#lang racket
; Common Racket Bugs:
; "procedure-name": contract violation
;   expected: ...
;   given: ...

; tl;dr: you are calling a built-in Racket procedure with the wrong type of argument,
;        there is probably some underlying flaw in your logic and you may need to add
;        additional base cases

; -------------------------------------------------------------------------------------------
; CORE PROBLEM: 
; -------------------------------------------------------------------------------------------

; This is one of the more common bugs that novices to Racket will run into.
; So what does it mean?

; Well the short answer is that you called a built-in Racket procedure with the wrong
; argument types. Let's look at the example below:

(define (own-length lst)
  (+ 1 (own-length (rest lst))))
  
(own-length '())

; Trying running this code!

; Contract violations can be a bit trickier to fix compared to the other common errors.
; Let's take a look at the error message:
; rest: contract violation
;   expected: (and/c list? (not/c empty?))
;   given: '()

; Next, let's break this down line by line.

; The first line, "rest: contract violation", tells you what procedure is giving the error,
; so in this case we know that there is something wrong with our call to (rest).

; The second line, "expected: (and/c list? (not/c empty?))", tell us what kind of argument
; (rest) wants as input. The "and/c" part means that there are multiple requirements that must be
; satisfied. Next, "list?" means that the input should be a list, and "(not/c empty?)" means that
; the input should not be a empty list. So altogether, "expected: (and/c list? (not/c empty?))"
; is telling us that (rest) expects a list that is not empty.

; The third line, "given: '()", tells us what (rest) actually got as an argument. As you can see,
; we gave (rest) an empty list '(). No wonder there's an error! We just talked about above how
; (rest) explicity wants to be given a non-empty list as an input. 

; So now is the tough part, fixing your code. A contract violation almost always points to an
; underlying flaw in your logic. Often, it means that you did not account for enough cases
; when you were writing your code. For example, in the code above, something should have been
; done to prevent (rest) having being called on an empty list. In this particular example, it
; makes sense to check that the list is empty, before calling rest. Thus, this code is missing
; a base case, and it probably needs an (if) or (cond) statement.

; See if you can fix the code above!

; -------------------------------------------------------------------------------------------
; MORE COMPLICATED EXAMPLE: 
; -------------------------------------------------------------------------------------------

; (sum-list lst) should add up all the numbers in a list, any non-numbers in the list are ignored
(define (sum-list lst)
  (cond
    [(empty? lst) 0]
    [else (+ (first lst) (sum-list (rest lst)))]))

(sum-list '(1 2 3))
(sum-list '(1 a 2 3))
                     
; Trying running this code!

; You should see an error similar to the one you saw above. Here, the error message is telling you
; that the (+) command expected to get a number, but it got 'a instead.
; How would you fix this code?

; You should be able to fix it by adding another base case, in case the first thing in the list
; is not a number. 

; -------------------------------------------------------------------------------------------
; Feedback: 
; -------------------------------------------------------------------------------------------

; Did this help you fix your bug? If not, feel free to shoot us an e-mail or come to
; office hours. You can also send feedback about this help file to: nathan.lin@yale.edu
; Thanks!