free variable

Dyna is a declarative language for writing weighted dynamic programs. It compiles supplied rules into C++ classes — here’s an example of Dijkstra’s algorithm. Although it seems to be targeted to natural-language processing, I can think of several applications in artificial language processing as well.

(If you really want to nerd out with the declarative programming, please see this implementation of Dijkstra’s algorithm in the interactive fiction language Inform.)