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HPlayground is a subset of Transient running widgets in the Web browser with the Haste compiler. The widgets are first class, that means that there is an widget algebra using standard haskell combinators.

latest additions:

Create applications in the browser as fast as easyly as console applications and have reactive, window-oriented and spreadsheet-like behaviours for free.

So you translate your inputs and outputs from console calls to HPlayground widgets and with no more modifications you have reactive and spreadsheet behaviours.

HPlayground uses the same widgets and combinators used in MFlow. MFlow is a server-side framework.

This program creates two input boxes and presents the sum below them:

    import Haste.HPlay.View
    import Control.Applicative

    main= runBody action

    action :: Widget ()
    action = do
         r  <- (+) <$> inputInt Nothing `wake` OnKeyPress <++ br
                   <*> inputInt Nothing `wake` OnKeyPress <++ br
         p  (show r) ++> noWidget

The coming extension applicativeDo for the GHC compiler will make this syntax much easier. Each widget creates his own rendering and manages his own events, that can be propagated or not down through the monadic computation and triggers modifications in the DOM.


There is an IDE for Haste and HPlayground with many examples running at:


You can install this IDE locally or in an Heroku instance. Follow the instructions at:


Additionally you can see a more complex example: the hplay-todo, the todoMVC project for HPlayground.

The todo application running

How it works

Under the hood there is the good old formlet concept. It uses monadic and applicative combinators the same ones used by MFlow in the server side. While the server side widgets of MFlow produce blaze-html output converted to bytestrings, HPlayground constructs a builder function that creates a tree in the HTML DOM when executed. This builder (perch) is monoidal so the formlet can aggregate subtrees. When some event happens in the widget subtree, the widget executes its code and reconstructs itself. If it returns a valid result and it is in a monadic computation, the tree continues recreating itself downstream by executing further widgets in the monadic sequence. If the event is raised within a widget that does not generate a valid result (return empty), the remaining widgets continue unchanged and unevaluated.


Non-local modifications of the DOM work with the new "at" primitive. Option buttons, checkboxes and drop-down buttons work with the same syntax as in MFlow.

The Cell module has Lens-like primitives for updating form elements and experimental math operations with form elements as spreadsheet cells. Currently it is at the beginning.

How to run

Docker file


contains everything necessary to use HPlayground

execute it as:

sudo docker run -it -p 80:80 agocorona/tryplayg

it runs the examples IDE at port 80, where you can also create and compile new programs with a web browser.

To access the IDE, in Windows and Mac you can obtain the IP of the docker instance with:

boot2docker ip

use the IP address as the URL in the browser.

Install from scratch

install the ghc compiler

install Haste:

>cabal install haste-compiler

clone HPlayground:

>git clone http://github.com/agocorona/hplayground

install hplayground:

>haste-inst install

or install it from Hackage using cabal:

>haste-inst install hplayground

it will also install haste-perch


>cd src
>hastec Main.hs --output-html

hastec uses ghc internally so you can expect ordinary ghc error messages in your development.

Browse the Main.html file. In windows simply execute it in the command line:


you can also see it executing at


Main.html and Main.js are included in the repo so you can execute it in your PC.

Execute it in the same directory where Main.js is, since it references it assuming that it is in the current folder.h