diff --git a/example/formula_evaluation_example.dart b/example/formula_evaluation_example.dart
index 068c699..0b60770 100644
--- a/example/formula_evaluation_example.dart
+++ b/example/formula_evaluation_example.dart
@@ -37,8 +37,8 @@ void main() {
     print('   Mass: 10.0 kg');
     print('   Acceleration: 9.8 m/s²');
     print('   Calculated Force: $force N');
-    print('   Output variable: ${evaluator.getOutputVariableName(newtonFormula)}');
-    print('   Output magnitude: ${evaluator.getOutputVariableMagnitude(newtonFormula)}');
+    print('   Output variable: ${newtonFormula.output.name}');
+    print('   Output magnitude: ${newtonFormula.output.unit}');
   } catch (e) {
     print('   Error: $e');
   }
diff --git a/lib/ai/formula_screen.dart b/lib/ai/formula_screen.dart
index 81d82ad..7b12262 100644
--- a/lib/ai/formula_screen.dart
+++ b/lib/ai/formula_screen.dart
@@ -105,11 +105,11 @@ class _FormulaScreenState extends State<FormulaScreen> {
         // Convert input to base unit if needed
         // Always convert from dropdown unit to variable's base unit
         late final convertedValue;
-        if( value is Number ) {
+        if( value is Number && input.unit != null ) {
           convertedValue = widget.corpus.convert(
             value,
             _selectedUnits[input.name]!,
-            input.unit,
+            input.unit as String,
           );
         }
         else{
@@ -124,11 +124,17 @@ class _FormulaScreenState extends State<FormulaScreen> {
       final result = evaluator.evaluate(widget.formula, inputValues);
 
       // Convert output to selected unit if needed
-      _result = widget.corpus.convert(
-        result,
-        widget.formula.output.unit,
-        _selectedOutputUnit!,
-      ).toStringAsFixed(2);
+      String? unit = widget.formula.output.unit;
+      if( unit != null ) {
+        _result = widget.corpus.convert(
+          result,
+          unit,
+          _selectedOutputUnit!,
+        ).toStringAsFixed(2);
+      }
+      else{
+        _result = result;
+      }
 
       //print( "_evaluateFormula: result:${result} _result:${_result}");
 
diff --git a/lib/corpus.dart b/lib/corpus.dart
index c9c724f..0710d8b 100644
--- a/lib/corpus.dart
+++ b/lib/corpus.dart
@@ -34,7 +34,7 @@ class Corpus{
 
       if( checkUnits ){
         for( final inputVar in formula.input + [formula.output] ){
-          if( !_allUnits.containsKey(inputVar.unit) ){
+          if( inputVar.unit != null && !_allUnits.containsKey(inputVar.unit) ){
             throw ArgumentError( "Unit not found: ${inputVar.unit}");
           }
         }
@@ -71,7 +71,10 @@ class Corpus{
     }
   }
 
-  List<String> unitsOfSameMagnitude(String unit){
+  List<String> unitsOfSameMagnitude(String? unit){
+    if( unit == null ){
+      return ["unitless"];
+    }
     final base = getUnit(unit).baseUnit;
     return _baseToUnits[base] as List<String>;
   }
@@ -113,7 +116,7 @@ class Corpus{
     }
 
     final ret = _convertUsingCode(x, unit.codeFromUnitToBase as String);
-    return ret as Number;
+    return ret;
   }
 
   Number _convertFromBase(Number x, String toUnit) {
@@ -128,7 +131,7 @@ class Corpus{
     }
 
     final ret = _convertUsingCode(x, unit.codeFromBaseToUnit as String);
-    return ret as Number;
+    return ret;
   }
 
