* Some refactoring of the exception handling code so that we can catch

  Nix expression assertion failures.

2 files changed, 16 insertions, 5 deletions

diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index ee34cb18a5..0d970e69e3 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -25,6 +25,13 @@ Error::Error(const format & f)
 }
 
 
+Error & Error::addPrefix(const format & f)
+{
+    err = f.str() + err;
+    return *this;
+}
+
+
 SysError::SysError(const format & f)
     : Error(format("%1%: %2%") % f.str() % strerror(errno))
 {
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index aac2acd173..05cf777f1e 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -26,6 +26,7 @@ public:
     ~Error() throw () { };
     const char * what() const throw () { return err.c_str(); }
     const string & msg() const throw () { return err; }
+    Error & addPrefix(const format & f);
 };
 
 class SysError : public Error
@@ -34,11 +35,14 @@ public:
     SysError(const format & f);
 };
 
-class UsageError : public Error
-{
-public:
-    UsageError(const format & f) : Error(f) { };
-};
+#define MakeError(newClass, superClass) \
+    class newClass : public superClass                  \
+    {                                                   \
+    public:                                             \
+        newClass(const format & f) : superClass(f) { }; \
+    };
+
+MakeError(UsageError, Error)
 
 
 typedef list<string> Strings;