diff --git a/Makefile b/Makefile
index 7f13f8db..0aab4000 100644
--- a/Makefile
+++ b/Makefile
@@ -1,10 +1,12 @@
+CC=gcc
+
 all: build run
 
 build:
 	cd src; \
 	yacc -d parse.y; \
 	lex scan.l
-	gcc -Wall -o bin/picrin -I./include -lreadline src/main.c src/state.c src/gc.c src/pair.c src/port.c src/symbol.c src/value.c src/y.tab.c src/lex.yy.c src/bool.c src/vm.c src/init.c src/number.c
+	 $(CC) -Wall -o bin/picrin -I./include -lreadline src/main.c src/state.c src/gc.c src/pair.c src/port.c src/symbol.c src/value.c src/y.tab.c src/lex.yy.c src/bool.c src/vm.c src/init.c src/number.c
 
 clean:
 	rm -f src/y.tab.c src/y.tab.h src/lex.yy.c
diff --git a/src/main.c b/src/main.c
index d068904d..88ff9ea8 100644
--- a/src/main.c
+++ b/src/main.c
@@ -17,13 +17,17 @@ main()
 {
   pic_state *pic;
   char code[CODE_MAX_LENGTH] = "", line[LINE_MAX_LENGTH];
-  char last_char, *read_line, *prompt;
-  int char_index;
+  char *read_line, *prompt;
   pic_value v;
   struct pic_proc *proc;
   int ai;
   bool r;
 
+#if ! PIC_ENABLE_READLINE
+  char last_char;
+  int char_index;
+#endif
+
   pic = pic_open();
 
   ai = pic_gc_arena_preserve(pic);
@@ -89,9 +93,11 @@ main()
     pic_gc_arena_restore(pic, ai);
   }
 
+#if ! PIC_ENABLE_READLINE
  eof:
   puts("");
   goto exit;
+#endif
 
  overflow:
   puts("** [fatal] line input overflow");
diff --git a/src/parse.y b/src/parse.y
index 8c1176d7..1864e620 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -25,7 +25,7 @@ struct parser_control {
 %token <datum> tSYMBOL tNUMBER tBOOLEAN
 
 %type <datum> datum simple_datum symbol compound_datum
-%type <datum> number list list_data
+%type <datum> number boolean list list_data
 
 %%
 
@@ -111,7 +111,7 @@ incomplete_data
 
 %%
 
-int
+void
 yyerror(struct parser_control *p, const char *msg)
 {
   puts(msg);
diff --git a/src/scan.l b/src/scan.l
index 790cf19d..ad3c46fc 100644
--- a/src/scan.l
+++ b/src/scan.l
@@ -7,11 +7,13 @@
 struct parser_control {
   pic_state *pic;
   pic_value value;
+  bool incomp;
 };
 
 #define YY_DECL int yylex (struct parser_control *p)
 %}
 
+%option noinput
 %option nounput
 
   /* boolean */
diff --git a/src/state.c b/src/state.c
index 5fee9658..71d1f9ac 100644
--- a/src/state.c
+++ b/src/state.c
@@ -43,7 +43,7 @@ pic_open()
   pic->icapa = PIC_IREP_SIZE;
 
   /* globals */
-  pic->globals = (struct pic_value *)malloc(sizeof(pic_value) * PIC_GLOBALS_SIZE);
+  pic->globals = (pic_value *)malloc(sizeof(pic_value) * PIC_GLOBALS_SIZE);
   pic->glen = 0;
   pic->gcapa = PIC_GLOBALS_SIZE;
 
diff --git a/src/value.c b/src/value.c
index 52aca41a..7fa303d2 100644
--- a/src/value.c
+++ b/src/value.c
@@ -1,6 +1,7 @@
 #include <stdlib.h>
+#include <stdbool.h>
 
-#include "picrin.h"
+#include "picrin/value.h"
 
 enum pic_tt
 pic_type(pic_value v)
@@ -9,15 +10,18 @@ pic_type(pic_value v)
   case PIC_VTYPE_NIL:
     return PIC_TT_NIL;
   case PIC_VTYPE_TRUE:
+    return PIC_TT_BOOL;
   case PIC_VTYPE_FALSE:
     return PIC_TT_BOOL;
-  case PIC_VTYPE_FLOAT:
-    return PIC_TT_FLOAT;
   case PIC_VTYPE_UNDEF:
     return PIC_TT_UNDEF;
+  case PIC_VTYPE_FLOAT:
+    return PIC_TT_FLOAT;
   case PIC_VTYPE_HEAP:
     return ((struct pic_object *)v.u.data)->tt;
   }
+  /* logic flaw (suppress warnings gcc will emit) */
+  abort();
 }
 
 pic_value
diff --git a/src/vm.c b/src/vm.c
index 97c81621..a54c0c5d 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -143,6 +143,9 @@ print_irep(pic_state *pic, struct pic_irep *irep)
   printf("## irep %p [clen = %zd, ccapa = %zd]\n", irep, irep->clen, irep->ccapa);
   for (i = 0; i < irep->clen; ++i) {
     switch (irep->code[i].insn) {
+    case OP_POP:
+      puts("OP_POP");
+      break;
     case OP_PUSHNIL:
       puts("OP_PUSHNIL");
       break;
@@ -375,6 +378,11 @@ pic_gen(pic_state *pic, struct pic_irep *irep, pic_value obj, struct pic_env *en
     irep->clen++;
     break;
   }
+  case PIC_TT_PROC:
+  case PIC_TT_UNDEF:
+  case PIC_TT_PORT: {
+    pic_raise(pic, "invalid expression given");
+  }
   }
 }
 
@@ -431,7 +439,7 @@ pic_gen_lambda(pic_state *pic, pic_value obj, struct pic_env *env)
   pic_free(pic, inner_env);
 
 #if VM_DEBUG
-  printf("LAMBDA_%d:\n", pic->ilen);
+  printf("LAMBDA_%zd:\n", pic->ilen);
   print_irep(pic, irep);
   puts("");
 #endif
@@ -476,6 +484,7 @@ pic_codegen(pic_state *pic, pic_value obj, struct pic_env *env)
 
 #define PUSH(v) (*++pic->sp = (v))
 #define POP() (*pic->sp--)
+#define POPN(i) ((void)(pic->sp-=i))
 
 #define PUSHCI() (++pic->ci)
 #define POPCI() (pic->ci--)
@@ -506,7 +515,7 @@ pic_run(pic_state *pic, struct pic_proc *proc, pic_value args)
 
   VM_LOOP {
     CASE(OP_POP) {
-      POP();
+      POPN(1);
       NEXT;
     }
     CASE(OP_PUSHNIL) {