Quoted: However, if you want to use unboxed arrays, you need to do something like: instance MArray IOUArray Foo IO where unsafeRead (arr :: IOUArray i Foo) i = do (arr2 :: IOUArray i Int) <- castIOUArray arr v1 <- unsafeRead arr2 (i*2) v2 <- unsafeRead arr2 (i*2+1) return (Foo v1 v2) End quoting I understand you just create one large array with the elements v1 and v2 next to eachother (I 'd never thought it would be that easy :) ). There is only a small problem: there is no such function as unsafeRead or unsaferead, at at least not in GHC (I saw unsafeThaw, but that also didn't work). P.S. I just turned on the fglasgow-exts, but I would preferabily do it without any extensions, how can I rewrite the same code, without the extensions? Greets Ron __________________________________ Do you Yahoo!? The New Yahoo! Search - Faster. Easier. Bingo. http://search.yahoo.com