core: Add string-ref.

* src/strings.c (string_ref): New function.
* mes/module/mes/scm.mes (string-ref): Remove.

mes/module/mes/scm.mes

@@ -194,9 +194,6 @@
 (define (make-string n . fill)
   (list->string (apply make-list n fill)))
 
-(define (string-ref s k)
-  (list-ref (string->list s) k))
-
 (define (string-set! s k v)
   (list->string (list-set! (string->list s) k v)))
 

src/mes.c

@@ -1258,6 +1258,8 @@ eval_apply ()
           goto begin;
         }
     }
+  // write_error_ (CAR (r1));
+  // eputs ("\n");
   push_cc (CAR (r1), r1, r0, cell_vm_apply2);
   goto eval;
  apply2:

src/strings.c

@@ -18,6 +18,8 @@
  * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <string.h>
+
 #define MAX_STRING 4096
 
 char const*
@@ -267,3 +269,16 @@ string_length (SCM string)
   assert (TYPE (string) == TSTRING);
   return MAKE_NUMBER (LENGTH (string));
 }
+
+SCM
+string_ref (SCM str, SCM k)
+{
+  assert (TYPE (str) == TSTRING);
+  assert (TYPE (k) == TNUMBER);
+  size_t size = LENGTH (str);
+  size_t i = VALUE (k);
+  if (i >= size)
+    error (cell_symbol_system_error, cons (MAKE_STRING0 ("value out of range"), k));
+  char const *p = CSTRING (str);
+  return MAKE_CHAR (p[i]);
+}