5 files changed, 19 insertions, 6 deletions
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 1ee6c5a7f6aa..eb1d8d432c69 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -59,6 +59,7 @@ std::ostream & operator << (std::ostream & str, const Value & v)
         str << "]";
         break;
     case tThunk:
+    case tApp:
     case tCopy:
         str << "<CODE>";
         break;
@@ -901,12 +902,18 @@ string EvalState::forceString(Value & v)
 }
 
 
-string EvalState::forceString(Value & v, PathSet & context)
+void copyContext(const Value & v, PathSet & context)
 {
-    string s = forceString(v);
     if (v.string.context)
         for (const char * * p = v.string.context; *p; ++p) 
             context.insert(*p);
+}
+
+
+string EvalState::forceString(Value & v, PathSet & context)
+{
+    string s = forceString(v);
+    copyContext(v, context);
     return s;
 }
 
@@ -937,9 +944,7 @@ string EvalState::coerceToString(Value & v, PathSet & context,
     string s;
 
     if (v.type == tString) {
-        if (v.string.context) 
-            for (const char * * p = v.string.context; *p; ++p) 
-                context.insert(*p);
+        copyContext(v, context);
         return v.string.s;
     }
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 4e0af70ba64b..03bf43a3b9a4 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -163,6 +163,8 @@ void mkString(Value & v, const char * s);
 void mkString(Value & v, const string & s, const PathSet & context = PathSet());
 void mkPath(Value & v, const char * s);
 
+void copyContext(const Value & v, PathSet & context);
+
 
 typedef std::map<Path, Hash> DrvHashes;
 
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 1c72441b2792..b1043a32684e 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -15,6 +15,7 @@ MakeError(AssertionError, EvalError)
 MakeError(ThrownError, AssertionError)
 MakeError(Abort, EvalError)
 MakeError(TypeError, EvalError)
+MakeError(ImportError, EvalError) // error building an imported derivation
 
 
 /* Position objects. */
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 9d36fb6a05d7..42c8586116aa 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -37,7 +37,11 @@ static void prim_import(EvalState & state, Value * * args, Value & v)
             throw EvalError(format("cannot import `%1%', since path `%2%' is not valid")
                 % path % *i);
         if (isDerivation(*i))
-            store->buildDerivations(singleton<PathSet>(*i));
+            try {
+                store->buildDerivations(singleton<PathSet>(*i));
+            } catch (Error & e) {
+                throw ImportError(e.msg());
+            }
     }
 
     state.evalFile(path, v);
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index e751fd300fdb..8955a8a33931 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -69,6 +69,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
         case tString:
             /* !!! show the context? */
+            copyContext(v, context);
             doc.writeEmptyElement("string", singletonAttrs("value", v.string.s));
             break;