Yitzchak Gale
I tend to use Control.Monad.State for stateful stuff like this. It is found in the mtl package http://hackage.haskell.org/package/mtl-2.0.1.0
I agree that this is a good approach.
If the only state you need is this, it might be easier just to add the counter as an additional parameter to your function, and return its incremented value as part of your return type. For example, you can return a tuple (v, n) where v is the value you were returning before and n is the updated counter.
If you also need other kinds of state, or if adding the counter complicates the types throughout your program, then the State monad is the way to go.
I agree that this is a good approach for this particular case, if the counter is really attached to the given computation and is not interesting elsewhere. However for state in general there is an approach, which is often forgotten: implicit configurations. This allows you to pull state around without mentioning it explicitly in any way, somewhat related to the ImplicitParams extension, but saner and with a sound theoretical foundation. The advantage is that the state is encapsulated in a type class and thus you can have parallel state threads, invisibly pulling around state attached to certain type constructors. The paper "Functional Pearl: Implicit configurations" by Oleg Kiselyov and Chung-chieh Shan introduces you to this abstraction. Greets -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/