GHCi hacking

[Ulf Norell]

Hi.

I want to use GHCi as the interface to my Application. The simple  
solution is to have the Application store its state in global IORefs.  
A user can then start up ghci with -package Application and use the  
provided functions to communicate with the Application. This works  
nicely.

Now the tricky part. I want the user to be able to implement her own  
layers on top of the Application API. A typical user interaction  
could look something like this:

$ ghci -package Application
*Application:API> startApplication
*Application:API> :load UserLayer
*main:UserLayer> myCleverFunction 42

The problem, of course, is that as soon as the user says ':load' ghci  
forgets all about the state of the Application. My solution to the  
problem is to compile my own version of ghci (copy InteractiveUI.hs  
and use -package ghc) and remove the call to rts_revertCAFs when  
loading new modules. This seems to work, but since I don't really  
have a clue what I'm doing I wanted to ask a few questions:

1) What is a CAF?
2) What breaks down if you don't revert them?
3) Does not reverting the CAFs really solve my problem?

/ Ulf