Hello all, I have a pure function which uses immutable arrays from Data.Array, but it spends about 95% of its time doing array updates. The arrays are used in a single-threaded manner (no need for the old values after an array update), and the arrays are not returned; just one of the elements. So I want to convert it to use STArray, to see if there's a performance gain, but it's not clear how I should tie everything together. I'm believe I don't want to use runSTArray, because I'm not interested in getting an array back from the pure function - just one of the elements. Here's an contrived example: module Main where import Data.Array.ST import Control.Monad.ST main = print (compute 5) compute :: Int -> Int compute n = runST ( do arr <- newArray (-1, 1) n readArray 1 arr ) to which GHC responds: No instance for (MArray a Int (ST s)) arising from use of `readArray' at test.hs:11:4-12 Probable fix: add (MArray a Int (ST s)) to the expected type of an expression or add an instance declaration for (MArray a Int (ST s)) In the result of a 'do' expression: readArray 1 arr In the first argument of `runST', namely `(do arr <- newArray (- 1, 1) n readArray 1 arr)' In the definition of `compute': compute n = runST (do arr <- newArray (- 1, 1) n readArray 1 arr) Alistair ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************