 
@@ -140,17 +143,19 @@ class Corpus{
       final ret = _evaluate(completeSourceExpression);
       return ret;
     }
-    catch(e1,stack){
+    catch(e1, stack1){
       try{
         completeSourceStatement = _converterFromCodeStringAsStatement(x, code);
         final ret = _evaluate(completeSourceStatement);
         return ret;
       }
-      catch( e2, stack ){
+      catch( e2, stack2 ){
         print(completeSourceExpression);
         print(e1);
+        print(stack1);
         print(completeSourceStatement);
         print(e2);
+        print(stack2);
         throw FormulaEvaluationException( "Evaluation as statement and expression failed" );
       }
     }
diff --git a/lib/defaults/default_corpus.dart b/lib/defaults/default_corpus.dart
index 7df8d82..d740ba8 100644
--- a/lib/defaults/default_corpus.dart
+++ b/lib/defaults/default_corpus.dart
@@ -17,6 +17,7 @@ Future<Corpus> createDefaultCorpus() async{
       "lib/defaults/units/force.d4rt.units",
       "lib/defaults/units/mass.d4rt.units",
       "lib/defaults/units/pressure.d4rt.units",
+      "lib/defaults/units/scalar.d4rt.units",
       "lib/defaults/units/temperature.d4rt.units",
       "lib/defaults/units/time.d4rt.units",
       "lib/defaults/units/velocity.d4rt.units",
diff --git a/lib/defaults/formulas.d4rt b/lib/defaults/formulas.d4rt
index b0ea338..fafa242 100644
--- a/lib/defaults/formulas.d4rt
+++ b/lib/defaults/formulas.d4rt
@@ -126,13 +126,13 @@ Where:
     "name": "Apgar Score",
     "description": "Newborn health assessment scoring system\n\nScores 0-2 for:\n1. Heart rate\n2. Breathing\n3. Muscle tone\n4. Reflexes\n5. Skin color\nTotal score 0-10",
     "input": [
-      {"name": "HeartRate", "unit": "integer"},
-      {"name": "Breathing", "unit": "integer"},
-      {"name": "MuscleTone", "unit": "integer"},
-      {"name": "Reflexes", "unit": "integer"},
-      {"name": "SkinColor", "unit": "integer"}
+      {"name": "HeartRate", "values": ["hr1", "hr2", "hr3"] },
+      {"name": "Breathing", "values": ["hr1", "hr2", "hr3"] },
+      {"name": "MuscleTone", "values": ["hr1", "hr2", "hr3"] },
+      {"name": "Reflexes", "values": ["hr1", "hr2", "hr3"] },
+      {"name": "SkinColor", "values": ["hr1", "hr2", "hr3"] }
     ],
-    "output": {"name": "Result", "unit": "Apgar score"},
+    "output": {"name": "Result", "unit": "scalar"},
     "d4rtCode": """
       var total = HeartRate + Breathing + MuscleTone + Reflexes + SkinColor;
       var interpretation = switch (total) {
diff --git a/lib/defaults/units/scalar.d4rt.units b/lib/defaults/units/scalar.d4rt.units
new file mode 100644
index 0000000..c314f66
--- /dev/null
+++ b/lib/defaults/units/scalar.d4rt.units
@@ -0,0 +1,3 @@
+[
+  {"name": "scalar", "symbol": "", "isBase": true}
+]
\ No newline at end of file
diff --git a/lib/formula_evaluator.dart b/lib/formula_evaluator.dart
index 0665b38..9a844b9 100644
--- a/lib/formula_evaluator.dart
+++ b/lib/formula_evaluator.dart
@@ -121,13 +121,7 @@ class FormulaEvaluator {
     }
   }
 
-  String getOutputVariableName(Formula formula) {
-    return formula.output.name;
-  }
 
-  String getOutputVariableMagnitude(Formula formula) {
-    return formula.output.unit;
-  }
 
   List<String> getInputVariableOrder(Formula formula) {
     return formula.inputVarNames()..sort();
@@ -166,9 +160,9 @@ class FormulaEvaluator {
       }
     }
     buffer.writeln("""
-      late var ${getOutputVariableName(formula)};
+      late var ${formula.output.name};
       ${formula.d4rtCode}
-      return ${getOutputVariableName(formula)};
+      return ${formula.output.name};
       }
       """
     );
diff --git a/lib/formula_models.dart b/lib/formula_models.dart
index ebdf047..05bb6a8 100644
--- a/lib/formula_models.dart
+++ b/lib/formula_models.dart
@@ -101,12 +101,18 @@ class UnitSpec {
 
 class VariableSpec {
   final String name;
-  final String unit;
+  final String? unit;
+  final List<dynamic>? allowedValues;
 
-  VariableSpec({required this.name, required this.unit});
+  VariableSpec({required this.name, this.unit, this.allowedValues}){
+    final valuesValid = allowedValues != null && allowedValues?.isNotEmpty == true;
+    if( unit == null && !valuesValid ){
+      throw new ArgumentError("$name: at least unit or allowedValues should be valid");
+    }
+  }
 
   @override
-  String toString() => 'var($name: $unit)';
+  String toString() => 'var($name: $unit${allowedValues != null ? ' allowed: $allowedValues' : ''})';
 
   @override
   bool operator ==(Object other) =>
@@ -114,10 +120,11 @@ class VariableSpec {
       other is VariableSpec &&
           runtimeType == other.runtimeType &&
           unit == other.unit &&
-          name == other.name;
+          name == other.name &&
+          const DeepCollectionEquality().equals(allowedValues, other.allowedValues);
 
   @override
-  int get hashCode => Object.hash(unit, name);
+  int get hashCode => Object.hash(unit, name, allowedValues != null ? const DeepCollectionEquality().hash(allowedValues!) : 0);
 }
 
 class Formula {
@@ -196,8 +203,25 @@ class Formula {
   factory Formula.fromSet(Map<Object?, Object?> theSet) {
     VariableSpec parseVar(Map<Object?, Object?> varSpec) {
       String name = SetUtils.stringValue(varSpec, "name");
-      String unit = SetUtils.stringValue(varSpec, "unit");
-      return VariableSpec(name: name, unit: unit);
+      String? unit;
+      if (varSpec.containsKey("unit")) {
+        unit = SetUtils.stringValue(varSpec, "unit");
+      }
+      final allowed = varSpec['values'] as List<dynamic>?;
+      if (allowed != null) {
+        final types = allowed.map((v) => v.runtimeType).toSet();
+        if (types.length > 1) {
+          throw ArgumentError('Allowed values must be all Strings or all Numbers');
+        }
+        if (!types.contains(String) && !types.contains(double) && !types.contains(int)) {
+          throw ArgumentError('Allowed values must be Strings or Numbers');
+        }
+      }
+      return VariableSpec(
+        name: name,
+        unit: unit,
+        allowedValues: allowed?.toList(growable: false),
+      );
     }
 
     String name = SetUtils.stringValue(theSet, "name");
diff --git a/test/d4rt_test.dart b/test/d4rt_test.dart
index bc68e6c..db0aeb4 100644
--- a/test/d4rt_test.dart
+++ b/test/d4rt_test.dart
@@ -29,6 +29,7 @@ main(){
        }
       """;
     final interpreter = D4rt();
+    interpreter.grant(FilesystemPermission.readPath("/etc/passwd"));
     final result = interpreter.execute(source: completeSource);
 
     expect(result, contains("root"));
diff --git a/test/formula_evaluator_test.dart b/test/formula_evaluator_test.dart
index 727759f..0b4c04d 100644
--- a/test/formula_evaluator_test.dart
+++ b/test/formula_evaluator_test.dart
@@ -152,7 +152,7 @@ void main() {
           d4rtCode: 'force = x;',
         );
 
-        expect(evaluator.getOutputVariableName(formula), 'force');
+        expect(formula.output.name, 'force');
       });
 
       test(
@@ -165,7 +165,7 @@ void main() {
             d4rtCode: 'force = x;',
           );
 
-          expect(evaluator.getOutputVariableMagnitude(formula), 'Newton');
+          expect(formula.output.unit, 'Newton');
         },
       );
 
@@ -177,8 +177,8 @@ void main() {
           d4rtCode: 'result = x;',
         );
 
-        expect(evaluator.getOutputVariableName(validFormula), 'result');
-        expect(evaluator.getOutputVariableMagnitude(validFormula), 'Newton');
+        expect(validFormula.output.name, 'result');
+        expect(validFormula.output.unit, 'Newton');
       });
 
     });