[Haskell-cafe] Callback functions?

[Stefan O'Rear]

On Tue, Jul 03, 2007 at 12:26:47AM +0200, Hugh Perkins wrote:
> This sounds something like using a continuation function.
> 
> In imperative languages, we might have something like:
> 
> class MyDisplayForm
> {
>   void ButtonGo_Press()
>   {
>       Processor.Go( new CallbackDelegate( this.SetStatus ) );
>   }
> 
>   public void SetStatus( string message )
>   {
>      StatusLbl.Text = message;
>      Application.DoEvents();
>   }
> }
> 
> 
> So: we call Processor.Go, passing in a pointer to the function SetStatus (a
> "delegate" in C# terminology, an "interface" in Java, a pointer in C, ...).
> 
> The Processor.Go function is going to update the status bar in MyDisplayForm
> as it goes along with useful messages "Processing blah.txt...", etc.
> 
> It does that by calling the passed in "callback" function, with the
> appropriate status message as a parameter.
> 
> This is also implicit to the Observer pattern, which may or may not be
> useful in FP (?)
> 
> 
> Anyway, so the question is: how do we write callback functions in
> FP/Haskell?  Can someone provide a simple, but functional example?

mapM_ print [1,2,3]

But usually we don't do that, because returning the list directly is
simpler and because of laziness, almost as fast.

Stefan