cleanup grade()

Author: PhilippImhof

classes/local/answer_parser.php

@@ -66,6 +66,12 @@ public function __construct($tokenlist, array $knownvariables = [], bool $caretm
             }
         }
 
+        // If we only have one single token and it is an empty string, we set it to the $EMPTY token.
+        $firsttoken = reset($tokenlist);
+        if (count($tokenlist) === 1 && $firsttoken->value === '') {
+            $tokenlist[0] = new token(token::EMPTY, '$EMPTY', $firsttoken->row, $firsttoken->column);
+        }
+
         // Once this is done, we can parse the expression normally.
         parent::__construct($tokenlist, $knownvariables);
     }

classes/local/evaluator.php

@@ -907,14 +907,19 @@ private function evaluate_the_right_thing($input, bool $godmode = false) {
     /**
      * Evaluate a single expression or an array of expressions.
      *
-     * @param expression|for_loop|array $input
+     * @param expression|for_loop|array|false $input
      * @param bool $godmode whether to run the evaluation in god mode
      * @return token|array
      */
     public function evaluate($input, bool $godmode = false) {
         if (($input instanceof expression) || ($input instanceof for_loop)) {
             return $this->evaluate_the_right_thing($input, $godmode);
         }
+        // For convenience, the evaluator accepts FALSE as an input, This allows
+        // passing reset($array) with a possibly empty array.
+        if ($input === false) {
+            return new token(token::EMPTY, '$EMPTY');
+        }
         if (!is_array($input)) {
             throw new Exception(get_string('error_evaluate_invocation', 'qtype_formulas', 'evaluate()'));
         }

question.php

@@ -1466,7 +1466,7 @@ public function add_special_variables(array $studentanswers, float $conversionfa
             // We calculate the sum of squares of all model answers.
             $ssqmodelanswer = 0;
             foreach ($this->get_evaluated_answers() as $answer) {
-                if ($answer === '$EMPTY' || trim($answer) === '') {
+                if ($answer === '$EMPTY') {
                     continue;
                 }
                 $ssqmodelanswer += $answer ** 2;
@@ -1554,13 +1554,12 @@ public function grade(array $response, bool $finalsubmit = false): array {
                 // failed evaluation, e.g. caused by an invalid answer.
                 $this->evaluator->clear_stack();
 
-                // Evaluate, if not empty.
-                if ($answer === '""' || (empty($answer) && !is_numeric($answer))) {
-                    $evaluated = new token(token::EMPTY, '$EMPTY');
-                } else {
-                    $evaluated = $this->evaluator->evaluate($parser->get_statements())[0];
-                }
-                $evaluatedresponse[] = token::unpack($evaluated); // $evaluated;
+                // Evaluate. If the answer was empty (an empty string or the '$EMPTY'), the parser
+                // will create an appropriate evaluable statement or return an empty array. The evaluator,
+                // on the other hand, will know how to deal with the "false" return value from reset()
+                // and return the $EMPTY token.
+                $statements = $parser->get_statements();
+                $evaluatedresponse[] = token::unpack($this->evaluator->evaluate(reset($statements)));
             } catch (Throwable $t) {
                 // TODO: convert to non-capturing catch
                 // If parsing, validity check or evaluation fails, we consider the answer as wrong.