word(I)
word(0) = "Hello".
foo(X, bar(Y))
phrase(X,I,K) max= phrase(Y,I,K) \* rule(X,Y).
phrase(X,I,I+1) max= rule(X, word(I)).
In picking Clojure, there were two main things that I was looking for in a language: compile-time macros and a runtime eval function. The reason for wanting compile time macros is that without them, you essentially create another DSL to generate code in you language of choice (e.g. tablegen and pdll in the llvm project). As such I was mainly considering at LISP-like language for the implementation of dyna3. Clojure's emphasis on immutable data structures also fit nicely with the design for R-exprs.
Lack of types in Clojure wasn't an issue in terms of getting things working. However, I did end up getting very annoyed with the runtime speed and quality of the generated code from the Clojure implementation. Features such as `^:dynamic` and `defprotocol` were too slow, so I rolled my own by wrapping a Java class and using macros. I also had to replace Clojure's builtin map with my own implementation of a map for performance-critical code. The R-exprs themselves are implemented using `deftype`, which generates a Java class rather than using maps to hold the relevant data.