Declarative programming and debugging

Declarative programming -- from regexes to XAML to mega-configuration like WSE or Enterprise Library -- can be pretty sweet, but my goodness, it's awful to debug. Get something wrong and it just doesn't work, or goes completely bonkers: we don't seem to have the tools to localise problems.

I don't have a solution; in fact I don't even have an idea of what a solution would look like. By the very nature of declarative programming, conventional ideas such as breakpoints and tracing are hard to apply. But there are increasingly complex declarative languages out there and it would be nice to have some way to track down errors more easily.