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 wrote:

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

http://hackage.haskell.org/trac/ghc/ticket/3259

Thanks for being so persistent Mario!

Simon has diagnosed the bug. It's really rather subtle.

Duncan

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