GHCi debugger status

[Simon Marlow]

Claus Reinke wrote:

> Consider this code and session:
> 
>    f x y z | x<y       = z
>        | otherwise = z*y
> 
...
> Things to note:
> 
> - when reaching the breakpoint "in" 'f', one isn't actually in 'f' yet - 
> nothing
>    about 'f' can be inspected
> - at no point in the session was 'x' inspectable, even though it is 
> likely to
>    contain information needed to understand 'f', especially when we are 
>    deep in a recursion of a function that can be called from many places;
>    this information doesn't happen to be needed in the current branch, 
> but    debugging the current expression always happens in a context, and 
>    accessing information about this context is what the GHCi debugger    
> doesn't seem to support well
> 
> In this particular example, the second item is most likely a bug (the 
> free variables of the guard were never offered for inspection).

Indeed it was a bug, the same as #2740, and I've just fixed it.  Thanks for 
boiling it down to a nice small example.

Cheers,
	Simon