print null as empty string, enable functions eval and print

bootstrap.php

@@ -10,25 +10,16 @@ function ml_get_lisp(): array
 
     $lisp = new MadLisp\Lisp($tokenizer, $reader, $eval, $printer);
 
-    // environment
-
+    // Environment
     $env = new MadLisp\Env();
 
+    // Register core functions
+    $lisp->register($env);
+
+    // Register libraries
     (new MadLisp\Lib\Math())->register($env);
     (new MadLisp\Lib\Compare())->register($env);
     (new MadLisp\Lib\Types())->register($env);
 
-    /*
-    $env->set('eval', function (...$args) use ($eval, $env) {
-        $results = $eval->eval($args, $env);
-        return $results[count($results) - 1];
-    });
-
-    $env->set('print', function (...$args) use ($printer) {
-        $printer->print($args);
-        return null;
-    });
-    */
-
     return [$lisp, $env];
 }

src/Lisp.php

@@ -26,4 +26,24 @@ class Lisp
 
         $this->printer->print($results);
     }
+
+    public function register(Env $env): void
+    {
+        $env->set('eval', new CoreFunc('eval', 'Evaluate arguments.', 1, -1,
+            function (...$args) use ($env) {
+                $results = $this->eval->eval($args, $env);
+
+                // Return last evaluated value
+                return $results[count($results) - 1];
+            }
+        ));
+
+        $env->set('print', new CoreFunc('print', 'Print arguments.', 1, -1,
+            function (...$args) {
+                $this->printer->print($args);
+
+                return null;
+            }
+        ));
+    }
 }

src/Printer.php

@@ -28,7 +28,7 @@ class Printer
         } elseif ($a === false) {
             return 'false';
         } elseif ($a === null) {
-            return 'null';
+            return '';
         } elseif (is_string($a)) {
             return '"' . $a . '"';
         } else {