diff --git a/biscuit-auth/src/datalog/expression.rs b/biscuit-auth/src/datalog/expression.rs
index d8d9b74b..314bcbcb 100644
--- a/biscuit-auth/src/datalog/expression.rs
+++ b/biscuit-auth/src/datalog/expression.rs
@@ -1141,6 +1141,15 @@ mod tests {
             Op::Value(Term::Integer(42)),
             Op::Unary(Unary::Ffi("test_un".to_owned())),
             Op::Binary(Binary::And),
+            Op::Value(Term::Integer(42)),
+            Op::Unary(Unary::Ffi("test_closure".to_owned())),
+            Op::Binary(Binary::And),
+            Op::Value(Term::Str(i)),
+            Op::Unary(Unary::Ffi("test_closure".to_owned())),
+            Op::Binary(Binary::And),
+            Op::Value(Term::Integer(42)),
+            Op::Unary(Unary::Ffi("test_fn".to_owned())),
+            Op::Binary(Binary::And),
         ];
 
         let values = HashMap::new();
@@ -1172,7 +1181,29 @@ mod tests {
                 }
             })),
         );
+        let closed_over_int = 42;
+        let closed_over_string = "test".to_string();
+        extern_funcs.insert(
+            "test_closure".to_owned(),
+            ExternFunc::new(Rc::new(move |left, right| match (&left, &right) {
+                (builder::Term::Integer(left), None) => {
+                    Ok(builder::boolean(*left == closed_over_int))
+                }
+                (builder::Term::Str(left), None) => {
+                    Ok(builder::boolean(left == &closed_over_string))
+                }
+                _ => {
+                    println!("{left:?}, {right:?}");
+                    Err("expecting a single integer".to_string())
+                }
+            })),
+        );
+        extern_funcs.insert("test_fn".to_owned(), ExternFunc::new(Rc::new(toto)));
         let res = e.evaluate(&values, &mut tmp_symbols, &extern_funcs);
         assert_eq!(res, Ok(Term::Bool(true)));
     }
+
+    fn toto(_left: builder::Term, _right: Option<builder::Term>) -> Result<builder::Term, String> {
+        Ok(builder::Term::Bool(true))
+    }
 }