 0.6, the communication with the web browser has been overhauled completely. On a technical level, Threepenny implements a HTTP server that sends JavaScript code to the web browser and receives JSON data back. However, this is not the right level of abstraction to look at the problem. What we really want is a foreign function interface for JavaScript, i.e. we want to be able to call arbitrary JavaScript functions from our Haskell code. As of this version, Threepenny implements just that: The module Foreign.JavaScript gives you the essential tools you need to interface with the JavaScript engine in a web browser, very similar to how the module Foreign and related modules from the base library give you the ability to call C code from Haskell. You can manipulate JavaScript objects, call JavaScript functions and export Haskell functions to be called from JavaScript.

However, the foreign calls are still made over a HTTP connection (Threepenny does not compile Haskell code to JavaScript). This presents some challenges, which I have tried to solve with the following design choices:

To see Threepenny in action, have a look at the following applications:

Daniel Austin’s FNIStash
Editor for Torchlight 2 inventories.
Chaddai’s CurveProject
Plotting curves for math teachers.

Get the library here:

Note that the API is still in flux and is likely to change radically in the future. You’ll have to convert frequently or develop against a fixed version.

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