[Haskell-cafe] is value evaluated?

[Joe Fredette]

That strikes me as being bad in a "I'm violating the Halting Problem" 
sort of way- but I'm not sure how. Is there some contradictory 
construction that
could be built from such a function?

Nikhil Patil wrote:
> Hi, 
>
> I am curious to know if there is a function in Haskell to find if a certain
> value has already been evaluated. The function I need would have the type:
>
>   
>> (?!) :: a -> Bool
>>     
>
> And I expect it to be such that the following terminates after printing the
> first 101 fibonacci numbers.
>
>   
>> fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
>>
>> main = do print $ fibs !! 100
>>           print $ takeWhile (?!) fibs
>>     
>
> Although I guess I can imagine the following not terminating:
>
>   
>>           print $ filter (?!) fibs
>>     
>
> I would find such a function immensely useful in "printing out" my infinite lists.
>
> Thanks!
>
> nikhil
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>   
-------------- next part --------------
A non-text attachment was scrubbed...
Name: jfredett.vcf
Type: text/x-vcard
Size: 296 bytes
Desc: not available
Url : http://www.haskell.org/pipermail/haskell-cafe/attachments/20090508/30a96a9a/jfredett.vcf