Henning Thielemann wrote:

Currently, runSTArray can handle only one array:

runSTArray :: Ix i => (forall s . ST s (STArray s i e)) -> Array i e

Would it be possible to access multiple arrays successively by a function like

runSTArraySplit :: Ix i => (forall s . ST s (STArray s i e, a)) -> (Array i e, ST t a)

Hm, this would not work, because type variables 's' in 'a' had to be converted to 't' as well. Are there other ideas to tackle this problem?

I tried making a type class for this, namely class EvalST st pure | st -> pure where freezeST :: st s -> ST s pure together with evalST :: EvalST st pure => (forall s. ST s (st s)) -> pure evalST f = runST (f >>= freezeST) I'll attach the complete module. Feel free to use, modify, etc. Unfortunately, the extra 's' type argument to the 'st' constructor makes this a bit cumbersome to use. In particular I needed wrapper types for arrays and pairs. On the positive side, the approach allows dealing with pure values, boxed and unboxed arrays, and STRefs (not implemented) in a uniform way. Example: *EvalST> evalST (do a <- newArray (0,0) 0; return (STPair (STPure 1) (STArray' a))) :: (Int, Array Int Int) (1,array (0,0) [(0,0)]) Is there any way to make evalST (do a <- newArray (0,0) 0; return (STPure 1, a)) :: (Int, Array Int Int) work? That's the prettiest interface I can imagine right now. Bertram