diff --git a/Makefile b/Makefile
index 122990f..f407181 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
-run: prompt.c loperations.o lnumbers.o lexpressions.o lio.o mpc.o
-	cc -std=c99 -Wall prompt.c loperations.o lnumbers.o lexpressions.o lio.o mpc.o -ledit -lm -o prompt
+run: prompt.c loperations.o lnumbers.o lexpressions.o lio.o lerror.o mpc.o
+	cc -std=c99 -Wall prompt.c loperations.o lnumbers.o lexpressions.o lio.o lerror.o mpc.o -ledit -lm -o prompt
 loperations.o: lval/operations.c lval/operations.h
 	cc -std=c99 -Wall -c lval/operations.c -o loperations.o
 lnumbers.o: lval/numbers.c lval/numbers.h
@@ -8,6 +8,8 @@ lexpressions.o: lval/expressions.c lval/expressions.h
 	cc -std=c99 -Wall -c lval/expressions.c -o lexpressions.o
 lio.o: lval/io.c lval/io.h
 	cc -std=c99 -Wall -c lval/io.c -o lio.o
+lerror.o: lval/error.c lval/error.h
+	cc -std=c99 -Wall -c lval/error.c -o lerror.o
 mpc.o: mpc.c mpc.h
 	cc -std=c99 -Wall -lm -c mpc.c 
 clean:
diff --git a/lval.h b/lval.h
index 6046b6f..0fe7489 100644
--- a/lval.h
+++ b/lval.h
@@ -4,6 +4,8 @@
 
 // Bring in the lval struct and lispy types
 #include "lval/base.h"
+// Error functionality
+#include "lval/error.h"
 // Adds functionality for the numeric data type
 #include "lval/numbers.h"
 // Adds functionality for the expression (Q or S) data type
diff --git a/lval/error.c b/lval/error.c
new file mode 100644
index 0000000..22f3936
--- /dev/null
+++ b/lval/error.c
@@ -0,0 +1,11 @@
+#include <stdlib.h>
+#include <string.h>
+#include "error.h"
+
+lval* lval_err(char* m) {
+	lval* v = (lval *) malloc(sizeof(lval));
+	v->type = LVAL_ERR;
+	v->err = (char *) malloc(strlen(m) + 1);
+	strcpy(v->err, m);
+	return v;
+}
\ No newline at end of file
diff --git a/lval/error.h b/lval/error.h
new file mode 100644
index 0000000..19b9a45
--- /dev/null
+++ b/lval/error.h
@@ -0,0 +1,11 @@
+#ifndef LVAL_ERROR
+#define LVAL_ERROR
+#include <stdbool.h>
+#include "base.h"
+
+lval* lval_err(char* m);
+
+#define LASSERT(args, cond, err) \
+  if (!(cond)) { lval_del(args); return lval_err(err); }
+
+#endif
diff --git a/lval/expressions.c b/lval/expressions.c
index 4ade6ff..9ab6c8c 100644
--- a/lval/expressions.c
+++ b/lval/expressions.c
@@ -2,8 +2,9 @@
 #include <string.h>
 #include <stdlib.h>
 
-#include "operations.h"
 #include "expressions.h"
+#include "operations.h"
+#include "error.h"
 
 // Think about where to put this declaration later
 lval* builtin_op(lval* v, char* op);
diff --git a/lval/numbers.c b/lval/numbers.c
index c5b14d7..ee151b3 100644
--- a/lval/numbers.c
+++ b/lval/numbers.c
@@ -1,5 +1,6 @@
 #include <stdlib.h>
 #include "numbers.h"
+#include "error.h"
 
 lval* lval_long(long x) {
 	lval* v = (lval *) malloc(sizeof(lval));
diff --git a/lval/operations.c b/lval/operations.c
index d6049c3..e011b34 100644
--- a/lval/operations.c
+++ b/lval/operations.c
@@ -12,14 +12,6 @@ lval* lval_sym(char* s) {
 	return v;
 }
 
-lval* lval_err(char* m) {
-	lval* v = (lval *) malloc(sizeof(lval));
-	v->type = LVAL_ERR;
-	v->err = (char *) malloc(strlen(m) + 1);
-	strcpy(v->err, m);
-	return v;
-}
-
 lval* lval_read(mpc_ast_t* t) {
 	// If symbol or number, convert
 	if (strstr(t->tag, "long")) { return lval_read_long(t); }
diff --git a/lval/operations.h b/lval/operations.h
index 7acea43..501e2e4 100644
--- a/lval/operations.h
+++ b/lval/operations.h
@@ -4,9 +4,8 @@
 #include "../mpc.h"
 #include "base.h"
 
-// Constructor for other data types
+// Constructor for symbol data type
 lval* lval_sym(char* s);
-lval* lval_err(char* m);
 
 /*
     Methods to read (parse AST), evaluate, 
diff --git a/prompt.c b/prompt.c
index 0fb3be7..0107d18 100644
--- a/prompt.c
+++ b/prompt.c
@@ -53,8 +53,9 @@ int main (int argc, char** argv) {
 			"number   : /[0-9]+/;						                     "
 			"long     : /-?[0-9]+/;	                    	                 "
 			"double   : <long> '.' <number>;                                 "
-			"symbol   : '+' | '-' | '*' | '/' | '%'                \
-				     | '^' | \"min\" | \"max\"; 		                     "
+			"symbol   : '+' | '-' | '*' | '/' | '%'                          \
+				      | '^' | \"min\" | \"max\" 	                         \
+					  |	\"list\" | \"head\" | \"tail\" | \"join\" | \"eval\" "
 		
 			"sexpr    : '(' <expr>* ')';					                 "
 			"qexpr    : '{' <expr>* '}';                                     "