This is a functional logic language that has a purely declarative logic. In that sense, Mercury is related to Prolog and Haskell and, in fact, can be seen as a subset of the former, albeit with strong types and modes. 

Mercury is a functional logic programming language made for real-world uses. The first version was developed at the University of Melbourne, Computer Science department, by Fergus Henderson, Thomas Conway, and Zoltan Somogyi, under Somogyi's supervision, and released on April 8, 1995.