[Haskell-cafe] Trouble with the ST monad
Andre Nathan
andre at digirati.com.br
Mon Dec 29 13:57:56 EST 2008
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
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