first steps for evaluator

classes/answer_parser.php

@@ -0,0 +1,55 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+namespace qtype_formulas;
+
+/**
+ * Parser for answer expressions for qtype_formulas
+ *
+ * @package    qtype_formulas
+ * @copyright  2022 Philipp Imhof
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+
+class answer_parser extends parser {
+    /**
+     * FIXME Undocumented function
+     *
+     * @param [type] $tokenlist
+     * @param [type] $knownvariables
+     */
+    public function __construct(array $tokenlist, array $knownvariables = []) {
+        // FIXME: add parameter for answer type (number, numeric, numerical formula, algebraic formula)
+
+        // When parsing an answer expression, we have to replace all ^ operators (XOR) by
+        // ** operators (exponentiation) in order to maintain backwards compatibility.
+        foreach ($tokenlist as $token) {
+            if ($token->type === token::OPERATOR && $token->value === '^') {
+                $token->value = '**';
+            }
+        }
+
+        // FIXME: add some filtering, according to answer type, e.g.
+        // * number (no operators and stuff at all, except for unary -)
+        // * numeric (allow +, -, *, /, ** or ^, parens and pi)
+        // * numerical formula (allow some functions; cf. func_algebraic in legacy variables.php)
+        // * algebraic formula (allow everything, but with ^ still being **)
+
+        // Once this is done, we can parse the expression normally.
+        parent::__construct($tokenlist);
+    }
+}