* An attribute set update operator (//). E.g.,

  {x=1; y=2; z=3;} // {y=4;}  =>  {x=1; y=4; z=3;}

src/libexpr/eval.cc

@@ -134,6 +134,18 @@ ATerm expandRec(ATerm e, ATermList rbnds, ATermList nrbnds)
 }
 
 
+static Expr updateAttrs(Expr e1, Expr e2)
+{
+    /* Note: e1 and e2 should be in normal form. */
+
+    ATermMap attrs;
+    queryAllAttrs(e1, attrs);
+    queryAllAttrs(e2, attrs);
+
+    return makeAttrs(attrs);
+}
+
+
 string evalString(EvalState & state, Expr e)
 {
     e = evalExpr(state, e);
@@ -264,6 +276,10 @@ Expr evalExpr2(EvalState & state, Expr e)
     if (atMatch(m, e) >> "OpOr" >> e1 >> e2)
         return makeBool(evalBool(state, e1) || evalBool(state, e2));
 
+    /* Attribut set update (//). */
+    if (atMatch(m, e) >> "OpUpdate" >> e1 >> e2)
+        return updateAttrs(evalExpr(state, e1), evalExpr(state, e2));
+
     /* Barf. */
     throw badTerm("invalid expression", e);
 }

src/libexpr/lexer.l

@@ -57,6 +57,7 @@ inherit     { return INHERIT; }
 \&\&        { return AND; }
 \|\|        { return OR; }
 \-\>        { return IMPL; }
+\/\/        { return UPDATE; }
 
 {ID}        { yylval->t = ATmake("<str>", yytext); return ID; /* !!! alloc */ }
 {INT}       { int n = atoi(yytext); /* !!! overflow */

src/libexpr/parser.y

@@ -42,6 +42,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, void * data, char * s)
 %left OR
 %left AND
 %nonassoc EQ NEQ
+%right UPDATE
 %left NEG
 
 %%
@@ -69,6 +70,7 @@ expr_op
   | expr_op AND expr_op { $$ = ATmake("OpAnd(<term>, <term>)", $1, $3); }
   | expr_op OR expr_op { $$ = ATmake("OpOr(<term>, <term>)", $1, $3); }
   | expr_op IMPL expr_op { $$ = ATmake("OpImpl(<term>, <term>)", $1, $3); }
+  | expr_op UPDATE expr_op { $$ = ATmake("OpUpdate(<term>, <term>)", $1, $3); }
   | expr_app
   ;