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?

> updated `genArray :: IO [Int]` to `genArrat :: ST s [Int]` — RnMss, Dec 23 '12 at 15:42

score 7 · Accepted Answer · answered Dec 23 '12 at 14:16

7

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

answered Dec 23 '12 at 14:16

Sjoerd Visscher

11,840
2
47
59

Oh I'm sorry, it's an `ST s [Int]` instead of `IO [Int]`. – RnMss Dec 23 '12 at 15:40

score 1 · Answer 2 · answered Dec 23 '12 at 14:11

1

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

answered Dec 23 '12 at 14:11

Alfonso Villén

373
1
9

Your answer solves this problem for good, which, however, I mistyped. Sorry for this. – RnMss Dec 23 '12 at 15:46

How to convert a STArray to a List in Haskell?

2 Answers2