How to convert a STArray to a List in Haskell?

Question

Is there a function can do what the function arrayToList do:

import Data.Array.ST
import Control.Monad.ST

genArray :: ST s [Int]
genArray = do
   a <- new Array (0, 99) 0 :: ST s (STArray s Int Int)
   writeArray a 0 1
   {- ... write something to the array ... -}
   return arrayToList(a)

If not, how to write one?

Answer 1

You don't need IO for this, constructing a list is a pure operation:

genArray :: [Int]
genArray = runST $ do
  a <- newArray (0, 99) 0 :: ST s (STArray s Int Int)
  writeArray a 0 1
  {- ... write something to the array ... -}
  getElems a

Answer 2

Use stToIO and getElems:

genArray :: IO [Int]
genArray = stToIO $ do
    a <- newArray (0,99) 0 :: ST s (STArray s Int Int)
    writeArray a 0 1
    getElems a