Jan-Willem Maessen - Sun Labs East wrote:
[...] The actual problem is function arguments. GHC can't do worker-wrapper unless a function is strict. Often we end up with something like this:
data Tree a = Branch !Int a (Tree a) (Tree a)
lookfor :: Int -> Tree a -> Maybe a lookfor Leaf key = Nothing lookfor (Branch k v l r) key | key==k = Just v | key < k = lookfor l key | otherwise = lookfor r key
Here we don't use "key" in the first clause, so GHC isn't allowed to unbox. Most of the uses of Int# that I've seen in library code have been workarounds to this problem. [...]
If that's the only concern, there is no real problem. One can do: a) Help GHC a bit by hand with something like lookfor Leaf key | key == key = Nothing or lookfor Leaf key = key `seq` Nothing Not very nice, but a "traditional" way of improving strictness. b) Use GHC's -O2 flag, enabling specialization over constructors, which is exactly what you're looking for, see the "Game Plan" comment at the beginning of http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/specialise/SpecConstr.lhs?rev=1.20&content-type=text/x-cvsweb-markup Using "-O2 -ddump-simpl" with your (slightly corrected) example, one can see that the Ints are indeed unboxed during recursion. Cheers, S.