(bitpacket TypeA (type 8) (check (= type 1) ("somedata" bitstring))
(bitpacket TypeB (type 8) (check (= type 2) ("anotherdata" bitstring))

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

(define packetA (make-type 1)) ; result "\x01somedata"
(define packetB (make-type 2)) ; result "\x02anotherdata"