Is functional GUI programming possible? [closed]

Question

The Haskell approach seems to be to just wrap imperative GUI toolkits (such as GTK+ or wxWidgets) and to use "do" blocks to simulate an imperative style

That's not really the "Haskell approach" -- that's just how you bind to imperative GUI toolkits most directly -- via an imperative interface. Haskell just happens to have fairly prominent bindings.

There are several moderately mature, or more experimental purely functional/declarative approaches to GUIs, mostly in Haskell, and primarily using functional reactive programming.

Some examples are:

• reflex-platform, https://github.com/reflex-frp/reflex-platform
• grapefruit, http://hackage.haskell.org/package/grapefruit-ui-gtk
• reactive, http://hackage.haskell.org/package/reactive-glut
• wxFruit, http://hackage.haskell.org/package/wxFruit
• reactive-banana, http://hackage.haskell.org/package/reactive-banana

For those of you not familiar with Haskell, Flapjax, http://www.flapjax-lang.org/ is an implementation of functional reactive programming on top of JavaScript.

My question is, is it possible to have a functional approach to GUI programming?

The key words you are looking for are "functional reactive programming" (FRP).

Conal Elliott and some others have made a bit of a cottage industry out of trying to find the right abstraction for FRP. There are several implementations of FRP concepts in Haskell.

You might consider starting with Conal's most recent "Push-Pull Functional Reactive Programming" paper, but there are several other (older) implementations, some linked from the haskell.org site. Conal has a knack for covering the entire domain, and his paper can be read without reference to what came before.

To get a feel for how this approach can be used for GUI development, you might want to look at Fudgets, which while it is getting a bit long in the tooth these days, being designed in the mid 90s, does present a solid FRP approach to GUI design.

Windows Presentation Foundation is a proof that functional approach works very well for GUI programming. It has many functional aspects and "good" WPF code (search for MVVM pattern) emphasizes the functional approach over imperative. I could bravely claim that WPF is the most successful real-world functional GUI toolkit :-)

WPF describes the User interface in XAML (although you can rewrite it to functionally looking C# or F# too), so to create some user interface you would write:

<!-- Declarative user interface in WPF and XAML --> 
<Canvas Background="Black">
   <Ellipse x:Name="greenEllipse" Width="75" Height="75" 
      Canvas.Left="0" Canvas.Top="0" Fill="LightGreen" />
</Canvas>

Moreover, WPF also allows you to declaratively describe animations and reactions to events using another set of declarative tags (again, same thing can be written as C#/F# code):

<DoubleAnimation
   Storyboard.TargetName="greenEllipse" 
   Storyboard.TargetProperty="(Canvas.Left)"
   From="0.0" To="100.0" Duration="0:0:5" />

In fact, I think that WPF has many things in common with Haskell's FRP (though I believe that WPF designers didn't know about FRP and it is a bit unfortunate - WPF sometimes feels a bit weird and unclear if you're using the functional point of view).

I would actually say that functional programming (F#) is much better tool for user interface programming than for example C#. You just need to think about the problem a little bit differently.

I discuss this topic in my functional programming book in Chapter 16, but there is a free excerpt available, which shows (IMHO) the most interesting pattern that you can use in F#. Say you want to implement drawing of rectangles (user pushes the button, moves the mouse and releases the button). In F#, you can write something like this:

let rec drawingLoop(clr, from) = async { 
   // Wait for the first MouseMove occurrence 
   let! move = Async.AwaitObservable(form.MouseMove) 
   if (move.Button &&& MouseButtons.Left) = MouseButtons.Left then 
      // Refresh the window & continue looping 
      drawRectangle(clr, from, (move.X, move.Y)) 
      return! drawingLoop(clr, from) 
   else
      // Return the end position of rectangle 
      return (move.X, move.Y) } 

let waitingLoop() = async { 
   while true do
      // Wait until the user starts drawing next rectangle
      let! down = Async.AwaitObservable(form.MouseDown) 
      let downPos = (down.X, down.Y) 
      if (down.Button &&& MouseButtons.Left) = MouseButtons.Left then 
         // Wait for the end point of the rectangle
         let! upPos = drawingLoop(Color.IndianRed, downPos) 
         do printfn "Drawn rectangle (%A, %A)" downPos upPos }

This is a very imperative approach (in the usual pragmatic F# style), but it avoids using mutable state for storing the current state of drawing and for storing inital location. It can be made even more functional though, I wrote a library that does that as part of my Master thesis, which should be available on my blog in the next couple of days.

Functional Reactive Programming is a more functional approach, but I find it somewhat harder to use as it relies on quite advanced Haskell features (such as arrows). However, it is very elegant in a large number of cases. It's limitation is that you cannot easily encode a state machine (which is a useful mental model for reactive programs). This is very easy using the F# technique above.

Whether you're in a hybrid functional/OO language like F# or OCaml, or in a purely functional language like Haskell where side-effects are relegated to the IO monad, it's mostly the case that a ton of the work required to manage a GUI is much more like a "side effect" than like a purely functional algorithm.

That said, there has been some really solid research put into functional GUIs. There are even some (mostly) functional toolkits such as Fudgets or FranTk.

You might check out the series by Don Syme on F# where he demo's creating a gui. the following link is to third part of the series (you can link from there to the other two parts).

Using F# for WPF development would be a very interesting GUI paradigm...

http://channel9.msdn.com/shows/Going+Deep/C9-Lectures-Dr-Don-Syme-Introduction-to-F-3-of-3/

One of mind-opening ideas behind Functional Reactive Programming is to have an event handling function producing BOTH reaction to events AND the next event handling function. Thus an evolving system is represented as a sequence of event handling functions.

For me, learning of Yampa became a crucial poing to get that functions-producing-functions thing properly. There are some nice papers about Yampa. I recommend The Yampa Arcade:

http://www.cs.nott.ac.uk/~nhn/Talks/HW2003-YampaArcade.pdf (slides, PDF) http://www.cs.nott.ac.uk/~nhn/Publications/hw2003.pdf (full article, PDF)

There is a wiki page on Yampa at Haskell.org

http://www.haskell.org/haskellwiki/Yampa

Original Yampa home page:

http://www.haskell.org/yampa (unfortunately is broken at the moment)