1

I have a struct that represents a move 

(struct move (tile x y rotations) #:prefab)

Pass is also allowed, so I have a bool that test for an action:

(define (action? v)
  (if (or (pass? v)
          (move? v))
      #t
      ;; else
      #f
  )
)

I can declare a move like

(define test (move (tile 'blue (list (posn 0 0 ) (posn 1 0) (posn 2 0) (posn 3 0) (posn 3 1))) 2 4 0))

And when I test for action? it passes:

(action? test) #t

However, when I use this file on a different module, the move structure is not accepted as a valid move:

state-transition: contract violation expected: (and/c action? ???) given: '#s(move #s(tile blue (#s(posn 0 0) #s(posn 1 0) #s(posn 2 0) #s(posn 3 0) #s(posn 3 1))) 2 4 0) which isn't: ???

Here is the state-transition contract provided by the teacher:

[state-transition (->i ([s state?]
          [a (s) (and/c action?
                (cut state-action-legal? <> s))])
         [result state?])]

So I have basically the same object in two places. In one of them it passes the test, and in the other one it fails and a contract is broken. Why is this happening?

Edit

Here is the definition of state-action-legal?

(define (state-action-legal? state action)
  (not (state-action-violation state action))
)

And state-action-violation returns string indicating the reason the action is not legal or #F if the action is legal. I know they are both working properly because we tested them manually and the teacher provided unit tests to ensure proper implementation and these functions pass all of them.

End edit

Thank you for your help.

gamda
  • 580
  • 6
  • 24
  • It shouldn't affect this output, but is there a reason you're doing `(if something #t #f)` as opposed to `something`? I.e., why not `(define (action? v) (or (pass? v) (move? v)))`? – Joshua Taylor Oct 29 '13 at 21:23
  • 1
    Force of habit. Most contracts specify the return value must be #t or #f and and? returns the last true value, often a list, that would break the contract. By the time we wrote this function, we had broken a lot of contracts for that reason and just wrote the functions that way from the beginning. – gamda Oct 30 '13 at 02:15
  • The error says that the failing contract may not necessarily be on `action?`, but on the `???` part, which seems to be `state-action-legal?`. Can you give the definition of this function? As I understand it, it seems that you are not allowed to do the specified action `a` in the state `s`, even if it can be an `action?`. – Metaxal Nov 01 '13 at 11:20
  • @Metaxal I included the definition of the function. The action is being selected from a list of legal actions so I know that the move is legal. I tested it by creating a move with the same attributes and testing it manually. Thanks for your help. – gamda Nov 11 '13 at 03:03
  • You need to find out what is the state `s` for which `(state-transition s test)` fails. Then try`(state-action-violation s test)`. I think it will not return `#f`. Btw, passing all given unit tests never ensures that the function works properly. It can only give you more confidence. – Metaxal Nov 11 '13 at 17:35

0 Answers0