On 10 October 2004 02:01, Peter Simons wrote:

in a module I am writing, I am using a 'StateT st IO' monad with a state like this:

data MyState st = ST !Int !st

My own monad is yet-another wrapper for ... another state monad. And that's getting inconvenient.

So I wondered whether it would be good to define a class that unified all those StateTs into _one_ state, like:

...

class KnowsMyStuff a where foo :: a -> Int bar :: a -> Float etc :: a -> [String]

Then I could write my functions so that they'd work on any MonadIO which has some way of getting those instances defined.

If I did that, how much performance would I lose? Accessing those values would probably require one more level of indirection, that can't be as fast as having a specific data type, right?

Or is there some optimizer magic at work here?

If your code is using a method from a known instance of a class, and the method has a small enough definition, then chances are good that it'll get inlined and the extra layer of abstraction will dissappear. I'm guessing this is the case with your example. If performance is important, then you should probably check the simplifier output with -ddump-simpl to make sure that the code is what you expect. We always enjoy investigating benchmarks, so if you have some code you think isn't going fast enough, then post it here and we'll help you find out where the performance problems are. Cheers, Simon