I had similar experiences as you when attempting to write "high
performance Haskell"; the language makes you want to use high-level
abstracted functions but the optimizer (while amazing, to be honest)
seems to miss a few cases that it seems like it should hit.
The problem seems to be that the compiler is extremely good at
optimizing systems-level code, but that any "control-structure"
function needs to be extremely inlined to be successful. You might
try re-writing "sequence" or "foldM" a few different ways using the
same "test" function and see what you can get.
One thing that I notice about this code is that if you switch Right
and Left you will get the default behavior for the Either monad:
worldSceneSwitch point = case redSphere (0,50,0) 50 point of
v@(Left _) -> v
Right d1 -> case greenSphere (25,-250,0) 50 point of
v@(Left _) -> v
Right d2 -> Right $ d1 `min` d2
is the same as
worldSceneSwitch point = do
d1 <- redSphere (0, 50, 0) 50 point
d2 <- greenSphere (25, -250, 0) 50 point
Right (d1 `min` d2)
Here is the same concept using foldM:
minimumM (x : xs) = do
v0 <- x
foldM (return . min) v0 xs
worldSceneSwitch point = minimumM [
redSphere (0, 50, 0) 50 point,
greenSphere (25, -250, 0) 50 point
]
However, the performance here will be terrible if minimumM and foldM
do not get sufficiently inlined; you do not want to be allocating list
thunks just to execute them shortly thereafter.
-- ryan
On Sat, Jul 19, 2008 at 6:48 AM, Mitar
Hi!
I had to change code somewhat. Now I have a function like:
worldScene point = case redSphere (0,50,0) 50 point of v@(Right _) -> v Left d1 -> case greenSphere (25,-250,0) 50 point of v@(Right _) -> v Left d2 -> Left $ d1 `min` d2
(Of course there could be more objects.)
Any suggestion how could I make this less hard-coded? Something which would take a list of objects (object functions) and then return a Right if any object function return Right or a minimum value of all Lefts. But that it would have similar performance? If not on my GHC version (6.8.3) on something newer (which uses fusion or something). Is there some standard function for this or should I write my own recursive function to run over a list of object functions? But I am afraid that this will be hard to optimize for compiler.
(It is important to notice that the order of executing object functions is not important so it could be a good candidate for parallelism.)
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