instances of class Stack are defined for s. empty's return type is s a 

empty does not take any arguments. GHCi is confused about empty's return type.

option 1:

try adding return type at prompt e.g.:

empty::ListStack Int

instead of just empty

option 2:
​add a as Stack class parameter i.e.:

class Stack s a where

   ...

then if you define only 1 instance, interactive will probably assume that you expect empty to return that (the only one defined) type

see if this works