diff --git a/include/picrin.h b/include/picrin.h
index 5246acee..4aedbb40 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -164,19 +164,6 @@ bool pic_interned_p(pic_state *, pic_sym);
 
 char *pic_strdup(pic_state *, const char *);
 char *pic_strndup(pic_state *, const char *, size_t);
-pic_str *pic_str_new(pic_state *, const char * /* nullable */, size_t);
-pic_str *pic_str_new_cstr(pic_state *, const char *);
-char pic_str_ref(pic_state *, pic_str *, size_t);
-pic_str *pic_strcat(pic_state *, pic_str *, pic_str *);
-pic_str *pic_substr(pic_state *, pic_str *, size_t, size_t);
-
-pic_value pic_format(pic_state *, const char *, ...);
-pic_value pic_vformat(pic_state *, const char *, va_list);
-pic_value pic_vfformat(pic_state *, XFILE *, const char *, va_list);
-
-struct pic_vector *pic_vec_new(pic_state *, size_t);
-struct pic_vector *pic_vec_new_from_list(pic_state *, pic_value);
-void pic_vec_extend_ip(pic_state *, struct pic_vector *, size_t);
 
 int pic_parse_file(pic_state *, FILE *, pic_value *);
 int pic_parse_cstr(pic_state *, const char *, pic_value *);
diff --git a/include/picrin/pair.h b/include/picrin/pair.h
index 74399277..8f1a1dbb 100644
--- a/include/picrin/pair.h
+++ b/include/picrin/pair.h
@@ -9,6 +9,15 @@
 extern "C" {
 #endif
 
+struct pic_pair {
+  PIC_OBJECT_HEADER
+  pic_value car;
+  pic_value cdr;
+};
+
+#define pic_pair_p(v) (pic_type(v) == PIC_TT_PAIR)
+#define pic_pair_ptr(o) ((struct pic_pair *)pic_ptr(o))
+
 pic_value pic_cons(pic_state *, pic_value, pic_value);
 pic_value pic_car(pic_state *, pic_value);
 pic_value pic_cdr(pic_state *, pic_value);
diff --git a/include/picrin/string.h b/include/picrin/string.h
new file mode 100644
index 00000000..14d0caa8
--- /dev/null
+++ b/include/picrin/string.h
@@ -0,0 +1,36 @@
+/**
+ * See Copyright Notice in picrin.h
+ */
+
+#ifndef PICRIN_STRING_H__
+#define PICRIN_STRING_H__
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+struct pic_string {
+  PIC_OBJECT_HEADER
+  char *str;
+  size_t len;
+};
+
+#define pic_str_p(v) (pic_type(v) == PIC_TT_STRING)
+#define pic_str_ptr(o) ((struct pic_string *)pic_ptr(o))
+
+pic_str *pic_str_new(pic_state *, const char * /* nullable */, size_t);
+pic_str *pic_str_new_cstr(pic_state *, const char *);
+
+char pic_str_ref(pic_state *, pic_str *, size_t);
+pic_str *pic_strcat(pic_state *, pic_str *, pic_str *);
+pic_str *pic_substr(pic_state *, pic_str *, size_t, size_t);
+
+pic_value pic_format(pic_state *, const char *, ...);
+pic_value pic_vformat(pic_state *, const char *, va_list);
+pic_value pic_vfformat(pic_state *, XFILE *, const char *, va_list);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
diff --git a/include/picrin/value.h b/include/picrin/value.h
index 5bda9c06..d5428305 100644
--- a/include/picrin/value.h
+++ b/include/picrin/value.h
@@ -113,37 +113,25 @@ struct pic_object {
   PIC_OBJECT_HEADER
 };
 
-typedef struct pic_pair {
-  PIC_OBJECT_HEADER
-  pic_value car;
-  pic_value cdr;
-} pic_pair;
-
-typedef struct pic_string {
-  PIC_OBJECT_HEADER
-  char *str;
-  size_t len;
-} pic_str;
-
-typedef struct pic_vector {
-  PIC_OBJECT_HEADER
-  pic_value *data;
-  size_t len;
-} pic_vec;
+struct pic_pair;
+struct pic_string;
+struct pic_vector;
 
 struct pic_proc;
 struct pic_port;
 struct pic_blob;
 
+/* set aliases to basic types */
+typedef struct pic_pair pic_pair;
+typedef struct pic_string pic_str;
+typedef struct pic_vector pic_vec;
+
 #define pic_float(v) ((v).u.f)
 #define pic_int(v) ((v).u.i)
 #define pic_sym(v) ((v).u.sym)
 #define pic_char(v) ((v).u.c)
 
 #define pic_obj_ptr(o) ((struct pic_object *)pic_ptr(o))
-#define pic_pair_ptr(o) ((struct pic_pair *)pic_ptr(o))
-#define pic_str_ptr(o) ((struct pic_string *)pic_ptr(o))
-#define pic_vec_ptr(o) ((struct pic_vector *)pic_ptr(o))
 
 #define pic_nil_p(v) (pic_vtype(v) == PIC_VTYPE_NIL)
 #define pic_true_p(v) (pic_vtype(v) == PIC_VTYPE_TRUE)
@@ -153,9 +141,6 @@ struct pic_blob;
 #define pic_int_p(v) (pic_vtype(v) == PIC_VTYPE_INT)
 #define pic_sym_p(v) (pic_vtype(v) == PIC_VTYPE_SYMBOL)
 #define pic_char_p(v) (pic_vtype(v) == PIC_VTYPE_CHAR)
-#define pic_pair_p(v) (pic_type(v) == PIC_TT_PAIR)
-#define pic_str_p(v) (pic_type(v) == PIC_TT_STRING)
-#define pic_vec_p(v) (pic_type(v) == PIC_TT_VECTOR)
 
