The built-in ability to pattern match on data nested deeply within the application state (or the XML document) is radically powerful. Haskell, for example, has

The built-in ability to pattern match on data nested deeply within the application state (or the XML document) is radically powerful. Haskell, for example, has nothing like this. Eve is a really interesting language where you define global pattern matches, using datalog-style unification rather than XPath, and these "templates" can result in state changes. So for example you can trivially define global invariants over the entire state, kind of like defining a database integrity constraint but for your program's state. As an aside, I'm recently feeling disillusioned with Haskell—after loving it since 2004—as any kind of ultimate or next-level general purpose language, precisely because of the lack of this dimension. Dealing with relational data in Haskell is quite tedious. Even writing multipass compilers in Haskell is not very pleasant.