Now if anyone could enlighten me about the "instance Monad Tracker a" instead of "instance Monad Tracker" part, everything will be clear!
A Monad always takes one type argument -- the a in IO a, Maybe a, etc. So Tracker can't be a Monad (it needs two arguments), but (Tracker a) is, for any a. This is basically partial application at the type level. Formally, we say a Monad needs to have "kind" * -> *. Kinds are like types for the type level. A kind of * indicates an actual type which can have values, etc. A kind of k1 -> k2 indicates a type operator which wants as an argument a type of kind k1, and will produce a type of kind k2. As with types, the kind arrow is right-associative and partial application is allowed. We can see the mismatch in the following (hypothetical) GHCi session: Prelude> :info Monad class Monad (m::* -> *) where ... Prelude> :kind Tracker Tracker :: * -> * -> * Prelude> :kind Tracker Bool Tracker Bool :: * -> * Hope that helps, keegan