[Haskell-cafe] is value evaluated?

[Nikhil Patil]

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