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
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