diff --git a/lval/expressions.c b/lval/expressions.c
index ab0c205..3c4f579 100644
--- a/lval/expressions.c
+++ b/lval/expressions.c
@@ -3,6 +3,7 @@
 #include <stdlib.h>
 
 #include "expressions.h"
+#include "numbers.h"
 #include "operations.h"
 #include "error.h"
 
@@ -143,3 +144,11 @@ lval* builtin_join(lval* a) {
 	lval_del(a);
 	return x;
 }
+
+lval* builtin_len(lval* a) {
+	LASSERT(a, a->cell[0]->type == LVAL_QEXPR, "Function 'len' passed incorrect type")
+	lval* x = lval_long(a->cell[0]->count);
+	
+	lval_del(a);
+	return x;
+}
diff --git a/lval/expressions.h b/lval/expressions.h
index c1482ca..9bda991 100644
--- a/lval/expressions.h
+++ b/lval/expressions.h
@@ -36,6 +36,6 @@ lval* builtin_list(lval* a);
 lval* builtin_eval(lval* a);
 lval* lval_join(lval* x, lval* y);
 lval* builtin_join(lval* a);
-
+lval* builtin_len(lval* a);
 
 #endif
diff --git a/prompt.c b/prompt.c
index f629658..9a5d647 100644
--- a/prompt.c
+++ b/prompt.c
@@ -55,7 +55,9 @@ int main (int argc, char** argv) {
 			"double   : <long> '.' <number>;                                 "
 			"symbol   : '+' | '-' | '*' | '/' | '%'                          \
 				      | '^' | \"min\" | \"max\" 	                         \
-					  |	\"list\" | \"head\" | \"tail\" | \"join\" | \"eval\";"
+					  |	\"list\" | \"head\" | \"tail\" 						 \
+					  | \"join\" | \"eval\" | \"len\"						 \
+					  | \"init\" | \"cons\";                                 "
 		
 			"sexpr    : '(' <expr>* ')';					                 "
 			"qexpr    : '{' <expr>* '}';                                     "
@@ -166,6 +168,7 @@ lval* builtin(lval* a, char* func) {
 	if (strcmp("tail", func) == 0) { return builtin_tail(a); }
 	if (strcmp("join", func) == 0) { return builtin_join(a); }
 	if (strcmp("eval", func) == 0) { return builtin_eval(a); }
+	if (strcmp("len", func) == 0) { return builtin_len(a); }
 	if (strstr("+-/*^%", func)) { return builtin_op(a, func); }
 	if (strcmp("min", func) == 0) { return builtin_op(a, func); }
 	if (strcmp("max", func) == 0) { return builtin_op(a, func); }