2 files changed, 45 insertions, 18 deletions

diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 3df9dd47f0bb..b8d0929285fe 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -16,6 +16,7 @@ MakeError(ThrownError, AssertionError)
 MakeError(Abort, EvalError)
 MakeError(TypeError, EvalError)
 MakeError(ImportError, EvalError) // error building an imported derivation
+MakeError(FindError, EvalError) // error building a nix-path component
 MakeError(UndefinedVarError, Error)
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index f270ca302f45..fecaf37b6ed3 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -37,36 +37,52 @@ std::pair<string, string> decodeContext(const string & s)
 }
 
 
-/* Load and evaluate an expression from path specified by the
-   argument. */
-static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args, Value & v)
+struct InvalidPathError : EvalError
 {
-    PathSet context;
-    Path path = state.coerceToPath(pos, *args[1], context);
+    Path path;
+    InvalidPathError(const Path & path) :
+        EvalError(format("path `%1%' is not valid") % path), path(path) {};
+};
 
+
+static void realiseContext(const PathSet & context)
+{
     PathSet drvs;
     for (auto & i : context) {
         std::pair<string, string> decoded = decodeContext(i);
         Path ctx = decoded.first;
         assert(isStorePath(ctx));
         if (!store->isValidPath(ctx))
-            throw EvalError(format("cannot import `%1%', since path `%2%' is not valid, at %3%")
-                % path % ctx % pos);
+            throw InvalidPathError(ctx);
         if (isDerivation(ctx))
             drvs.insert(decoded.first + "!" + decoded.second);
     }
     if (!drvs.empty()) {
-        try {
-            /* For performance, prefetch all substitute info. */
-            PathSet willBuild, willSubstitute, unknown;
-            unsigned long long downloadSize, narSize;
-            queryMissing(*store, drvs,
-                willBuild, willSubstitute, unknown, downloadSize, narSize);
+        /* For performance, prefetch all substitute info. */
+        PathSet willBuild, willSubstitute, unknown;
+        unsigned long long downloadSize, narSize;
+        queryMissing(*store, drvs,
+            willBuild, willSubstitute, unknown, downloadSize, narSize);
 
-            store->buildPaths(drvs);
-        } catch (Error & e) {
-            throw ImportError(e.msg());
-        }
+        store->buildPaths(drvs);
+    }
+}
+
+
+/* Load and evaluate an expression from path specified by the
+   argument. */
+static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    PathSet context;
+    Path path = state.coerceToPath(pos, *args[1], context);
+
+    try {
+        realiseContext(context);
+    } catch (InvalidPathError & e) {
+        throw EvalError(format("cannot import `%1%', since path `%2%' is not valid, at %3%")
+            % path % e.path % pos);
+    } catch (Error & e) {
+        throw ImportError(e.msg());
     }
 
     if (isStorePath(path) && store->isValidPath(path) && isDerivation(path)) {
@@ -660,6 +676,7 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
 
     SearchPath searchPath;
 
+    PathSet context;
     for (unsigned int n = 0; n < args[0]->list.length; ++n) {
         Value & v2(*args[0]->list.elems[n]);
         state.forceAttrs(v2, pos);
@@ -672,13 +689,22 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         i = v2.attrs->find(state.symbols.create("path"));
         if (i == v2.attrs->end())
             throw EvalError(format("attribute `path' missing, at %1%") % pos);
-        PathSet context;
         string path = state.coerceToPath(pos, *i->value, context);
 
         searchPath.push_back(std::pair<string, Path>(prefix, path));
     }
 
     string path = state.forceStringNoCtx(*args[1], pos);
+
+    try {
+        realiseContext(context);
+    } catch (InvalidPathError & e) {
+        throw EvalError(format("cannot find `%1%', since path `%2%' is not valid, at %3%")
+            % path % e.path % pos);
+    } catch (Error & e) {
+        throw FindError(e.msg());
+    }
+
     mkPath(v, state.findFile(searchPath, path).c_str());
 }