Haskell make (class a) instance of other class?

The problem with your fromInteger is that I needs to be converted to a Val a .

The issue with the instance definition of Num is that you say you are showing how any ExprTree is an instance of Num (meaning for any type a) but instead you show how ExprTree Integer is an instance of Num. In other words, you can't specialize that type variable if unless you want to declare your instance like this.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.