Re: [Haskell-cafe] Trouble with the ST monad

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.

Thanks! If only I knew that was the problem... It wouldn't have costed me the whole afternoon :P Is there any difference between using freeze/thaw from Data.Array.MArray versus freezeSTArray/thawSTArray from GHC.Arr? Best, Andre