* New builtin functions builtins.{hasAttr, getAttr} to check for

attribute existence and to return an attribute from an attribute
  set, respectively.  Example: `hasAttr "foo" {foo = 1;}'.  They
  differ from the `?' and `.' operators in that the attribute name is
  an arbitrary expression.  (NIX-61)

src/libexpr/primops.cc

@@ -703,6 +703,20 @@ static Expr primCurrentTime(EvalState & state, const ATermVector & args)
 }
 
 
+static Expr primGetAttr(EvalState & state, const ATermVector & args)
+{
+    string attr = evalString(state, args[0]);
+    return evalExpr(state, makeSelect(args[1], toATerm(attr)));
+}
+
+
+static Expr primHasAttr(EvalState & state, const ATermVector & args)
+{
+    string attr = evalString(state, args[0]);
+    return evalExpr(state, makeOpHasAttr(args[1], toATerm(attr)));
+}
+
+
 static Expr primRemoveAttrs(EvalState & state, const ATermVector & args)
 {
     ATermMap attrs(128); /* !!! */
@@ -749,6 +763,8 @@ void EvalState::addPrimOps()
     addPrimOp("abort", 1, primAbort);
 
     addPrimOp("map", 2, primMap);
+    addPrimOp("__getAttr", 2, primGetAttr);
+    addPrimOp("__hasAttr", 2, primHasAttr);
     addPrimOp("removeAttrs", 2, primRemoveAttrs);
     addPrimOp("relativise", 2, primRelativise);
 }

tests/lang/eval-okay-attrs2.exp

@@ -0,0 +1 @@
+Int(987)

tests/lang/eval-okay-attrs2.nix

@@ -0,0 +1,10 @@
+let {
+  as = { x = 123; y = 456; } // { z = 789; } // { z = 987; };
+
+  A = "a";
+  Z = "z";
+
+  body = if builtins.hasAttr A as
+         then builtins.getAttr A as
+         else assert builtins.hasAttr Z as; builtins.getAttr Z as;
+}