[GUI] Setting properties of a widget.

[Vincenzo Ciancia]

On Sat, 29 Mar 2003 08:39:52 -0800 (PST)
David Sankel <camio@yahoo.com> wrote:

> 
>  I'm not sure what you mean by filter?  Something like
>  an onTitleChange callback?

Given a Variable a, I think that it's a good idea to have an ability to
filter the values with something like

addFilter :: (Variable a) -> (a -> IO (Maybe a)) -> IO (IO ())

the IO action returned is to uninstall the filter (wich is like a
callback).

but this is just an idea, there might be reasons not to do it, or to do
it in a simpler way. 

If there are no filters (like in MVar), you can always provide a set/get
pair of functions wich allow filtering, but there is no way to avoid the
ability to bypass the filters.

Of course, I could entirely rewrite an MVar layer on top of the old one,
but that sounds silly.

V.