On Sun, 2008-12-21 at 16:47 -0800, Ryan Ingram wrote:
The problem is that you are trying to return a mutable array out of an ST computation. This lets the "mutability" of the computation escape. That's what the "s" type variable is for; without it, runST is just unsafePerformIO.
I'm trying something similar now... I have defined a data type for mutable matrices as data STMatrix s a = STMatrix { elements :: Array Int (STArray s Int a) , nrows :: Int , ncols :: Int } and one for immutable matrices: data Matrix a = Matrix { elements :: Array Int (Array int a) , nrows :: Int , ncols :: Int } What I wanted was a way to freeze an STMatrix into a Matrix so that I could work with it out of the ST monad after doing all of the modifications I need on the elements. I came up with the following: doFreeze :: STMatrix s a -> ST s (Matrix a) doFreeze mat = do let m = nrows mat n = ncols mat rows <- foldM (freezeRow mat) [] [m-1,m-2..0] return $ listMatrix (m, n) rows where freezeRow mat rs i = do r <- unsafeFreeze (elements mat ! i) return (r:rs) where "listMatrix" builds a Matrix from a list of Arrays. However, when I this: freezeMatrix = runST . doFreeze I get the "less polymorphic than expected" error from ghc. I fail to see why though. Since "freezeRow" returns a list of immutable Arrays, where is the mutability of the computation escaping here? Thanks in advance, Andre