Hello, the following code doesn't compile <snip> module Matrix where import Control.Monad.ST import Data.Array.ST import Data.Array.Unboxed type Block s = STUArray s Int Double data MMatrix s = MMatrix Int Int (Block s) newMatrix_ :: Int -> Int -> ST s (MMatrix s) newMatrix_ m n = do b <- newArray_ (1, m*n) return (MMatrix m n b) runMatrix = do _A <- newMatrix_ 3 3 _B <- newMatrix_ 3 3 matMul _A _B return "Success" main = show $ runST runMatrix matMul :: MMatrix s -> MMatrix s -> ST s (MMatrix s) --matMul a b = do let foo = 2*5 --return a matMul a b = do { let foo = 2*5; return a } </snip> under ghc 6.4.1, yielding the error message: question.hs:25:41: parse error on input `<-' Failed, modules loaded: none. The offending is line the one containing "let foo = 2*5", which is a little test I've done of let-clauses. Now, suppose instead that for the last function, matMul, I replace the version that's commented out. No errors! Could someone enlighten me as to why? I'm a bit confused, as I thought the two forms are equivalent save for formatting... This is on the back of a email discussion that I was reading about let-clauses, in which someone declared that they where better than where clauses for monadic code. If anyone could comment on this, I'd appreciate it as well. Many thanks in advance, Martin