[Haskell-cafe] Designing widgets in Reactive style

Heinrich Apfelmus apfelmus at quantentunnel.de
Tue Jul 8 20:04:24 UTC 2014

JP Moresmau wrote:
> Hello, I'm a bit confused on how to design graphical widgets to take
> advantage of FRP design. I'm trying to do things in threepenny-gui but I
> suppose the same question applies to any framework.
> Suppose I want to create a "file selection widget". Currently I have a
> function that takes a (FilePath -> UI()) function as a parameter: what to
> do when a file is selected; my function returns the "widget", so that the
> calling code can incorporate it in its UI. This works fine, but is not FRP,
> right?
> Should my function take no arguments and return a (Widget,Event FilePath)
> tuple, and let the calling code decide what to do with that event? Should
> it be a Behavior (the currently selected file of the widget)? I *think* I
> understand the principles of FRP, but I'm not getting what it means in
> terms of design. What are the advantages of returning an Event and letting
> the calling code do "on myEvent myWidget $ \file -> ..." instead of just
> passing the handler function as a parameter?
> I don't know if I make much sense. In any case, any pointer to non trivial
> FRP GUI code would be good!

I have distilled some principles for FRP widget design and wrote them 
down here:


They are not quite complete, though, in particular the section on 
implementation details is lacking, mainly because I'm not entirely sure 
on this myself.

In your particular example, I would use an `Event FilePath` and make it 
part of the `Widget` type, so that you have a function

    chosePath :: Widget -> Event FilePath

The advantage of using first-class events, or FRP, in the first place is 
also briefly explained in the aforementioned document.

 From a pragmatic point of view, you could say that you certainly don't 
lose anything by offering an Event instead of registering a callback 
function. :)

Best regards,
Heinrich Apfelmus


More information about the Haskell-Cafe mailing list