diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 38d737725..daf53846f 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -336,5 +336,13 @@ string showType(const Value & v);
 /* If `path' refers to a directory, then append "/default.nix". */
 Path resolveExprPath(Path path);
 
+struct InvalidPathError : EvalError
+{
+    Path path;
+    InvalidPathError(const Path & path);
+};
+
+/* Realise all paths in `context' */
+void realiseContext(const PathSet & context);
 
 }
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index a1374a05e..50d03f369 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -39,16 +39,10 @@ std::pair<string, string> decodeContext(const string & s)
 }
 
 
-struct InvalidPathError : EvalError
-{
-    Path path;
-    InvalidPathError(const Path & path) :
-        EvalError(format("path ‘%1%’ is not valid") % path), path(path) {};
-    ~InvalidPathError() throw () { };
-};
+InvalidPathError::InvalidPathError(const Path & path) :
+    EvalError(format("path ‘%1%’ is not valid") % path), path(path) {}
 
-
-static void realiseContext(const PathSet & context)
+void realiseContext(const PathSet & context)
 {
     PathSet drvs;
     for (auto & i : context) {