patch applied (ghc): TickBox representation change

[Simon Marlow]

Simon Peyton-Jones wrote:

> I had a look at what you did.   Generally looks good.  Much better than the Note version, as I hope you agree!   (You seem to have deleted quite a bit of code!)
> 
> Here's a qn.  I thought you were going to do ticks like this:
>         dsExpr (HsTick a e) = tick{a} e
> but instead you have
>         dsExpr (HsTick a e) = case tick{a} of DEFAULT -> e
> I think that both are probably fine, but the implications are not clear to me.

It's perhaps because I suggested we use the case form.  If we had

   tick{a} e

then we really don't want e compiled to a thunk.  So somehow this must turn into 
the case form in the back end:

  case tick{a} of DEFAULT -> e

but just declaring tick{a} to be strict won't do this: it'll give you

  case e of DEFAULT -> tick{a} e

Perhaps this still works, I don't know.  To me it seems like a longer way 
around; the case form more directly says what you want to happen.  But maybe I'm 
missing something?

   The former seems more robust somehow.  For example if we have
>         case tick{a} of DEFAULT -> ....(case tick{a} of DEFAULT -> ...)
> GHC might eliminate the inner tick.  Would that be OK (after all, you're only checking coverage, and it's covered)?

One of the suggestions was to use a PrimOp (like the foreign call op) for tick, 
which would prevent the simplifier from eliminating the inner tick because the 
PrimOp would be declared as having side effects.

Cheers,
	Simon