From 6c98e6a5dec2bcbc25ddeb2c279aa4a0b274bd6a Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Tue, 27 Nov 2012 15:01:32 +0100
Subject: Optionally ignore null-valued derivation attributes

This allows adding attributes like

  attr = if stdenv.system == "bla" then something else null;

without changing the resulting derivation on non-<bla> platforms.

We once considered adding a special "ignore" value for this purpose,
but using null seems more elegant.
---
 src/libexpr/eval.cc | 1 +
 1 file changed, 1 insertion(+)

(limited to 'src/libexpr/eval.cc')

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 31aaad5485f0..9eb5fc758985 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -139,6 +139,7 @@ EvalState::EvalState()
     , sSystem(symbols.create("system"))
     , sOverrides(symbols.create("__overrides"))
     , sOutputName(symbols.create("outputName"))
+    , sIgnoreNulls(symbols.create("__ignoreNulls"))
     , baseEnv(allocEnv(128))
     , baseEnvDispl(0)
     , staticBaseEnv(false, 0)
-- 
cgit 1.4.1