Fix order of variables in bindings

src/machine/lib_machine.rs

@@ -90,8 +90,10 @@ impl Iterator for QueryState<'_> {
 
         let mut bindings: BTreeMap<String, Value> = BTreeMap::new();
 
-        for (var_key, term_to_be_printed) in &term_write_result.var_dict {
-            let var_name = var_key.to_string();
+        let var_dict = &term_write_result.var_dict;
+
+        for (var_key, term_to_be_printed) in var_dict.iter() {
+            let mut var_name = var_key.to_string();
             if var_name.starts_with('_') {
                 let should_print = var_names.values().any(|x| match x.borrow().clone() {
                     Var::Named(v) => v == var_name,
@@ -102,15 +104,32 @@ impl Iterator for QueryState<'_> {
                 }
             }
 
-            let term = Value::from_heapcell(machine, *term_to_be_printed, &mut var_names.clone());
+            let mut term =
+                Value::from_heapcell(machine, *term_to_be_printed, &mut var_names.clone());
 
             if let Value::Var(ref term_str) = term {
-                if *term_str == var_key.to_string() {
+                if *term_str == var_name {
                     continue;
                 }
+
+                // Var dict is in the order things appear in the query. If var_name appears
+                // after term in the query, switch their places.
+                let var_name_idx = var_dict
+                    .get_index_of(&VarKey::VarPtr(Var::Named(var_name.clone()).into()))
+                    .unwrap();
+                let term_idx =
+                    var_dict.get_index_of(&VarKey::VarPtr(Var::Named(term_str.clone()).into()));
+                if let Some(idx) = term_idx {
+                    if idx < var_name_idx {
+                        let new_term = Value::Var(var_name);
+                        let new_var_name = term_str.into();
+                        term = new_term;
+                        var_name = new_var_name;
+                    }
+                }
             }
 
-            bindings.insert(var_key.to_string(), term);
+            bindings.insert(var_name, term);
         }
 
         // NOTE: there are outstanding choicepoints, backtrack
@@ -792,4 +811,22 @@ mod tests {
             ]))
         );
     }
+
+    #[test]
+    #[cfg_attr(miri, ignore)]
+    fn order_of_variables_in_binding() {
+        let mut machine = Machine::new_lib();
+
+        let result = machine.run_query("X = Y, Z = W.".into());
+
+        assert_eq!(
+            result,
+            Ok(QueryResolution::Matches(vec![QueryMatch::from(
+                btreemap! {
+                    "X" => Value::Var("Y".into()),
+                    "Z" => Value::Var("W".into()),
+                }
+            ),]))
+        );
+    }
 }