Most of this discussion on patterns (except for views) seems too much focused on concrete data types. (regexps for lists: what a horrible thought :-) This is just the thing to avoid in software design. (Don't expose implementation details, use interfaces etc.)
There's nothing in HaRP that would not work seamlessly with any sequential datatype through an interface of destructors, and clearly that's the Right (TM) way to go. The current implementation is just proof of concept. :-) IMO your comment only further speaks for my proposal to add guards to pattern synonyms. With an interface of destructors, you could define patterns that don't say anything about the underlying implementation, e.g. Head x = xs | x <- head xs where the head function comes from some interface for sequences. This is not something that can be done currently, nor with the initial proposal for pattern synonyms. /Niklas