Dominic Steinitz wrote:
If you arrange the types to try to do all the operations inside the IO monad you can't chain together more than 1 binary operation. eg.
do S <- A + B Z <- Q * S
vs
do S <- Q * (A + B)
Are there any suggestions for this dilemma? Am I using the wrong monad for this task?
I'm not sure if this is what you are asking but isn't liftM2 or some variant what you need?
Dominic.
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OK so check out what really happens with liftM2. Suppose I have an IO containing an involved matrix computation called s. For simplicity we might assume that s :: IO (Int) and the Int is an index into an array containing a bunch of matrices in C land. Assume that s is determined by a succession of many IO operations that have lots of side effects and are fairly computationally intensive. Also assume that s is unevaluated. Now do an operation like q = liftM2 MultMatrix s s What happens is that s is 'evaluated' twice when q is evaluated e.g. do qint <- q That becomes evident when we look at liftM2's definition liftM2 f = \a b -> do { a' <- a; b' <- b; return (f a' b') } the statements a' <- a and b' <- b will cause s to be evaluated twice. Therein lies my problem. -- View this message in context: http://www.nabble.com/Sequencing-Operations-in-a-Monad-tf4446788.html#a12687... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.