Mon Dec 29 14:19:33 EST 2008

```(I'm kinda a newbie, take my explanation with a grain of salt:)

The problem is that you're trying to take a STMatrix from some other
ST computation and freeze it in a new ST computation. The isolation
between separate computations is done via the rank-2 type variable "s"
in all those ST functions.

freezeMatrix :: forall s. STMatrix s a -> Matrix a
freezeMatrix = runST . freezeMatrix -- does not unify

runST :: (forall s. ST s a) -> a

Which is trying to unify the type variable "s" from the STMatrix you
pass in with the explicitly polymorphic "s" in runST -- Note the
parentheses -- these are different "s"s and cannot be unified

Try this:

freezeMatrix :: (forall s . STMatrix s a) -> Matrix a
freezeMatrix f :: runST (freezeMatrix f)

Also, instead of using an array of arrays, maybe an array with (Int,
Int) as the Ix might be a bit smoother?

-Ross

Here is a working version:

{-# LANGUAGE Rank2Types #-}

import Data.Array
import Data.Array.ST

data STMatrix s a = STMatrix
{ stm_elements :: Array Int (STArray s Int a)
, stm_nrows    :: Int
, stm_ncols    :: Int
}

data Matrix a = Matrix
{ m_elements :: Array Int (Array Int a)
, m_nrows    :: Int
, m_ncols    :: Int
}

listMatrix :: (Int, Int) -> [Array Int a] -> Matrix a
listMatrix (n,m) rs = Matrix { m_elements = listArray (0, length rs -
1) rs
, m_nrows = m
, m_ncols = n }

doFreeze :: STMatrix s a -> ST s (Matrix a)
doFreeze mat = do
let m = stm_nrows mat
n = stm_ncols mat
rows <- foldM (freezeRow mat) [] [m-1,m-2..0]
return \$ listMatrix (m, n) rows
where
freezeRow mat rs i = do
r <- unsafeFreeze (stm_elements mat ! i)
return (r:rs)

freezeMatrix :: (forall s. STMatrix s a) -> Matrix a
freezeMatrix f = runST (doFreeze f)

On Dec 29, 2008, at 1:57 PM, Andre Nathan wrote:

> 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?
>