Lazy ST is capable of returning values lazily. Not naively -- eg. if
you are writing elements to an STRef and then returning the contents
of the STRef at the end, then of course it will not return gradually
(who's to say that the last thing you do before you return isn't to
write [] to the STRef?)
However, if you do it this way:
import Control.Monad.ST.Lazy
import Data.STRef.Lazy
main = print $ runST work
where
work = do
ref <- newSTRef 0
let loop = do
x <- readSTRef ref
writeSTRef ref $! x+1
fmap (x:) loop
loop
You will find that it is perfectly lazy. You just have to communicate
that the computation *must* yield an element regardless of what the
remainder is. "fmap (x:) rest" is the typical way I yield elements
from lazy ST.
Luke
On Thu, Feb 24, 2011 at 7:55 PM, Sterling Clover
On Feb 24, 2011, at 3:45 PM, Andrew Coppin wrote:
OK, so I had a function that looks like
transform :: [Word8] -> [Word16]
It works nicely, but I'd like to use mutable state inside. No problem! Use the ST monad. Something like
transform :: [Word8] -> [Word16] transform xs = runST (work xs) where work :: [Word8] -> ST s [Word16]
Ah, yes, well there is one *small* problem... If you do that, the function becomes too strict.
unsafeInterleaveST? http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad-ST.... --Sterl _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe