Hi, I'm wondering how usually you parse command line arguments list safely. If the given argument is wrong, the program can still print out some error information instead of giving something like Prelude.read: no parse Let's make the discussion concrete. Suppose we have the following code. -->8----------8<-- import System.Environment data Conf = Conf { number :: Int } parseArgs [] = error "need at least one argument" parseArgs (x:_) = Conf { number = read x } work (Conf { number = n }) = n * (n-1) `div` 2 main = print . work . parseArgs =<< getArgs -->8----------8<-- If the "read x" fails in line 8, the program dies with Prelude.read: no parse What is the standard way to make the error message useful? Xiao-Yong -- c/* __o/* <\ * (__ */\ <