diff --git a/bootstrap.php b/bootstrap.php
index a6a61f2..b0869df 100644
--- a/bootstrap.php
+++ b/bootstrap.php
@@ -25,5 +25,8 @@ function ml_get_lisp(): array
     (new MadLisp\Lib\Time())->register($env);
     (new MadLisp\Lib\Types())->register($env);
 
+    // Functions defined in lisp itself
+    $lisp->re('(def loadf (fn (f) (if (file? f) (eval (read (str "(do " (fread f) ")"))) (error (str "file " f " does not exist")))))', $env);
+
     return [$lisp, $env];
 }
diff --git a/src/Evaller.php b/src/Evaller.php
index 459f166..d40acb7 100644
--- a/src/Evaller.php
+++ b/src/Evaller.php
@@ -62,8 +62,8 @@ class Evaller
                     $value = $this->eval($ast->get(2), $env);
                     return $env->set($ast->get(1)->getName(), $value);
                 } elseif ($ast->get(0)->getName() == 'do') {
-                    if ($ast->count() < 2) {
-                        throw new MadLispException("do requires at least 1 argument");
+                    if ($ast->count() == 1) {
+                        return null;
                     }
 
                     for ($i = 1; $i < $ast->count() - 1; $i++) {
diff --git a/src/Lisp.php b/src/Lisp.php
index 0dd9063..bae56a4 100644
--- a/src/Lisp.php
+++ b/src/Lisp.php
@@ -16,13 +16,18 @@ class Lisp
         $this->printer = $printer;
     }
 
-    public function rep(string $input, Env $env): void
+    public function re(string $input, Env $env)
     {
         $tokens = $this->tokenizer->tokenize($input);
 
         $expr = $this->reader->read($tokens);
 
-        $result = $this->eval->eval($expr, $env);
+        return $this->eval->eval($expr, $env);
+    }
+
+    public function rep(string $input, Env $env): void
+    {
+        $result = $this->re($input, $env);
 
         $this->printer->print($result);
     }
@@ -53,5 +58,11 @@ class Lisp
                 return null;
             }
         ));
+
+        $env->set('error', new CoreFunc('error', 'Throw an exception using argument (string) as message.', 1, 1,
+            function (string $error) {
+                throw new MadLispException($error);
+            }
+        ));
     }
 }