[xmonad] The future of PerWorkspace

[Nicolas Pouillard]

Excerpts from Brent Yorgey's message of Fri Feb 01 23:08:30 +0100 2008:
> On Fri, Feb 1, 2008 at 5:04 PM, Nicolas Pouillard <
> nicolas.pouillard at gmail.com> wrote:
> 
> > Excerpts from Brent Yorgey's message of Fri Feb 01 22:53:20 +0100 2008:
> > > On Thu, Jan 31, 2008 at 2:42 PM, Spencer Janssen <sjanssen at cse.unl.edu>
> > > wrote:
> > >
> > > > On Wed, Jan 30, 2008 at 07:29:58PM -0500, Brent Yorgey wrote:
> > > > >   1. PerWorkspace is an inelegant hack with several icky problems:
> > > >
> > > > Agreed.  It is approaching the limits of xmonad's layout design.
> >  However,
> > > > I
> > > > think we can accomplish PerWorkspace behavior without changing too
> > much.
> > > >
> > > > \begin{code}
> > > > data PerWS = PerWS { selected :: Maybe Layout
> > > >                   , choices  :: Map WorkspaceId Layout
> > > >                   , default  :: Layout }
> > > > \end{code}
> > > >
> > >
> > > So, I took a crack at implementing (something like) this today.  The
> > problem
> > > I ran into is that (Layout a) is not an instance of Read, so
> > PerWorkspace
> > > cannot derive Read either.  Is there any way around this?  Or do I have
> > to
> > > go back to caching a (Maybe Bool) and using that to decide on which of
> > two
> > > layouts to use, instead of directly caching a (Maybe (Layout a))?
> > >
> > > Ideas?
> >
> > What about "instance Read a => Read (Layout a)"?
> >
> 
> Well, since Layout is an existential wrapper, that's not possible -- it
> hides the type of the wrapped layout.  I know that lots of work has already
> gone into the xmonad core getting around this issue. =)

Yes, I didn't understand why this line compile and seems to solve the problem.
Indeed I've encountered this same issue while doing another xmonad extension.

-- 
Nicolas Pouillard aka Ertai
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