From 07a08bddf001271b4b7eff2a119c395f40119966 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date: Tue, 3 Sep 2013 15:45:32 +0200
Subject: [PATCH] nix-env: Load files in ~/.nix-defexpr on demand

So if you do "nix-env -qa -A nixos", then other channels won't be
parsed/evaluated at all.
---
 src/libexpr/eval.cc    | 6 ++++++
 src/libexpr/eval.hh    | 6 ++++++
 src/nix-env/nix-env.cc | 9 ++++++---
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 50f36ce4e..af2bf92e3 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -222,6 +222,12 @@ void EvalState::addPrimOp(const string & name,
 }
 
 
+void EvalState::getBuiltin(const string & name, Value & v)
+{
+    v = *baseEnv.values[0]->attrs->find(symbols.create(name))->value;
+}
+
+
 /* Every "format" object (even temporary) takes up a few hundred bytes
    of stack space, which is a real killer in the recursive
    evaluator.  So here are some helper functions for throwing
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 29c8341df..b08bec8d9 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -203,6 +203,12 @@ private:
     void addPrimOp(const string & name,
         unsigned int arity, PrimOpFun primOp);
 
+public:
+
+    void getBuiltin(const string & name, Value & v);
+
+private:
+
     inline Value * lookupVar(Env * env, const VarRef & var, bool noEval);
 
     friend class ExprVar;
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index e4cc66e76..9f51e919b 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -131,9 +131,12 @@ static void getAllExprs(EvalState & state,
                 continue;
             }
             attrs.insert(attrName);
-            // FIXME: make loading lazy.
-            Value & v2(*state.allocAttr(v, state.symbols.create(attrName)));
-            state.evalFile(path2, v2);
+            /* Load the expression on demand. */
+            Value & vFun(*state.allocValue());
+            Value & vArg(*state.allocValue());
+            state.getBuiltin("import", vFun);
+            mkString(vArg, path2);
+            mkApp(*state.allocAttr(v, state.symbols.create(attrName)), vFun, vArg);
         }
         else if (S_ISDIR(st.st_mode))
             /* `path2' is a directory (with no default.nix in it);