configurable none value

include/config.h

@@ -10,6 +10,9 @@
 /* enable readline module */
 #define PIC_ENABLE_READLINE 1
 
+/* treat false value as none */
+#define PIC_NONE_IS_FALSE 1
+
 /* initial memory size (to be dynamically extended if necessary) */
 #define PIC_ARENA_SIZE 100
 #define PIC_HEAP_PAGE_SIZE (10000)

include/picrin/value.h

@@ -143,6 +143,7 @@ pic_value pic_float_value(double);
 pic_value pic_int_value(int);
 pic_value pic_symbol_value(pic_sym);
 pic_value pic_char_value(char c);
+pic_value pic_none_value();
 
 #define pic_float(v) ((v).u.f)
 #define pic_int(v) ((v).u.i)