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
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