graph-contracts.rkt

#lang racket

;; TODO: represent bases in binary

(struct node* (sequence offset))

(provide
 (contract-out [struct node* ((sequence string?) (offset (or/c natural? null)))]))

(provide (contract-out
          [node (-> string?
                    (or/c natural? null?)
                    (struct/c node* string? (or/c natural? null?)))]))

;; define node as a fn
(define/contract (node sequence [offset null])
  (node* sequence offset))

(node* 123 "dd") ;; succeeds

alternative using typed/racket

#lang typed/racket

(provide (all-defined-out))

(struct node*
  ([sequence : String]
   [offset : (Option Natural)]))

(define (node [seq : String]
              [offset : (Option Natural) #f])
  (node* seq offset))

(node "foo" 123) ;; succeeds
(node "foo" #f) ;; succeeds
(node "foo") ;; succeeds
;(node 123 "dd") ;; type mismatch

Or if you want to use null:

#lang typed/racket

(provide (all-defined-out))

(define-type Natural-or-Null (U Natural Null))

(struct node*
  ([sequence : String]
   [offset : Natural-or-Null]))

(define (node [seq : String]
              [offset : Natural-or-Null null])
  (node* seq offset))