Tristan Allwood wrote:
Does anyone know if there is a function that tells you if a haskell
value has been forced or not? e.g. isWHNF :: a -> IO Bool
apfelmus wrote:
Note that this function [isWHNF :: a -> Bool] is not
referentially transparent
Indeed. Does it still mess up with the result in IO Bool (as was my
intent)?
It depends, but I think yes. I mean, given extensional equality,
isWHNF 2
isWHNF (1+1)
still have to be the same IO actions, in the sense that there cannot be
a guarantee that the first always returns True during execution
without the second returning always True , too. That is, you may not
depend on such a property for proving that your program is correct,
although you may use it for performance (memory & time) characteristics
(I don't know how you would use isWHNF to do /that/, but it's a
hypothetical possibility). In other words, if your program output is
correct with a fake nondeterministic replacement like
isWHNF x = do
b' <- getMemoizedValueFor x
if b' then return True else do
b <- randomIO
when b $ setMemoizedValueFor x True
return b
then it's safe, otherwise it's not. But similarly to a memoization
function implemented with unsafePerformIO
memoize :: Ord a => (a -> b) -> (a -> b)
you may well use the "not so nondeterministic" property of isWHNF to
achieve a time & space improvement.
Regards,
apfelmus
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
