[Haskell-cafe] proper way to generate a random data in criterion

[Henk-Jan van Tuyl]

On Wed, 19 Oct 2011 21:21:48 +0200, Gregory Collins  
<greg at gregorycollins.net> wrote:

> On Wed, Oct 19, 2011 at 5:03 PM, Johan Tibell <johan.tibell at gmail.com>  
> wrote:
>>
>> It does. You need to use evaluate to have ensure actually be evaluated.
>>
>
> I'm almost certain you're wrong about this. The bang pattern on the
> return from ensure (!r1 <- ensure $ ...) forces r1 to WHNF, which goes
> through deepseq, and thus the whole list is forced. See
> https://gist.github.com/1299380 for a short counterexample.
>
> G

You don't need the ensure function or the bang patterns if you do it like  
this:

main =
   deepseq testList $
     do
       putStrLn "we survived without evaluating testList"
       let x = foldl' (+) 0 $ take 50 testList
       putStrLn $ "x was " ++ show x


Regards,
Henk-Jan van Tuyl


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