(bitpacket TypeA (1) ("somedata" bitstring))
(bitpacket TypeB (2) ("anotherdata bitstring))

(define make-type (type)
 (bitconstruct
   ((TypeA))
   ((TypeB))
   (else (error "invalid type")))

(define packetA (make-type 1))
(define packetB (make-type 2))