Trivial 15-line bison excercise: Implemented expressions including

user-defined numeric symbols. Whenever possible, use `expr' instead
of `NUM' to get full express ion power :-)

1 files changed, 31 insertions, 0 deletions

diff --git a/conf/confbase.Y b/conf/confbase.Y
index b20986ae..db6d0214 100644
--- a/conf/confbase.Y
+++ b/conf/confbase.Y
@@ -30,8 +30,17 @@ CF_DECLS
 %token <s> SYM
 %token <t> TEXT
 
+%type <i> expr
+
+%left '+' '-'
+%left '*' '/' '%'
+
+CF_KEYWORDS(DEFINE)
+
 CF_GRAMMAR
 
+/* Basic config file structure */
+
 config: conf_entries END {
 	return 0;
    }
@@ -44,6 +53,28 @@ conf_entries:
 
 CF_ADDTO(conf, /* EMPTY */)
 
+/* Expressions */
+
+expr:
+   NUM
+ | expr '+' expr { $$ = $1 + $3; }
+ | expr '-' expr { $$ = $1 - $3; }
+ | expr '*' expr { $$ = $1 * $3; }
+ | expr '/' expr { if ($3) $$ = $1 / $3; else cf_error("Division by zero"); }
+ | expr '%' expr { if ($3) $$ = $1 % $3; else cf_error("Division by zero"); }
+ | '(' expr ')' { $$ = $2; }
+ | SYM { if ($1->class != SYM_NUMBER) cf_error("Number expected"); else $$ = $1->aux; }
+ ;
+
+CF_ADDTO(conf, definition)
+definition:
+   DEFINE SYM '=' expr {
+     if ($2->class != SYM_VOID) cf_error("Symbol already defined");
+     $2->class = SYM_NUMBER;
+     $2->aux = $4;
+   }
+ ;
+
 CF_CODE
 
 CF_END