[Haskell-cafe] isWHNF :: a -> IO Bool ?

[Pepe Iborra]

> 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
>

Very true, isWHNF needs to follow indirections indeed. To decide  
whether to recurse, you probably intended to test for Indirections  
and not for Constructors:

> removingIndirections :: (forall c . c -> IO b) -> a -> IO b
> removingIndirections k a = do
>     closureData <- getClosureData a
>     if isIndirection (tipe closureData) then
>         removingIndirections (ptrs closureData ! 0)
>     else
>         k a

Thanks,
pepe