GUI framework that uses the web browser as a display.

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Stackage Nightly 2024-07-13:
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BSD-3-Clause licensed by Heinrich Apfelmus
Maintained by Heinrich Apfelmus <apfelmus at quantentunnel dot de>
This version can be pinned in stack with:threepenny-gui-,9193

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What’s this?

Threepenny is a GUI framework written in Haskell that uses the web browser as a display. It’s very easy to install. See the

Project homepage

for more information on what it does and can do for you as a library user.


The library comes with many examples, which can be found in the samples folder. Follow the link for more information on how to run them.

Desktop Apps

Threepenny is mainly intended for writing GUI applications that run on the local network, and it relies on a web browser being installed. You can drop the latter requirement and integrate it a bit more tightly with you desktop environment by using the Electron framework. There is no fully automated support for this yet, but the documentation includes a tutorial on how to use Electron with Threepenny.

Technical overview

JavaScript FFI

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.

The web server displays a HTML page, which in turn establishes WebSocket connection with the server. The server uses this connection to send JavaScript code that is executed in the client. In the library, this appears as a JavaScript Foreign Function Interface (FFI). The documentation includes more information on the design of the JavaScript FFI.


The frequent communication between browser and server means that Threepenny is best used as a GUI server running on localhost. You can use it on your local network as well.

If you want to reduce latency, the best option is to generate larger blocks of JavaScript code and run them on the client. Consider this approach similar to a shading language. You can import any JavaScript library and use it from the JavaScript FFI.

If you don’t want to write JavaScript, then you could choose a Haskell-like language like PureScript, Fay. You can also directly compile Haskell to JavaScript with Haste or GHCJS.

Future ideas

HTML rendering mode

It might be nice in the case of search engines to merely generate a DOM and render it, so that search engines can read the pages.

UI libraries

qooxdoo — provides a feature-complete widget set. One could wrap this in a type-safe API from Threepenny and get a complete, stable UI framework for free. Most of the “immediate feedback” like dragging things here, switching tabs there, are taken care of by the framework. All that would be left would be to provide the domain configuration and business/presentation logic.

There are plenty more like this, but this is the first that springs to mind that is good.


Many thanks to everyone who contributed, provided feedback or simply wrote an application using Threepenny! In particular, many thanks to:

Heinrich Apfelmus, Daniel Austin, Jeremy Barisch-Rooney, Steve Bigham, Simon Jakobi, Ken Friis Larsen, Daniel Mlot, Tim C. Schröder and many others

Special thanks to Simon Jakobi for co-maintaining this project.

Special thanks to Chris Done for starting the precursor project Ji.


Changelog for the threepenny-gui package – Maintenance release

  • Bump dependencies for compatibility with GHC-8.8.
  • Bump dependencies to allow hashable
  • Remove support for GHC 7.6 and 7.8. – Maintenance and snapshot release

  • Export keypress event.
  • Fix the spelling of the refX and refY SVG attributes. refX and refY are added to Graphics.UI.Threepenny.SVG.Attributes, the old lowercase versions are deprecated.
  • Invoke compatibility mode of IE11 for jQuery v3.2.1.
  • Compatibility with GHC-8.6.1 – Maintenance release

  • Remove redundant dependencies on network-uri and network
  • Bump dependencies to allow aeson
  • Exclude websockets from dependencies. – Maintenance release

  • Compatibility with GHC-8.4.1
  • Bump dependencies to allow aeson
  • Bump dependencies to allow exceptions 0.10.0
  • Bump dependencies to allow snap-server – Maintenance release

  • Bump dependencies to allow exceptions 0.9.0 — Maintenance release

  • Bump dependencies to allow async 2.2
  • Fix a compatibility issue with Cabal-2.0 — Snapshot release

  • Add getCookies function that retrieves the cookies sent with the HTTP request when the browser window connects (to the websocket). #137
  • Allow Electron process to be accessed from JavaScript FFI. #200 This means that Threepenny is now more useful when used with the Electron framework, see doc/ for more information on that.
  • Bump dependencies to allow file-embed — Snapshot release

  • Improve documentation and handling of call buffering (CallBufferMode). The default call buffer mode was documented incorrectly, it was BufferRun and is now FlushOften. #163, #191, #192
  • Add new default CallBufferMode: FlushOften. This mode is like BufferRun, but will flush the buffer at every onEvent as well, leading to less confusion when using the library in most circumstances. #191
  • Add new CallBufferMode: FlushPeriodically. This mode is like BufferRun, but will flush the call buffer every 300ms if nonempty. #192
  • Add support for custom DOM events (CustomEvent). #196
  • Expose JavaScript FFI functions toJSObject and liftJSWindow in Graphics.UI.Threepenny. This is useful for linking the lifetime of JavaScript objects to the lifetime of Element. #181
  • Use jsLog parameter to log exceptions. #185
  • Update bundled jQuery to version 3.2.1. #186 — Maintenance release

  • Bump dependencies for compatibility with GHC-8.2.1
  • Bump dependencies to allow websockets 0.12 — Snapshot release


  • Fix getElementById to properly return Nothing when no element with the given id attribute is present. #129.
  • Bring back loadFile and loadDirectory. #110
  • Add MonadUI typeclass for easier lifting in custom monad stacks. #173

