[Haskell-cafe] Re: isWHNF :: a -> IO Bool ?
jon.fairbairn at cl.cam.ac.uk
Thu Sep 27 11:59:47 EDT 2007
apfelmus <apfelmus at quantentunnel.de> 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
> 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 Jon.Fairbairn at cl.cam.ac.uk
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