This is related to the long thread on a bug in cross-module strictness analysis for the "lazy" function in GHC. Simon posted a response to that bug ( http://hackage.haskell.org/trac/ghc/ticket/3259 ), which shows the Haskell source for GHC's "par" function:
{-# INLINE par #-} par :: a -> b -> b par x y = case (par# x) of { _ -> lazy y }
While I understand that in GHC Core, case always reduces its argument
to WHNF, this isn't true of Haskell in general. What causes par# to
get executed? Does GHC overload case for unboxed return values to
force the evaluation of the scrutinee even if its return value isn't
used?
-- ryan
On Tue, May 26, 2009 at 10:29 AM, Duncan Coutts
On Mon, 2009-05-25 at 09:40 -0400, Mario Blazevic wrote:
$diff main.simpl imported.simpl ... 223c232 < a_s1rs [ALWAYS Just L] :: GHC.Integer.Internals.Integer ---
a_s1sV [ALWAYS Just S] :: GHC.Integer.Internals.Integer ...
Good find!
Does this S vs. L difference have anything to do with strictness and laziness?
Yes.
So, I think we should open a ghc but report with all the details, particularly the example's source, the ghc version and that highlight of that strictness difference.
I have logged the ticket at
Thanks for being so persistent Mario!
Simon has diagnosed the bug. It's really rather subtle.
Duncan
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe