Handle checking instantiation without model answer

amd/src/instantiation.js

@@ -416,8 +416,9 @@ const instantiate = async() => {
             }
         }])[0];
         if (response.status == 'error') {
-            let str = await String.get_string('previewerror', 'qtype_formulas');
-            document.getElementById('qtextpreview_display').innerHTML = `${str}<br>${response.message}`;
+            document.getElementById('qtextpreview_display').innerHTML = await String.get_string(
+                'previewerror', 'qtype_formulas', response.message
+            );
         } else {
             document.getElementById('qtextpreview_display').innerHTML = '';
             prepareTableColumns(response.data);

classes/external/instantiation.php

@@ -137,8 +137,13 @@ protected static function fetch_one_instance($context, $parsedglobalvars, $parse
                     $partevaluator->evaluate($parsedlocalvars[$i]);
                 }
                 $localvars[$i] = self::variable_context_to_array($partevaluator->export_variable_context());
-                // Finally, evaluate the answer(s).
-                $answers[$i] = $partevaluator->evaluate($parsedanswers[$i])[0];
+                // Finally, evaluate the answer(s). If the model answer was empty, we cannot move on. As we
+                // are in a try-catch, we simply throw an exception with the appropriate error message.
+                $evaluationresult = $partevaluator->evaluate($parsedanswers[$i]);
+                if (empty($evaluationresult)) {
+                    throw new Exception(get_string('error_answer_missing_in_part', 'qtype_formulas', $i + 1));
+                }
+                $answers[$i] = reset($evaluationresult);
             }
         } catch (Exception $e) {
             return $e->getMessage();
@@ -512,7 +517,7 @@ public static function render_question_text($questiontext, $parttexts, $globalva
             $renderedquestiontext = $evaluator->substitute_variables_in_text($params['questiontext']);
         } catch (Exception $e) {
             return [
-                'question' => get_string('previewerror', 'qtype_formulas') . ' ' . $e->getMessage(),
+                'question' => get_string('previewerror', 'qtype_formulas', $e->getMessage()),
                 'parts' => [],
             ];
         }
@@ -529,7 +534,7 @@ public static function render_question_text($questiontext, $parttexts, $globalva
                 $renderedparttexts[$i] = $partevaluator->substitute_variables_in_text($params['parttexts'][$i]);
             } catch (Exception $e) {
                 return [
-                    'question' => get_string('previewerror', 'qtype_formulas') . ' ' . $e->getMessage(),
+                    'question' => get_string('previewerror', 'qtype_formulas', $e->getMessage()),
                     'parts' => [],
                 ];
             }

lang/en/qtype_formulas.php

@@ -86,6 +86,7 @@
 $string['error_algebraic_relerr'] = 'Relative error (_relerr) cannot be used with answer type algebraic formula.';
 $string['error_algvar_numbers'] = 'Algebraic variables can only be initialized with a list of numbers.';
 $string['error_answer_missing'] = 'No answer has been defined.';
+$string['error_answer_missing_in_part'] = 'No answer has been defined for part {$a}.';
 $string['error_answerbox_duplicate'] = 'Answer box placeholders must be unique, found second instance of {$a}.';
 $string['error_bitshift_integer'] = 'Bit shift operator should only be used with integers.';
 $string['error_bitshift_negative'] = 'Bit shift by negative number {$a} is not allowed.';
@@ -288,7 +289,7 @@
 
 Students are required to use the same input format. Examples:
 <pre class="prettyprint">1 m<br>0.1 m^2<br>20 m s^(-1)<br>400 kg m/s<br>100 kW</pre>';
-$string['previewerror'] = 'No preview available. Check your definition of random variables, global variables, parts\' local variables and answers. Original error message:';
+$string['previewerror'] = 'No preview available. Check your definition of random variables, global variables, parts\' local variables and answers. Original error message: {$a}';
 $string['privacy:metadata'] = 'The Formulas question type plugin does not store any personal data.';
 $string['qtextpreview'] = 'Preview';
 $string['questiontext'] = 'Question text';

tests/externallib_test.php

@@ -306,6 +306,24 @@ public function test_render_question_text($expected, $input): void {
      */
     public static function provide_instantiation_data(): array {
         return [
+            [
+                ['status' => 'error', 'message' => "No answer has been defined for part 1."],
+                [
+                    'n' => 1, 'randomvars' => '', 'globalvars' => '',
+                    'localvars' => [''], 'answers' => [''],
+                ],
+            ],
+            [
+                ['status' => 'ok', 'data' => [[
+                    'randomvars' => [],
+                    'globalvars' => [],
+                    'parts' => [[['name' => '_0', 'value' => '1']]],
+                ]]],
+                [
+                    'n' => 1, 'randomvars' => '', 'globalvars' => '',
+                    'localvars' => [''], 'answers' => ['1'],
+                ],
+            ],
             [
                 ['status' => 'ok', 'data' => [[
                     'randomvars' => [],