[Xmonad] darcs patch: Define a read-state-only monad for Layouts.

[Andrea Rossato]

On Wed, Sep 05, 2007 at 12:28:08PM -0700, David Roundy wrote:
> Hi all,
> 
> Here's a proposed change for the Layout API.  I'm not yet
> sending updates for XMonadContrib, since I'd rather first
> hear what folks think.  The idea is to disallow changes to
> the XState in Layout hooks, so we won't run into trouble with
> the state changing while we're in the process of updating
> said state.  Basically, as far as I can tell, this is needed
> in order for the current Layouts API to be safe.
> 
> A catch is that some existing Layout code will break.  I
> would say that that code is fragile and really wants a
> generic hooks API.  As far as I know, the modules that I
> maintain will work with this change.  Any Layout that just
> lays out windows and does IO stuff will still work (e.g. even
> weird ones like WorkspaceDir won't be broken).  But, for
> example, Andreas' focus-follows-mouse Layout will break,
> since it relies on modifying the XState (to change focus)
> within the Layout (which is also why it breaks when combined
> with other Layouts).
> 
> I look forward to discussion.  In my opinion, we really need
> a cleaner Layout API.

I agree with you: enforcing a safer API with a read-only monad is
something we need, I believe. As you noticed, my attempts with
focus-follows-mouse show a real bad method of messing up layouts. I
think that something like that should be ruled by the API itself, if
possible.

Just my 2 cents.

Andrea