Actually, in 6.8 we can build isWHNF on top of the GHC-API. First, you need to import the ghc package:
ghci -package ghc GHCi, version 6.7: http://www.haskell.org/ghc/ :? for help
Then, you can define the isWHNF function as follows:
Prelude> :m +RtClosureInspect Prelude RtClosureInspect> let isWHNF = fmap (isConstr . tipe) . getClosureData
Prelude RtClosureInspect> :t isWHNF isWHNF :: a -> IO Bool
What the code above does is to inspect the info table associated to the value given, and check if the closure is a Constructor closure, i.e. in WHNF. We can put it to test now:
Prelude RtClosureInspect> let a = [1..10] Prelude RtClosureInspect> isWHNF a False Prelude RtClosureInspect> seq a () () Prelude RtClosureInspect> isWHNF a True
As a bonus because this code is included in GHC itself it should stay in sync with any changes in the GHC internal representations. Cheers pepe On 27/09/2007, at 14:51, Bernie Pope wrote:
Hi Tristan,
I've implemented it for earlier versions of GHC, by calling some C code which then peeps at the internal representation of a value.
From memory, I needed to pass a stable pointer to the value to the C code, so that it can be polymorphic, without having to make it a primitive in GHC.
Have a look at the "reify" code on this page: http:// www.cs.mu.oz.au/~bjpop/code.html - its more than what you want, but you can trim it down easily. Let me know if you get stuck.
The internal representation in GHC tends to change between releases, so it might need a bit of polishing up.
Cheers, Bernie.
On 27/09/2007, at 10:07 PM, Tristan Allwood wrote:
Hi,
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
If not, would it be hard/easy/possible to implement on-top-of or using GHC? I'm happy (if it's possible) to have a stab at implementing it myself, so any pointers to right directions would be helpful.
I'm thinking it could be useful to allow creation of sparse-check [1] like libraries without needing a separate logic encoding, or things along those lines / in that area.
Cheers,
Tris
[1] http://www-users.cs.york.ac.uk/~mfn/sparsecheck/index.html#lim
-- 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
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