commit 2b73c245b2da259ff72a3a29ccd7211a6429c0cf
Author: Pekka Laiho <pekka.i.laiho@gmail.com>
Date:   Tue May 26 15:43:59 2020 +0700

    start

diff --git a/lisp.php b/lisp.php
new file mode 100644
index 0000000..51433c6
--- /dev/null
+++ b/lisp.php
@@ -0,0 +1,119 @@
+<?php
+
+class MadLispException extends Exception {}
+
+define("SYMBOL", "SYMBOL");
+define("STRING", "STRING");
+define("NUMBER", "NUMBER");
+define("START", "START");
+define("END", "END");
+
+function ml_tokenize(string $a): array
+{
+    $tokens = [];
+    $current = '';
+    $string = false;
+    $parens = 0;
+
+    $addCurrent = function($addEmpty = false) use (&$string, &$tokens, &$current) {
+        if ($current !== '' || $addEmpty) {
+            $type = $string ? STRING : (is_numeric($current) ? NUMBER : SYMBOL);
+            $tokens[] = [$type, $current];
+            $current = '';
+        }
+    };
+
+    for ($i = 0; $i < strlen($a); $i++) {
+        $c = substr($a, $i, 1);
+
+        if ($c == '"') {
+            if ($string) {
+                // End of string
+                $addCurrent(true);
+                $string = false;
+            } else {
+                // Start of string
+                $string = true;
+            }
+        } elseif ($c == ' ' || $c == "\t" || $c == "\n" || $c == "\r") {
+            if ($string) {
+                // Include whitespace only inside strings
+                $current .= $c;
+            } else {
+                $addCurrent();
+            }
+        } elseif ($c == '(') {
+            $addCurrent();
+            $tokens[] = [START];
+            $parens++;
+        } elseif ($c == ')') {
+            if ($parens == 0) {
+                throw new MadLispException("unexpected closing parenthesis");
+            }
+            $addCurrent();
+            $tokens[] = [END];
+            $parens--;
+        } else {
+            // All other characters are included normally
+            $current .= $c;
+        }
+    }
+
+    // Add last also
+    $addCurrent();
+
+    // Check for errors
+    if (empty($tokens)) {
+        throw new MadLispException("no input");
+    } elseif ($tokens[0][0] !== START) {
+        throw new MadLispException("missing opening parenthesis");
+    } elseif ($parens != 0) {
+        throw new MadLispException("missing closing parenthesis");
+    } elseif ($string) {
+        throw new MadLispException("unterminated string");
+    }
+
+    return $tokens;
+}
+
+function ml_parse_expressions($tokens, &$index)
+{
+    $result = [];
+
+    for (; $index < count($tokens); ) {
+        if ($token[0] == START) {
+
+        }
+        if ($token[0] == END) {
+            break;
+        }
+    }
+
+    return $result;
+}
+
+function ml_read($a)
+{
+    $tokens = ml_tokenize($a);
+
+    $index = 0;
+    $expressions = ml_parse_expressions($tokens, $index);
+
+    return $expressions;
+}
+
+function ml_eval($a)
+{
+    return $a;
+}
+
+function ml_print($a)
+{
+    // print(implode('*', $a));
+    print_r($a);
+}
+
+function ml_rep($a)
+{
+    ml_print(ml_eval(ml_read($a)));
+}
diff --git a/repl.php b/repl.php
new file mode 100644
index 0000000..ec5860b
--- /dev/null
+++ b/repl.php
@@ -0,0 +1,17 @@
+<?php
+
+require('lisp.php');
+
+while (true) {
+    $input = readline('> ');
+
+    print('% ');
+
+    try {
+        ml_rep($input);
+    } catch (MadLispException $ex) {
+        print('error: ' . $ex->getMessage());
+    }
+
+    print(PHP_EOL);
+}