  We study the decidability of termination for two CHR dialects which,
similarly to the Datalog like languages, are defined by using a signature which
does not allow function symbols (of arity >0). Both languages allow the use of
the = built-in in the body of rules, thus are built on a host language that
supports unification. However each imposes one further restriction. The first
CHR dialect allows only range-restricted rules, that is, it does not allow the
use of variables in the body or in the guard of a rule if they do not appear in
the head. We show that the existence of an infinite computation is decidable
for this dialect. The second dialect instead limits the number of atoms in the
head of rules to one. We prove that in this case, the existence of a
terminating computation is decidable. These results show that both dialects are
strictly less expressive than Turing Machines. It is worth noting that the
language (without function symbols) without these restrictions is as expressive
as Turing Machines.

Logic in Computer Science