From 20866a7031ca823055a221653b77986faa167329 Mon Sep 17 00:00:00 2001
From: Shea Levy <shea@shealevy.com>
Date: Mon, 15 Jul 2013 15:53:14 -0400
Subject: Delay evaulation of `with` attrs until a variable lookup needs them

Evaluation of attribute sets is strict in the attribute names, which
means immediate evaluation of `with` attribute sets rules out some
potentially interesting use cases (e.g. where the attribute names of one
set depend in some way on another but we want to bring those names into
scope for some values in the second set).

The major example of this is overridable self-referential package sets
(e.g. all-packages.nix). With immediate `with` evaluation, the only
options for such sets are to either make them non-recursive and
explicitly use the name of the overridden set in non-overridden one
every time you want to reference another package, or make the set
recursive and use the `__overrides` hack. As shown in the test case that
comes with this commit, though, delayed `with` evaluation allows a nicer
third alternative.

Signed-off-by: Shea Levy <shea@shealevy.com>
---
 tests/lang/eval-okay-delayed-with.exp |  1 +
 tests/lang/eval-okay-delayed-with.nix | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+)
 create mode 100644 tests/lang/eval-okay-delayed-with.exp
 create mode 100644 tests/lang/eval-okay-delayed-with.nix

(limited to 'tests')

diff --git a/tests/lang/eval-okay-delayed-with.exp b/tests/lang/eval-okay-delayed-with.exp
new file mode 100644
index 000000000..eaacb55c1
--- /dev/null
+++ b/tests/lang/eval-okay-delayed-with.exp
@@ -0,0 +1 @@
+"b-overridden"
diff --git a/tests/lang/eval-okay-delayed-with.nix b/tests/lang/eval-okay-delayed-with.nix
new file mode 100644
index 000000000..82934d6a9
--- /dev/null
+++ b/tests/lang/eval-okay-delayed-with.nix
@@ -0,0 +1,26 @@
+let
+  pkgs_ = with pkgs; {
+    a = derivation {
+      name = "a";
+      system = builtins.currentSystem;
+      builder = "/bin/sh";
+      args = [ "-c" "touch $out" ];
+      inherit b;
+    };
+
+    b = derivation {
+      name = "b";
+      system = builtins.currentSystem;
+      builder = "/bin/sh";
+      args = [ "-c" "touch $out" ];
+    };
+
+    c = b;
+  };
+
+  packageOverrides = p: {
+    b = derivation (p.b.drvAttrs // { name = "b-overridden"; });
+  };
+
+  pkgs = pkgs_ // (packageOverrides pkgs_);
+in pkgs.a.b.name
-- 
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