* Added an operator `?' to test for attribute existence, e.g.,

`attrs ? x' yields true iff `attrs' has an attribute named `x'.
NixOS · Mar 28, 2004 · ac4d39f · ac4d39f · nixos-discourse · Jul 2, 2023
1 parent f958bcd
commit ac4d39f
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Showing 2 changed files with 10 additions and 1 deletion.
diff --git 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
@@ -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
   ;