JavaScript FFI

  • Implement batching / buffering for FFI calls. #131. Several calls to the runFunction function may now buffer the JavaScript code and send it to the browser window in a single message at a later time. This improves performance considerably.
  • Clarify semantics for exceptions. See the file doc/ for more.
    • The UI monad now supports exception handling by being an instance of the type classes MonadThrow and MonadCatch.
    • The function callFunction can now throw a JavaScriptException exception to the Haskell side.
    • The function runFunction now terminates the connection to the browser window whenever the JavaScript code within throws an exception.
  • Exceptions in the UI monad that are not handled are now printed properly for better debugging. #145
  • Clarify semantics of the disconnect event. It is now triggered more reliably. #133.
  • Remove unnecessary client response when exporting event handlers. #131.
  • Add option jsWindowReloadOnDisconnect to reload the browser window #130 whenever the WebSocket connection is broken. This is useful for e.g. mobile devices, which tend to disconnect WebSocket connections very often.


  • Add dependency on exceptions — Maintenance release

  • Bump dependencies to allow aeson 1.2
  • Bump dependencies to allow websockets 0.11 — Maintenance release

  • Bump dependencies to allow aeson 1.1
  • Bump dependencies to allow vector 0.12
  • Bump dependencies to allow websockets 0.10 — Maintenance and snapshot release

  • JavaScript FFI: Reduce communication from browser to server when creating Elements. New function unsafeCreateJSObject to create JavaScript objects without waiting for a client response. #131
  • JavaScript FFI: Implement escape sequence ‘%%’ when calling JavaScript functions. #132.
  • Change type of onEvent function to allow unregistering events.
  • Add function timestamp for simple performance measurements.
  • Update JavaScript dependencies to jQuery 2.2.3
  • Adapt to GHC 8.0.1. #138
  • Bump dependencies to allow aeson 1.0
  • Bump dependencies to allow data-default 0.7
  • Bump dependencies to allow snap-core 1.0 and snap-server 1.0
  • Bump dependencies to allow template-haskell 2.11
  • Bump dependencies to allow websockets-snap 0.10 — Maintenance release

  • Bump dependencies to allow base 4.9
  • Bump dependencies to allow aeson 0.11 — Maintenance release

  • Bump dependencies to allow async 2.1
  • Bump dependencies to allow transformers 0.5 — Maintenance release.

  • Elements that have become unreachable, for instance because they have been removed from the DOM and are no longer reachable in the Haskell code, will be garbage collected again. Fix #109, #113.
  • Adjust dependencies.
  • Add <meta> tag to indicate UTF8 encoding in html file. #116 — Maintenance release.

  • Temporary fix for #109, which was causing event handlers to be discarded. Unfortunately, this means that elements are currently not garbage collected after they have been removed from the DOM tree. — Maintenance release.

  • Remove unused direct dependencies, in particular ** attoparsec-enumerator ** utf8-string ** MonadCatchIO-transformers ** time — Maintenance release.

  • The ADDR environment variable is now parsed correctly.
  • Now builds on GHC 7.8 and GHC 7.10
  • The example source code in the samples folder has been reorganized and consolidated. — Snapshot release.

  • The internals of the JavaScript FFI has been reimplemented completely. A new module Foreign.JavaScript exports a bare JavaScript FFI in case you want to write a custom GUI framework. However, the module Graphics.UI.Threepenny is not compatible with it, even though it builds on top of it.
  • The fields of Config type for server configuration are now prefixed with js instead of tp. Example: jsPort, jsStatic.
  • The functions loadFile and loadDirectory have been removed, as I felt that the jsStatic option is sufficient for most use cases. — Snapshot release.

  • Possibility to specify IP address to bind the server to.
  • FFI now supports callbacks into Haskell. Remove callDeferredFunction function.
  • Graphics.UI.Threepenny.Canvas.SVG for creating SVG elements and attributes.
  • 2D graphics API in Graphics.UI.Threepenny.Canvas is beginning to grow.
  • Bool is now correctly marshalled to JavaScript.
  • Text can now be marshalled to JavaScrtip. — Maintenance release.

  • Dependency bytestring >=0.9.2 is now implemented correctly.
  • Allow newer versions of aeson dependency.
  • Allow newer versions of network, transformers and template-haskell dependencies.
  • Helper scripts in the samples directory now assume that you use a cabal sandbox for development.
  • The UI monad is now an instance of the Applicative class. — Maintenance release.

  • Dependency on text package now from version 0.11 to 1.1.*.
  • Dependency on aeson package replaces the former dependency on the json package.
  • Unicode characters are now transmitted correctly to the browser. #75, #62.
  • Change default port number to 8023. #64 — Bugfix release.

  • Fix CSS bug for grid function. — Maintenance release.

  • Adjust package dependencies. — Snapshot release.

  • New UI monad for easier JavaScript FFI and recursion in FRP.
  • Garbage collection for DOM elements. (Unfortunately, this doesn’t support using custom HTML files anymore, see issue #60.)
  • First stab at widgets.
  • Bump dependencies to allow websockets 0.8 — Snapshot release.

  • Browser communication with WebSockets.
  • First stab at FRP integration. — Snapshot release.

  • First stab at easy JavaScript FFI.

  • Initial release.