From ac4d39f9db28743b6c1e9def7a61241a50b02335 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <e.dolstra@tudelft.nl>
Date: Sun, 28 Mar 2004 21:15:01 +0000
Subject: [PATCH] * Added an operator `?' to test for attribute existence,
 e.g.,   `attrs ? x' yields true iff `attrs' has an attribute named `x'.

---
 src/libexpr/eval.cc  | 9 ++++++++-
 src/libexpr/parser.y | 2 ++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 2281ee7212..0623e49532 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -288,10 +288,17 @@ Expr evalExpr2(EvalState & state, Expr e)
     if (atMatch(m, e) >> "OpOr" >> e1 >> e2)
         return makeBool(evalBool(state, e1) || evalBool(state, e2));
 
-    /* Attribut set update (//). */
+    /* Attribute set update (//). */
     if (atMatch(m, e) >> "OpUpdate" >> e1 >> e2)
         return updateAttrs(evalExpr(state, e1), evalExpr(state, e2));
 
+    /* Attribute existence test (?). */
+    if (atMatch(m, e) >> "OpHasAttr" >> e1 >> name) {
+        ATermMap attrs;
+        queryAllAttrs(evalExpr(state, e1), attrs);
+        return makeBool(attrs.get(name) != 0);
+    }
+
     /* Barf. */
     throw badTerm("invalid expression", e);
 }
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index b1fe9e0931..cfcbc589f6 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -44,6 +44,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, void * data, char * s)
 %nonassoc EQ NEQ
 %right UPDATE
 %left NEG
+%nonassoc '?'
 %nonassoc '~'
 
 %%
@@ -81,6 +82,7 @@ expr_op
   | expr_op IMPL expr_op { $$ = ATmake("OpImpl(<term>, <term>)", $1, $3); }
   | expr_op UPDATE expr_op { $$ = ATmake("OpUpdate(<term>, <term>)", $1, $3); }
   | expr_op '~' expr_op { $$ = ATmake("SubPath(<term>, <term>)", $1, $3); }
+  | expr_op '?' ID { $$ = ATmake("OpHasAttr(<term>, <term>)", $1, $3); }
   | expr_app
   ;