Check which information of instances needs to be considered
Currently, the instance environment contains all implemented instance methods along with their arities. However, it seems as if the information about the arity is no longer needed. Only the interface check seems to compare the arities. Furthermore, there is a remark in the deriving check and the deriving transformation stating that the later generated instances have to match the arity entered in the instance environment. It is not clear whether this is still true.
Considering that the implemented methods in an instance do not affect the interface of a module, it might be unnecessary at all to save the implemented methods in the instance environment.