Hi Michael, On 25 Jun 2007, at 06:39, Michael T. Richter wrote:
do x <- performActionA y <- performActionB z <- performActionC return $ calculateStuff x y z
I don't know about you're exact example, but here's what I'd do. Control.Monad has functions like when, unless, and guard that you can use to check whether the "precondition" holds. I find an "ifM" combinator quite useful sometimes: ifM :: Monad m => m Bool -> m a -> m a -> ma ifM cond thenBranch elseBranch = do b <- cond if cond then thenBranch else elseBranch If everything checks out, you can then execute your A, B, and C actions. I don't think you really want arrows here. The right idiom is applicative functors (see Control.Applicative). You could then write the above as: calculateStuff <$> x <*> y <*> z Hope this helps, Wouter