[Haskell-cafe] Re: isWHNF :: a - IO Bool ?
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 let x = (map succ [0..]) in do putStrLn . show (isWHNF x)-- False putStrLn . show . head $ x putStrLn . show (isWHNF x)-- True putStrLn . show (isWHNF (Just undefined)) -- True Note that this function is not referentially transparent since isWHNF 2 = True but isWHNF (1+1) = False although 1+1 = 2. In other words, it messes up the language semantics (extensional equality) which is bad. Regards, apfelmus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: isWHNF :: a - IO Bool ?
On Thu, Sep 27, 2007 at 05:31:51PM +0200, apfelmus wrote: 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 let x = (map succ [0..]) in do putStrLn . show (isWHNF x)-- False putStrLn . show . head $ x putStrLn . show (isWHNF x) -- True putStrLn . show (isWHNF (Just undefined)) -- True Note that this function is not referentially transparent since isWHNF 2 = True but isWHNF (1+1) = False although 1+1 = 2. In other words, it messes up the language semantics (extensional equality) which is bad. Indeed. Does it still mess up with the result in IO Bool (as was my intent)? Ah, I do realise my example use case above needs some ='s inserting into it which may have led to some confusion. Tris -- Tristan Allwood PhD Student Department of Computing Imperial College London ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: isWHNF :: a - IO Bool ?
apfelmus [EMAIL PROTECTED] writes: 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 let x = (map succ [0..]) in do putStrLn . show (isWHNF x)-- False putStrLn . show . head $ x putStrLn . show (isWHNF x)-- True putStrLn . show (isWHNF (Just undefined)) -- True Note that this function is not referentially transparent since isWHNF 2 = True but isWHNF (1+1) = False although 1+1 = 2. In other words, it messes up the language semantics (extensional equality) which is bad. Isn't it OK if it's a - IO Bool ? (Admittedly, the test example above is wrong in that case). -- Jón Fairbairn [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: isWHNF :: a - IO Bool ?
On Thu, 2007-09-27 at 16:57 +0100, Tristan Allwood wrote: On Thu, Sep 27, 2007 at 05:31:51PM +0200, apfelmus wrote: 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 let x = (map succ [0..]) in do putStrLn . show (isWHNF x)-- False putStrLn . show . head $ x putStrLn . show (isWHNF x) -- True putStrLn . show (isWHNF (Just undefined)) -- True Note that this function is not referentially transparent since isWHNF 2 = True but isWHNF (1+1) = False although 1+1 = 2. In other words, it messes up the language semantics (extensional equality) which is bad. Indeed. Does it still mess up with the result in IO Bool (as was my intent)? In IO this should be fine, as IO is explicitly a non-determinism monad (along with everything else). jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: isWHNF :: a - IO Bool ?
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
