diff --git a/include/picrin.h b/include/picrin.h
index 61cfbb69..8c7cb34f 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -71,6 +71,12 @@ typedef struct {
 
 typedef pic_value (*pic_func_t)(pic_state *);
 
+enum pic_parser_res {
+  PIC_PARSER_INCOMPLETE = -1,
+  PIC_PARSER_ERROR = -2
+  /* if parser is successfully done, return the number of exprs (>= 0) */
+};
+
 void *pic_alloc(pic_state *, size_t);
 void *pic_realloc(pic_state *, void *, size_t);
 void *pic_calloc(pic_state *, unsigned, size_t);
diff --git a/src/init.c b/src/init.c
index 24b83265..811a0043 100644
--- a/src/init.c
+++ b/src/init.c
@@ -33,7 +33,7 @@ pic_load_stdlib(pic_state *pic)
   }
 
   n = pic_parse_file(pic, file, &vs);
-  if (n <= 0) {
+  if (n < 0) {
     fputs("fatal error: built-in.scm broken", stderr);
     abort();
   }
diff --git a/src/parse.y b/src/parse.y
index 3280ab6b..94adcac9 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -67,6 +67,10 @@ program
 	  p->incomp = true;
 	  p->value = pic_undef_value();
 	}
+| /* empty line */
+	{
+	  p->value = pic_nil_value();
+	}
 ;
 
 program_data
@@ -249,17 +253,17 @@ pic_parse_file(pic_state *pic, FILE *file, pic_value *v)
 
   if (p.yynerrs > 0) {
     p.value = pic_undef_value();
-    return -1;
+    return PIC_PARSER_ERROR;
   }
   if (p.incomp) {
-    return 0;
+    return PIC_PARSER_INCOMPLETE;
   }
 
   *v = p.value;
   return pic_length(pic, p.value);
 }
 
-int
+enum pic_parser_res
 pic_parse_cstr(pic_state *pic, const char *str, pic_value *v)
 {
   struct parser_control p;
@@ -274,10 +278,10 @@ pic_parse_cstr(pic_state *pic, const char *str, pic_value *v)
   yylex_destroy(p.yyscanner);
 
   if (p.yynerrs > 0) {
-    return -1;
+    return PIC_PARSER_ERROR;
   }
   if (p.incomp) {
-    return 0;
+    return PIC_PARSER_INCOMPLETE;
   }
 
   *v = p.value;
diff --git a/tools/main.c b/tools/main.c
index 81f2c46f..ff6a5545 100644
--- a/tools/main.c
+++ b/tools/main.c
@@ -103,11 +103,11 @@ repl(pic_state *pic)
 
     /* read */
     n = pic_parse_cstr(pic, code, &vs);
-    if (n == 0) {		/* wait for more input */
+    if (n == PIC_PARSER_INCOMPLETE) {		/* wait for more input */
       goto next;
     }
     code[0] = '\0';
-    if (n == -1) {		/* parse error */
+    if (n == PIC_PARSER_ERROR) {		/* parse error */
       goto next;
     }