evt-do.rkt

#lang racket/base

(require racket/match
         racket/port
         syntax/parse/define
         (for-syntax racket/base))

(define (read-message inp)
 (match (read-byte inp)
   [#x00 (define payload-size (read-byte inp))
         (define payload (read-bytes payload-size inp))
         (list payload)]
   [#x01 'failure]))


(define-syntax <- (λ (stx) (raise-syntax-error #f "Illegal use of <-" stx)))

(define-syntax-parser evt-do #:literals (<-)
  [(_ x) #'(if (evt? x)
               x
               (handle-evt always-evt (λ (_) x)))]
  [(_ [x <- e] rest ...)
   #'(replace-evt e (λ (x) (evt-do rest ...)))])

(define (read-byte-evt in)
  (handle-evt in read-byte))

(define (read-message-evt in)
  (evt-do [b <- (read-byte-evt in)]
          (match b
            [#x00 (evt-do [size <- (read-byte-evt in)]
                          [payload <- (read-bytes-evt size in)]
                          (list payload))]
            [#x01 'failure])))