On Sep 27, 2007, at 9:14 AM, Pepe Iborra wrote:
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.
Very cool. This is much nicer than when I asked much the same question a few years back (and I can think of all sorts of interesting things I can learn from the interface in that module). But what about indirection chasing? Surely we want isWHNF to return True if we have an indirection to a WHNF. Possibly one wants something a bit like this (untested, and rather depends on GHC's indirection semantics): removingIndirections :: (forall c . c -> IO b) -> a -> IO b removingIndirections k a = do closureData <- getClosureData a if isConstr (tipe closureData) then removingIndirections (ptrs closureData ! 0) else k a simpleIsWHNF :: a -> IO Boolean simpleIsWHNF = fmap (isConstr . tipe) . getClosureData isWHNF = removingIndirections simpleIsWHNF -Jan-Willem Maessen