Hi all,
the problem is simple but i can't do it :
I want to generate some values and 'accumulate' them. The meaning of
'accumulating' is not so important. The only point is that to
'accumulate' the values, I only have to know one value and the
accumulator (and not all the values).
The most simple example is that of adding a list of numbers.
I thought that I could use a haskell list to represent the values
so I could use foldr or mapM to do the accumulation (and not an
imperative-style loop).
Below are four attempts to solve the problem. It's not solved : in
ghci, I have '*** Exception: stack overflow' with 'test1 5000000'.
5000000 is not so much for my application.
How can I achieve what I want (and efficiently)?
---------------------------------------------------------------------}
import Control.Monad.State
import Control.Monad.ST
import Data.STRef
acc1, acc2, acc3 :: [Int] -> Int
----------------------------------------------------------------------
acc1 ints = foldr (+) 0 ints
----------------------------------------------------------------------
acc2 ints = execState (mapM add2 ints) 0
add2 :: Int -> State Int ()
add2 i = do
acc <- get
put (acc + i)
----------------------------------------------------------------------
acc3 ints = runST (acc3' ints :: ST s Int)
acc3' ints = do
accRef <- newSTRef 0
mapM (add3 accRef) ints
acc <- readSTRef accRef
return acc
add3 accRef i = do
modifySTRef accRef (+ i)
----------------------------------------------------------------------
test4' n = do
let g = gen n
accRef <- newSTRef 0
sRef <- newSTRef 1
acc <- acc4 g sRef accRef
return acc
acc4 g sRef accRef = do
ret <- g sRef
case ret of
Nothing -> do acc <- readSTRef accRef
return acc
Just i -> do add3 accRef i
acc4 g sRef accRef
gen n sRef = do
s <- readSTRef sRef
let ret = if s > n then Nothing else Just 1
writeSTRef sRef (s + 1)
return ret
----------------------------------------------------------------------
test1 n = acc1 $ replicate n 1
test2 n = acc2 $ replicate n 1
test3 n = acc3 $ replicate n 1
test4 n = runST (test4' n :: ST s Int)
Thanks a lot,
VO Minh Thu
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