Hi, currently we have the following implementation for (!!): #ifdef USE_REPORT_PRELUDE xs !! n | n < 0 = error "Prelude.!!: negative index" [] !! _ = error "Prelude.!!: index too large" (x:_) !! 0 = x (_:xs) !! n = xs !! (n-1) #else -- HBC version (stolen), then unboxified xs !! (I# n0) | isTrue# (n0 <# 0#) = error "Prelude.(!!): negative index\n" | otherwise = sub xs n0 where sub :: [a] -> Int# -> a sub [] _ = error "Prelude.(!!): index too large\n" sub (y:ys) n = if isTrue# (n ==# 0#) then y else sub ys (n -# 1#) #endif I propose to change the error messages for the non-report version to include index and list length, something that is functionally equivalent to: xs !! (I# n0) | isTrue# (n0 <# 0#) = indexError xs n0 | otherwise = sub xs n0 where sub :: [a] -> Int# -> a sub [] _ = indexError xs n0 sub (y:ys) n = if isTrue# (n ==# 0#) then y else sub ys (n -# 1#) indexError :: [a] -> Int# -> b indexError xs (I# -> n) | n < 0 = error ("Prelude.(!!): negative index " ++ show n) | otherwise = error ("Prelude.(!!): index " ++ show n ++ " too large for list of length " ++ show (length xs)) Some usage examples: *Main> [1, 2, 3] !! (-1) *** Exception: Prelude.(!!): negative index -1 *Main> [1, 2, 3] !! 3 *** Exception: Prelude.(!!): index 3 too large for list of length 3 This will require some refactoring, i.e. we need to move itos from GHC.Show to e.g. GHC.Base. Discussion period: 2 weeks Cheers, Simon