#lang plait
(require (typed-in racket/base
                   [char->integer : (Char -> Number)]
                   [integer->char : (Number -> Char)]
                   [number->string : (Number -> String)]
                   [vector->list : ((Vectorof Number) -> (Listof Number))])
         (typed-in racket/vector
                   [vector-copy : ((Vectorof Number) Number Number -> (Vectorof Number))]))

(define MEMORY (make-vector 100 -1))
(define next-addr 0)
(define (reset) (set! next-addr 0))
(define (write-and-bump v)
  (let ([n next-addr])
    (begin
      (vector-set! MEMORY n v)
      (set! next-addr (add1 next-addr))
      n)))

(define NUMBER-TAG 1337)
(define STRING-TAG 5712)

(define (store-num n)
  (let ([a0 (write-and-bump NUMBER-TAG)])
    (begin
      (write-and-bump n)
      a0)))

(define (read-num a)
  (if (= (vector-ref MEMORY a) NUMBER-TAG)
      (vector-ref MEMORY (add1 a))
      (error 'number (number->string a))))

(test/exn (begin (reset) (read-num 0)) "number")
(test (read-num (store-num 42)) 42)