diff --git a/builtins.c b/builtins.c
index 37c1434..f1248c0 100644
--- a/builtins.c
+++ b/builtins.c
@@ -10,11 +10,29 @@ struct Builtin {
     struct Object *(*func)(struct Object *args);
 };
 
-static struct Object *func_sum(struct Object *numbers);
+// Type predicates
+static struct Object *func_booleanQN(struct Object *args);
+static struct Object *func_charQN(struct Object *args);
+static struct Object *func_nullQN(struct Object *args);
+static struct Object *func_numberQN(struct Object *args);
+static struct Object *func_pairQN(struct Object *args);
+static struct Object *func_stringQN(struct Object *args);
+static struct Object *func_symbolQN(struct Object *args);
+// Other
+static struct Object *func_sum(struct Object *args);
 
 static struct Builtin builtins[] = {
-    { "+", func_sum },
-    { NULL, NULL },
+    // Type predicates
+    { "boolean?", func_booleanQN },
+    { "char?",    func_charQN    },
+    { "null?",    func_nullQN    },
+    { "number?",  func_numberQN  },
+    { "pair?",    func_pairQN    },
+    { "string?",  func_stringQN  },
+    { "symbol?",  func_symbolQN  },
+    // Other
+    { "+",        func_sum       },
+    { NULL,       NULL           },
 };
 
 struct Object *builtins_call(const char *name, struct Object *args)
@@ -28,21 +46,88 @@ struct Object *builtins_call(const char *name, struct Object *args)
     abort();
 }
 
-struct Object *func_sum(struct Object *const numbers)
+struct Object *func_booleanQN(struct Object *args)
+{
+    assert(args);
+    assert(args->type == TYPE_PAIR);
+    assert(args->pair.b == NULL);
+
+    return
+        Object_new_boolean(args->pair.a && args->pair.a->type == TYPE_BOOLEAN);
+}
+
+struct Object *func_charQN(struct Object *args)
+{
+    assert(args);
+    assert(args->type == TYPE_PAIR);
+    assert(args->pair.b == NULL);
+
+    return Object_new_boolean(args->pair.a && args->pair.a->type == TYPE_CHAR);
+}
+
+struct Object *func_nullQN(struct Object *args)
+{
+    assert(args);
+    assert(args->type == TYPE_PAIR);
+    assert(args->pair.b == NULL);
+
+    return Object_new_boolean(args->pair.a == NULL);
+}
+
+struct Object *func_numberQN(struct Object *args)
+{
+    assert(args);
+    assert(args->type == TYPE_PAIR);
+    assert(args->pair.b == NULL);
+
+    return
+        Object_new_boolean(args->pair.a && args->pair.a->type == TYPE_NUMBER);
+}
+
+struct Object *func_pairQN(struct Object *args)
+{
+    assert(args);
+    assert(args->type == TYPE_PAIR);
+    assert(args->pair.b == NULL);
+
+    return Object_new_boolean(args->pair.a && args->pair.a->type == TYPE_PAIR);
+}
+
+struct Object *func_stringQN(struct Object *args)
+{
+    assert(args);
+    assert(args->type == TYPE_PAIR);
+    assert(args->pair.b == NULL);
+
+    return
+        Object_new_boolean(args->pair.a && args->pair.a->type == TYPE_STRING);
+}
+
+struct Object *func_symbolQN(struct Object *args)
+{
+    assert(args);
+    assert(args->type == TYPE_PAIR);
+    assert(args->pair.b == NULL);
+
+    return
+        Object_new_boolean(args->pair.a && args->pair.a->type == TYPE_SYMBOL);
+}
+
+struct Object *func_sum(struct Object *const args)
 {
     struct Object *const object = Object_new_number(0);
 
-    if (!numbers) return object;
+    if (!args) return object;
 
-    assert(numbers->type == TYPE_PAIR);
-    assert(numbers->pair.a);
-    assert(numbers->pair.a->type == TYPE_NUMBER);
+    assert(args->type == TYPE_PAIR);
+    assert(args->pair.a);
+    assert(args->pair.a->type == TYPE_NUMBER);
 
-    object->i = numbers->pair.a->i;
+    object->i = args->pair.a->i;
 
-    if (!numbers->pair.b) return object;
+    if (!args->pair.b) return object;
 
-    struct Object *const b_sum = func_sum(numbers->pair.b);
+    struct Object *const b_sum = func_sum(args->pair.b);
 
     object->i += b_sum->i;
 
diff --git a/object.c b/object.c
index 681add9..900b8af 100644
--- a/object.c
+++ b/object.c
@@ -81,6 +81,12 @@ void Object_print(struct Object *const self, const unsigned indent)
         Object_print(self->pair.a, indent + 1);
         Object_print(self->pair.b, indent + 1);
         break;
+    case TYPE_BOOLEAN:
+        printf("%s\n", self->boolean ? "#t" : "#f");
+        break;
+    case TYPE_CHAR:
+        printf("%c\n", self->chr);
+        break;
     case TYPE_SYMBOL:
     case TYPE_STRING:
         printf("%s\n", self->s);
@@ -88,7 +94,5 @@ void Object_print(struct Object *const self, const unsigned indent)
     case TYPE_NUMBER:
         printf("%li\n", self->i);
         break;
-    default:
-        break;
     }
 }