I also recently implemented a Fisher-Yates shuffle in mutable vectors.
I probably should have used Gen (from mwc-random) in ST, but I already
had a GenIO hanging around for other reasons.
------------------------------------------------------------------------------
import Control.Monad.ST (unsafeIOToST)
import Data.Vector (Vector)
import qualified Data.Vector as V
import qualified Data.Vector.Mutable as MV
import System.Random.MWC
shuffle :: GenIO -> Vector k -> Vector k
shuffle rng v = if V.null v then v else V.modify go v
where
!n = V.length v
go mv = f (n-1)
where
-- note: inclusive
pickOne b = unsafeIOToST $ uniformR (0,b) rng
swap = MV.unsafeSwap mv
f 0 = return ()
f !k = do
idx <- pickOne k
swap k idx
f (k-1)
------------------------------------------------------------------------------
G.
On Sun, Apr 24, 2011 at 5:21 AM, Globules
brad clawsie
writes: hi all
i was wondering if anyone could post some minimal examples on using mutable Vectors in the ST monad. i've been digging around in the usual places but haven't been able to find anything to get me over the hump
thanks in advance brad
I was just looking into the same thing. This link at Rosetta Code has a list shuffling function, which is my first experiment with mutable Vectors:
http://rosettacode.org/wiki/Balanced_brackets#Haskell
The list is converted to a mutable Vector, the mapM_ performs a series of element swaps, then the result is frozen and converted back to a list.
- Globules
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Gregory Collins