[Haskell-cafe] Functional GUIs again

Fraser Wilson blancolioni at gmail.com
Mon Feb 16 13:44:53 EST 2009

Since I'm congenitally lazy, and writing a GUI by hand in the IO monad is
... not what I expect from a beautiful program, and because what I often
need is a GUI that manipulates a state, and because I don't understand
arrows, and having been intrigued by a recent cafe thread, I threw together
a prototype GUI library with the following features:

  - the GUI is defined on three levels: gadgets, widgets and styles
  - gadgets are functions on a state
  - widgets are data structures which define the layout
  - styles are ways to modify the appearance of widgets

The following quick example shows a text box and a button.  Clicking on the
button reverses the text in the text box.

> module Main where

> import Barrie

> demoWidget :: Widget
> demoWidget = vbox [ui "demo entry" textBox,
>                    ui "demo command" (labelButton "click me")]

> type DemoState = String

> type DemoGadget = Gadget DemoState

> demoGUI :: DemoGadget
> demoGUI = localG "demo gui" [editorG "demo entry" id const,
>                              commandG "demo command" reverse]

> main = gtkMain demoGUI demoWidget "Hello, world"

Two gadgets are used:

editorG :: String -> (a -> b) -> (b -> a -> a) -> Gadget a
commandG :: String -> (a -> a) -> Gadget a

The editor gadget can show a value from a state, and update a state with a
value.  The command gadget can transform a state to a new state.  gtkMain
connects the gadgets to a widget, which specifies layout using the vbox,
attaching the editor gadget to a text box, and the command gadget to a

Well, that's all pretty trivial.  The key thing for me was that I can easily
slap a GUI onto the the front of a class of applications, which happen to be
the sort of applications I've been writing lately.  Also, arbitrary parts of
the GUI can respond to things that happen miles away, without really having
to worry about it too much.  In barrie-0.1 and 0.2, which used stream-based
approaches, the problem of getting state from one end of the application to
the other was non-trivial.

I'll sketch another quick example:

> data BridgeGame = ...

And a bunch of things you can do with the state:

> makeBid :: Bid -> BridgeGame -> BridgeGame
> playCard :: Card -> BridgeGame -> BridgeGame

For bidding, each bid is represented by a gadget:

> bidG :: Bid -> Gadget BridgeGame
> bidG bid = enabled (bidOK bid) $ CommandG (show bid) (makeBid bid)

'enabled' switches the gadget on if its first argument returns true when
applied to the current state.  However, the decision about what to do with a
disabled gadget is made by its corresponding widget.

We get one button for each bid:

> biddingG :: Gadget BridgeGame
> biddingG = localG "bidding" (map bidG allBids)

And they can be displayed in any old order using a widget:

> biddingW :: Widget
> biddingW = vbox (map suitBids [Club, Diamond, Heart, Spade] ++ [ntBids])
>    where suitBids suit = hbox $ map (bidButton . flip Bid suit) [1 .. 7]
>          ntBids = hbox $ map (bidButton . NT) [1 .. 7]
>          bidButton bid = ui (show bid) $ labelButton (show bid)

(You're right, double, redouble and pass are not represented.  They make the
lines too long).

Screenshot here: http://thewhitelion.org/images/4D.png

I've just bid four diamonds, so everything lower than that is automatically

Currently, Barrie implements buttons, text boxes, labels,
vertical/horizontal layout, single-column lists and drop lists.  It current
uses Gtk2hs for rendering, but it's GUI-agnostic (in fact, the first
renderer was putStrLn/getLine).

You can have a look by using darcs:
   darcs get http://thewhitelion.org/darcs/barrie

Or get the tarball at

One note: this is not intended to be a theoretically sound approach, merely
a way of getting something done quickly.  I would expect it to be most
useful in putting a GUI front-end onto an existing application, in
particular, an application that is driven by user actions which update a
state; e.g. a calculator, a bridge game, a 4th edition D&D character creator
(but that leads to a critical mass of nerdiness, so it's off the table for


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