Bugfix clause/2: TypeError

The presence of Var within the unify method caused TypeErrors.

Author: VincentDerk

problog/clausedb.py

@@ -564,6 +564,29 @@ def _compile(self, struct, variables=None, scope=None):
         else:
             raise ValueError("Unknown structure type: '%s'" % struct)
 
+    def _create_ints(self, term):
+        """
+        Convert all occurrences of Var into ints.
+
+        For example, when term = Term('a', Var('V_1'), Constant(3)), this returns Term('a', 1, Constant(3))
+        This assumes each Var has functor V_{var_id} with var_id some int value.
+        This is opposite to the method _create_vars(term).
+        :param term: The term to replace all Var occurrences in.
+        :type term: Term
+        :return: The same term but with all occurrences of Var replaced with their corresponding int value.
+        :rtype: Term
+        """
+        if type(term) == Var:
+            assert term.functor[:2] == "V_"
+            # Convert V_{digit} to an int of value {digit}
+            return int(term.functor[2:])
+        else:
+            args = [self._create_ints(arg) for arg in term.args]
+            term = term.with_args(*args)
+            if term.probability is not None:
+                term = term.with_probability(self._create_ints(term.probability))
+            return term
+
     def _create_vars(self, term):
         if type(term) == int:
             return Var("V_" + str(term))

problog/engine_builtin.py

@@ -268,8 +268,10 @@ def _builtin_clause(head, body, database=None, **kwd):
             b = Term("true")
 
         try:
-            unify_value(head, h, {})
-            unify_value(body, b, {})
+            # Perform unifying checks before storing the results
+            # can't do unify_value with Vars, first replace them with ints (database._create_ints)
+            unify_value(head, database._create_ints(h), {})
+            unify_value(body, database._create_ints(b), {})
             result.append((h, b))
         except UnifyError:
             pass

test/clause_pred.pl

@@ -0,0 +1,11 @@
+%Expected outcome:
+% clause(a,b(V_0))  1
+% clause(c(d(V_0)),b(V_0))  1
+
+a :- b(X).
+
+query(clause(a,b(Y))).
+
+c(d(X)) :- b(X).
+
+query(clause(c(X),Y)).

test/cut_2.pl

@@ -0,0 +1,18 @@
+%Expected outcome:
+% ism(2,3,3)  1
+% ism(2,3,2)  1
+% ism(6,1,6)  1
+% ism(6,1,1)  0
+
+:- use_module(library(cut)).
+
+% if X =< Y, then Z = Y, otherwise Z = X
+m(1, X,Y,Z):- X =< Y, Z = Y.
+m(2, X,Y,X).
+
+ism(X, Y, Z) :- cut(m(X,Y,Z)).
+
+query(ism(2,3,Z)).
+query(ism(2,3,2)).
+query(ism(6,1,6)).
+query(ism(6,1,1)).