Source code in the transd programming language

#lang transd

MainModule: {

contains-duplicates: (λ s String() -> Bool()
    (with str String(s) (sort str) 
        (ret (neq (find-adjacent str) (end str))))
),

play: (λ locals:
    syms "0123456789" 
    len 4 
    thenum String() 
    guess String()

    (shuffle syms)
    (= thenum (substr syms 0 len))
    (textout "Your guess: ")
    (while (getline guess)
        (if (eq guess "q") break)
        (if (or (neq (size guess) len) 
                (neq (find-first-not-of guess syms) -1)
                (contains-duplicates guess))
            (lout guess " is not valid guess")
            (textout "Your guess: ")
            continue
        )
        (with bulls 0 cows 0 pl 0
            (for i in Range(len) do 
                (= pl (index-of thenum (subn guess i)))
                (if (eq pl i) (+= bulls 1)
                elsif (neq pl -1) (+= cows 1))
            )
            (lout "bulls: " bulls ", cows: " cows)
            (if (eq bulls len)
                (lout "Congratulations! You have found out the number!")
                (ret null)
            else (textout "Your guess: "))
        )
    )
    (lout "You quit the game.")
),

_start: (λ locals: s String()
    (lout "Welcome to \"Bulls and cows\"!") 
    (while true
        (while true
            (textout "Do you want to play? (yes|no) : ")
            (getline s)
            (if (not (size s)) 
                (lout "Didn't receive an answer. Exiting.") (exit)
            elsif (== (sub (tolower s) 0 1) "n") (lout "Bye!")(exit)
            elsif (== (sub (tolower s) 0 1) "y") break
            else (lout "(Hint: \"yes\" or \"no\".)"))
        )
        (play)
        (lout "Another game?")
        )
)
}