From 7d3e75827bc20d50a3b9172ed8891e5bffb30c93 Mon Sep 17 00:00:00 2001
From: Nikolay Amiantov <ab@fmap.me>
Date: Thu, 14 Apr 2016 18:13:50 +0300
Subject: [PATCH] getOutput function: init

---
 lib/attrsets.nix | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/lib/attrsets.nix b/lib/attrsets.nix
index 4161fa546c8f..9104ff35d194 100644
--- a/lib/attrsets.nix
+++ b/lib/attrsets.nix
@@ -438,6 +438,18 @@ rec {
   overrideExisting = old: new:
     old // listToAttrs (map (attr: nameValuePair attr (attrByPath [attr] old.${attr} new)) (attrNames old));
 
+  /* Get a package output.
+     If no output is found, fallback to `.out` and then to the default.
+
+     Example:
+       getOutput "dev" pkgs.openssl
+       => "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev"
+  */
+  getOutput = output: pkg:
+    if pkg.outputUnspecified or false
+      then pkg.${output} or pkg.out or pkg
+      else pkg;
+
 
   /*** deprecated stuff ***/