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