diff --git a/ascii.hpp b/ascii.hpp
index b951ad93..b49cff5b 100644
--- a/ascii.hpp
+++ b/ascii.hpp
@@ -17,6 +17,11 @@
 #ifndef __CRUFT_UTIL_ASCII_HPP
 #define __CRUFT_UTIL_ASCII_HPP
 
+#include "./annotation.hpp"
+
+#include <cstdint>
+#include <stdexcept>
+
 namespace util { namespace ascii {
     ///////////////////////////////////////////////////////////////////////////
     constexpr inline
@@ -26,6 +31,16 @@ namespace util { namespace ascii {
         return c >= '0' && c <= '9';
     }
 
+    //-------------------------------------------------------------------------
+    constexpr inline
+    uint8_t
+    to_integer (char c)
+    {
+        return likely (is_digit (c)) ?
+              c - '0'
+            : throw std::invalid_argument ("character is not a digit");
+    }
+
 
     //-------------------------------------------------------------------------
     constexpr inline