 #define pic_test(v) (! pic_false_p(v))
 
diff --git a/include/picrin/vector.h b/include/picrin/vector.h
new file mode 100644
index 00000000..80a4cb73
--- /dev/null
+++ b/include/picrin/vector.h
@@ -0,0 +1,29 @@
+/**
+ * See Copyright Notice in picrin.h
+ */
+
+#ifndef PICRIN_VECTOR_H__
+#define PICRIN_VECTOR_H__
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+struct pic_vector {
+  PIC_OBJECT_HEADER
+  pic_value *data;
+  size_t len;
+};
+
+#define pic_vec_p(v) (pic_type(v) == PIC_TT_VECTOR)
+#define pic_vec_ptr(o) ((struct pic_vector *)pic_ptr(o))
+
+struct pic_vector *pic_vec_new(pic_state *, size_t);
+struct pic_vector *pic_vec_new_from_list(pic_state *, pic_value);
+void pic_vec_extend_ip(pic_state *, struct pic_vector *, size_t);
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
diff --git a/src/error.c b/src/error.c
index de6167dc..8faa359f 100644
--- a/src/error.c
+++ b/src/error.c
@@ -8,6 +8,7 @@
 
 #include "picrin.h"
 #include "picrin/pair.h"
+#include "picrin/string.h"
 #include "picrin/error.h"
 
 const char *
@@ -30,7 +31,7 @@ raise(pic_state *pic, struct pic_error *e)
 }
 
 NORETURN static void
-error(pic_state *pic, struct pic_string *msg, pic_value irrs)
+error(pic_state *pic, pic_str *msg, pic_value irrs)
 {
   struct pic_error *e;
 
diff --git a/src/gc.c b/src/gc.c
index 36227a44..c3347cdf 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -6,6 +6,9 @@
 
 #include "picrin.h"
 #include "picrin/gc.h"
+#include "picrin/pair.h"
+#include "picrin/string.h"
+#include "picrin/vector.h"
 #include "picrin/irep.h"
 #include "picrin/proc.h"
 #include "picrin/port.h"
diff --git a/src/macro.c b/src/macro.c
index 6fe9f596..3b0642ea 100644
--- a/src/macro.c
+++ b/src/macro.c
@@ -4,6 +4,7 @@
 
 #include "picrin.h"
 #include "picrin/pair.h"
+#include "picrin/string.h"
 #include "picrin/proc.h"
 #include "picrin/macro.h"
 #include "picrin/lib.h"
diff --git a/src/parse.y b/src/parse.y
index 6072acf1..7374b51b 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -8,6 +8,8 @@
 
 #include "picrin.h"
 #include "picrin/pair.h"
+#include "picrin/string.h"
+#include "picrin/vector.h"
 #include "picrin/blob.h"
 #include "picrin/parse.h"
 
diff --git a/src/port.c b/src/port.c
index 719f51d3..0295ff24 100644
--- a/src/port.c
+++ b/src/port.c
@@ -8,6 +8,7 @@
 #include "picrin.h"
 #include "picrin/proc.h"
 #include "picrin/port.h"
+#include "picrin/string.h"
 #include "picrin/blob.h"
 
 pic_value
diff --git a/src/string.c b/src/string.c
index a491e4c8..58e3c69c 100644
--- a/src/string.c
+++ b/src/string.c
@@ -5,6 +5,7 @@
 #include <string.h>
 
 #include "picrin.h"
+#include "picrin/string.h"
 #include "picrin/pair.h"
 #include "picrin/port.h"
 
diff --git a/src/symbol.c b/src/symbol.c
index 0012672e..a4159b32 100644
--- a/src/symbol.c
+++ b/src/symbol.c
@@ -7,6 +7,7 @@
 #include <math.h>
 
 #include "picrin.h"
+#include "picrin/string.h"
 
 pic_sym
 pic_intern_cstr(pic_state *pic, const char *str)
diff --git a/src/system.c b/src/system.c
index c8d2ac7b..be814089 100644
--- a/src/system.c
+++ b/src/system.c
@@ -5,6 +5,7 @@
 #include <stdlib.h>
 
 #include "picrin.h"
+#include "picrin/string.h"
 #include "picrin/pair.h"
 
 static pic_value
diff --git a/src/vector.c b/src/vector.c
index b9586976..917f0878 100644
--- a/src/vector.c
+++ b/src/vector.c
@@ -3,6 +3,7 @@
  */
 
 #include "picrin.h"
+#include "picrin/vector.h"
 #include "picrin/pair.h"
 
 struct pic_vector *
diff --git a/src/vm.c b/src/vm.c
index 58360f41..2bd8f27e 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -9,6 +9,8 @@
 
 #include "picrin.h"
 #include "picrin/pair.h"
+#include "picrin/string.h"
+#include "picrin/vector.h"
 #include "picrin/proc.h"
 #include "picrin/port.h"
 #include "picrin/irep.h"
diff --git a/src/write.c b/src/write.c
index 5f7f08a7..80756369 100644
--- a/src/write.c
+++ b/src/write.c
@@ -5,6 +5,8 @@
 #include "picrin.h"
 #include "picrin/port.h"
 #include "picrin/pair.h"
+#include "picrin/string.h"
+#include "picrin/vector.h"
 #include "picrin/blob.h"
 #include "picrin/macro.h"