diff --git a/include/picrin.h b/include/picrin.h
index 2867439a..ea7d3228 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -166,6 +166,9 @@ char *pic_strdup(pic_state *, const char *);
 char *pic_strndup(pic_state *, const char *, size_t);
 struct pic_string *pic_str_new(pic_state *, const char *, size_t); /* the 2nd arg may be NULL for empty string */
 struct pic_string *pic_str_new_cstr(pic_state *, const char *);
+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);
diff --git a/src/error.c b/src/error.c
index 5e6fee08..c375f36c 100644
--- a/src/error.c
+++ b/src/error.c
@@ -8,8 +8,6 @@
 
 #include "picrin.h"
 #include "picrin/pair.h"
-#include "picrin/proc.h"
-#include "picrin/port.h"
 #include "picrin/error.h"
 
 const char *
@@ -20,86 +18,6 @@ pic_errmsg(pic_state *pic)
   return pic->err->msg->str;
 }
 
-static pic_value
-pic_vfformat(pic_state *pic, XFILE *file, const char *fmt, va_list ap)
-{
-  char c;
-  pic_value irrs = pic_nil_value();
-
-  while ((c = *fmt++)) {
-    switch (c) {
-    default:
-      xfputc(c, file);
-      break;
-    case '%':
-      c = *fmt++;
-      if (! c)
-        goto exit;
-      switch (c) {
-      default:
-        xfputc(c, file);
-        break;
-      case '%':
-        xfputc('%', file);
-        break;
-      case 'c':
-        xfprintf(file, "%c", va_arg(ap, int));
-        break;
-      case 's':
-        xfprintf(file, "%s", va_arg(ap, const char *));
-        break;
-      case 'd':
-        xfprintf(file, "%d", va_arg(ap, int));
-        break;
-      case 'p':
-        xfprintf(file, "%p", va_arg(ap, void *));
-        break;
-      case 'f':
-        xfprintf(file, "%f", va_arg(ap, double));
-        break;
-      }
-      break;
-    case '~':
-      c = *fmt++;
-      if (! c)
-        goto exit;
-      switch (c) {
-      default:
-        xfputc(c, file);
-        break;
-      case '~':
-        xfputc('~', file);
-        break;
-      case '%':
-        xfputc('\n', file);
-        break;
-      case 'S':
-        irrs = pic_cons(pic, pic_fdebug(pic, va_arg(ap, pic_value), file), irrs);
-        break;
-      }
-      break;
-    }
-  }
- exit:
-
-  return pic_reverse(pic, irrs);
-}
-
-static pic_value
-pic_vformat(pic_state *pic, const char *fmt, va_list ap)
-{
-  struct pic_port *port;
-  pic_value irrs;
-
-  port = pic_open_output_string(pic);
-
-  irrs = pic_vfformat(pic, port->file, fmt, ap);
-  irrs = pic_cons(pic, pic_obj_value(pic_get_output_string(pic, port)), irrs);
-
-  pic_close_port(pic, port);
-  return irrs;
-}
-
 NORETURN static void
 error(pic_state *pic, struct pic_string *msg, pic_value irrs)
 {
diff --git a/src/string.c b/src/string.c
index 2c44c415..67b98692 100644
--- a/src/string.c
+++ b/src/string.c
@@ -5,6 +5,8 @@
 #include <string.h>
 
 #include "picrin.h"
+#include "picrin/pair.h"
+#include "picrin/port.h"
 
 struct pic_string *
 pic_str_new(pic_state *pic, const char *cstr, size_t len)
@@ -24,6 +26,99 @@ pic_str_new(pic_state *pic, const char *cstr, size_t len)
   return str;
 }
 
+pic_value
+pic_vfformat(pic_state *pic, XFILE *file, const char *fmt, va_list ap)
+{
+  char c;
+  pic_value irrs = pic_nil_value();
+
+  while ((c = *fmt++)) {
+    switch (c) {
+    default:
+      xfputc(c, file);
+      break;
+    case '%':
+      c = *fmt++;
+      if (! c)
+        goto exit;
+      switch (c) {
+      default:
+        xfputc(c, file);
+        break;
+      case '%':
+        xfputc('%', file);
+        break;
+      case 'c':
+        xfprintf(file, "%c", va_arg(ap, int));
+        break;
+      case 's':
+        xfprintf(file, "%s", va_arg(ap, const char *));
+        break;
+      case 'd':
+        xfprintf(file, "%d", va_arg(ap, int));
+        break;
+      case 'p':
+        xfprintf(file, "%p", va_arg(ap, void *));
+        break;
+      case 'f':
+        xfprintf(file, "%f", va_arg(ap, double));
+        break;
+      }
+      break;
+    case '~':
+      c = *fmt++;
+      if (! c)
+        goto exit;
+      switch (c) {
+      default:
+        xfputc(c, file);
+        break;
+      case '~':
+        xfputc('~', file);
+        break;
+      case '%':
+        xfputc('\n', file);
+        break;
+      case 'S':
+        irrs = pic_cons(pic, pic_fdebug(pic, va_arg(ap, pic_value), file), irrs);
+        break;
+      }
+      break;
+    }
+  }
+ exit:
+
+  return pic_reverse(pic, irrs);
+}
+
+pic_value
+pic_vformat(pic_state *pic, const char *fmt, va_list ap)
+{
+  struct pic_port *port;
+  pic_value irrs;
+
+  port = pic_open_output_string(pic);
+
+  irrs = pic_vfformat(pic, port->file, fmt, ap);
+  irrs = pic_cons(pic, pic_obj_value(pic_get_output_string(pic, port)), irrs);
+
+  pic_close_port(pic, port);
+  return irrs;
+}
+
+pic_value
+pic_format(pic_state *pic, const char *fmt, ...)
+{
+  va_list ap;
+  pic_value objs;
+
+  va_start(ap, fmt);
+  objs = pic_vformat(pic, fmt, ap);
+  va_end(ap);
+
+  return objs;
+}
+
 struct pic_string *
 pic_str_new_cstr(pic_state *pic, const char *cstr)
 {