Re: [Haskell-beginners] Stymied by mutable arrays in the ST monad

On 25 May 2012 18:50, Matthew Moppett wrote:

I've been trying to use mutable arrays in the ST monad, and wrote out a little proof of concept function:

idST :: [Int] -> [Int] idST xs = runST $ do     array <- newListArray (1, (length xs)) xs     return (getElems array)

-- where idSt should be equivalent to id.

And I get the error message:

Couldn't match type `[Int]' with `Int'     In the return type of a call of `getElems'     In the first argument of `return', namely `(getElems array)'     In a stmt of a 'do' block: return (getElems array)

Obviously I'm making a very simple mistake here, but I can't seem to spot it. Can anyone offer some advice?

'getElems array' already has type 'ST s [Int]', you don't need another 'return'. Furthermore you need to help out with the type inference a little bit (it's similar to the read-show problem). This should work: idST :: [Int] -> [Int] idST xs = runST $ do array <- newListArray (1, (length xs)) xs :: ST s (STArray s Int Int) getElems array (You could also replace STArray by STUArray.)