[Haskell-cafe] Is there any way to prevent a computation from being shared?

Brian Bloniarz brian.bloniarz at gmail.com
Mon Nov 15 15:27:51 EST 2010

On 11/13/2010 08:55 AM, Petr Prokhorenkov wrote:
> import Data.List
> wtf d = head . dropWhile (< 10^100) . map (*d) $ enumFrom 2
> main = do
>     print $ wtf 1
>     print $ wtf 2 -- Everything is ok without this line
> Is there any way to overcome this?

I think this phenomenon is called the "full laziness transform":
an optimization that GHC will apply to avoid redundant computation.
More details in: "Let-Floating: Moving Bindings to Give
Faster Programs"

So I think you're seeing bug http://hackage.haskell.org/trac/ghc/ticket/917
where the increased sharing only leads to space leaks. You could
try the workaround, passing -fno-full-laziness (it might need to
come after -O on the command line).

I'm not sure if this'd be more or less fragile than disabling
inlining as Felipe suggested. I'd also suggest adding
yourself as a CC: on bug 917 so GHC HQ knows how many people
are being affected by this kind of behavior.

Hope this helps,

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