Chad.Scherrer:
> Hi,
>
> I'd like to be able to do replicateM, but over a vector instead of a list.
> Right now I'm doing this:
>
> import qualified Data.Vector.Generic as G
> import qualified Data.Vector.Unboxed.Mutable as M
> replicateM n action = do
> mu <- M.unsafeNew n
> let go !i | i < n = action >>= M.unsafeWrite mu i >> go (i+1)
> | otherwise = G.unsafeFreeze mu
> go 0
>
> I thought it might be useful to express this in terms of the available
> primitives, since this might fuse more easily, but I don't yet see a
> way to do it.
>
> Is there a better (more elegant and/or faster) way to write this?
>
The way you're writing it is fine and direct, but yes, I think the
challenge is to have have monadic generators that will fuse.
The direct style, I'd write as:
import qualified Data.Vector.Generic as G
import qualified Data.Vector.Generic.Mutable as M
import qualified Data.Vector.Generic.New as N
import qualified Data.Vector.Fusion.Stream.Monadic as S
replicateM :: (G.Vector v a) => Int -> IO a -> IO (v a)
replicateM n a = do
v <- M.new n
fill v 0
G.unsafeFreeze v
where
fill v i
| i < n = do
x <- a
M.unsafeWrite v i x
fill v (i+1)
| otherwise = return ()
But that doesn't fuse, obviously. So it seems we should be able to do something
like:
-- Almost there:
replicateS n a = {-unsafeFreeze-} (M.unstream (S.replicate n a))
Or use Data.Vector.Generic.New, but I couldn't get the monadic action to work
out, ending up in a twisty maze of PrimMonads.
Roman? Can we generate frozen arrays for monadic generators, and still fuse in
the current New/Mutable/MStream architecture?
-- Don
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