Julien Oster wrote:
Now if anyone could enlighten me about the "instance Monad Tracker a" instead of "instance Monad Tracker" part, everything will be clear!
Hello Julien, The way you defined it, Tracker is a type constructor with two arguments, both of which are types; another way to state this is to say that Tracker has kind (* -> * -> *). However, a monad is a type constructor with kind * -> *, i.e. it accepts only one type argument. This can be seen from the class definition, where m is always used with one argument (i.e. 'm a' or 'm b' and a and b are types). Partially applying Tracker to one argument ('T a') gives you a type constructor that has only one remaining 'open' argument and thus can be made an instance of class Monad. HTH, Ben