Arc uses vectors to implement tagged procedures. Type the name of a macro in on the repl, and it displays a vector:
arc> def
#3(tagged mac #<procedure:.../arc/arc.arc.scm:151:11>)
That's a vector with 3 elements: tagged, mac, and a procedure.
Arc doesn't currently let you construct or manipulate your own vectors, but they could be used to provide transparent meta-data attached to functions and variables, such as where they were defined, and what the source code was that did so.
