The definition of add-element you have given does not match its contract. It looks like the contract specifies two parameters (though your notation is a bit inconsistent, and you might think about that as well) but your implementation only has one parameter. I think if you are careful about your data definition and then use it properly in the design recipe, you might see where you are going wrong.

ok, I’ll work on it. Thanks !

@plragde @dan153 Hello,

I “think” I finally understand my confusions:

1. My interpretation of the data definition was ~wrong~ not the right one:

; A set is a function:
; A [Set X] is [X -> Boolean]
; Interpretation: if s is a set and x an element, (s x) produces #t if x belongs to s, ; #f otherwise.
; *MEANS is a given element in the set ? It is preferable to: can I add the  ;element to the set ?* (this "flawed" interpretation led me to thinking about ;storing elements in sets).
; (Strange how the brain can sometimes go astray...)


; Number -> Boolean
; Set of even numbers.
(check-expect (even-numbers 5) #f)
(check-expect (even-numbers 8) #t)

(define even-numbers
  (lambda (n) (even? n)))


; [Set X] X -> [Set X]
; Adds an element elt to a set s.
(check-expect [(add-element even-numbers 5) 5] #f)
(check-expect [(add-element even-numbers 5) 6] #f)
(check-expect [(add-element even-numbers 6) 6] #t)
(check-expect [(add-element even-numbers 6) 5] #f)

(define (add-element s elt)
  (lambda (x) (and (equal? x elt)
                  (s x))))

1. My understanding of the purpose of lambdas is not as good as I thought it was. I have to rework the chapter.

Thank you all !

@diallo.ms Good! In your defence, lambdas have diverse uses, and it takes some experience to realize just how much one can do with them.