Hi everyone,

After trying to implement some changes Sage suggested (all errors are my 
own), I am now encountering the following message (courtesy of DrRacket):

"head: unbound identifier in: head"

Could someone help me find a fix?

As before, I welcome suggestions on how to better do this the functional 
programming way.

Karona

;example.rkt

#lang racket

(require racket/match)

(struct message (str sender recipient))

(define (say chat-history m)
  (cons m
        chat-history))

(define (log chat-history m)
  (cons m
        chat-history))
  
(let loop ()
  (display "Input: ")
  (define input (message (read-line (current-input-port) 'any) 
"participant" "me"))
  (define str (message-str input))
  (log chat-history input)

  (cond
      
    [(regexp-match #px"(?i:Hello,* world!)" str)
     (say chat-history (message "I would not know about the rest of the 
world, but I can hear \
you just fine." "me" "participant" "me" "participant"))]

    [(regexp-match #px"(?i:I( am|'m) learning how to program in Racket,* 
world!)" str)
     (say chat-history (message "Racket is a great language, and it is 
lovely that you are \
learning it, but does literally everyone need to know?" "me" 
"participant"))]

    [(regexp-match #px".*,+\\s*world!" str)
     (say chat-history (message "Did the whole world really need to hear 
that?" "me" "participant"))]
    
    [else (say chat-history (message "Did you really just say something 
without addressing the \
world? I am so proud of you! :,)" "me" "participant"))])
  
  (define hd (head chat-history))
  (define s (message-str hd))
  (printf "Chatbot: ~a\n" s)
  (loop))

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