Is functional GUI programming possible?
I've recently caught the FP bug (trying to learn Haskell), and I've been really impressed with what I've seen so far (first-class functions, lazy evaluation, and all the other goodies). I'm no expert yet, but I've already begun to find it easier to reason "functionally" than imperatively for basic algorithms (and I'm having trouble going back where I have to).
The one area where current FP seems to fall flat, however, is GUI programming. 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. I haven't used F#, but my understanding is that it does something similar using OOP with .NET classes. Obviously, there's a good reason for this--current GUI programming is all about IO and side effects, so purely functional programming isn't possible with most current frameworks.
My question is, is it possible to have a functional approach to GUI programming? I'm having trouble imagining what this would look like in practice. Does anyone know of any frameworks, experimental or otherwise, that try this sort of thing (or even any frameworks that are designed from the ground up for a functional language)? Or is the solution to just use a hybrid approach, with OOP for the GUI parts and FP for the logic? (I'm just asking out of curiosity--I'd love to think that FP is "the future," but GUI programming seems like a pretty large hole to fill.)
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.
An example is
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).
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