The rescue clause does not work

by Creative Bachkhoa (modified: 2009 Oct 31)

I've tested the system by an example of exception handling in Eiffel <code> feature {NONE} -- Initialization i:INTEGER x:REAL y:ARRAY[REAL] make -- Run application. do create y.make(1,2) io.put_string ("The item at position: ") io.read_integer i:=io.last_integer find io.put_string (" is: ") io.put_real (x) io.put_string ("%NReturn to make is successful") end ----------------------------------------------- find require i<=2 local no_retry:BOOLEAN do if not no_retry then x:=y.item (i) io.put_real (x) end no_retry:=true rescue if not no_retry then io.put_string ("Can not execute instruction") no_retry:=true end end --------------------------- end
<code> When the precondition is violated, the rescue clause take no action Can anybody help me fix this error? I am using EiffelStudio - 6.4 GPL (Official Release) Thanks very much!

Comments
  • Bernd Schoeller (7 years ago 31/10/2009)

    No violation

    Why should the rescue clause be triggered? There is not contract violation in find. The precondition is an obligation to the caller, so the contract violation actually happens in the caller. Try adding a rescue clause to make.

    • Creative Bachkhoa (7 years ago 1/11/2009)

      "There is not contract violation in find"

       Thanks you for your reply.<br/>
       But, there is violation. When I input i the value 3. The precondition fails. But no retry is made<br/>
       There is an other example that make me trouble.

      create make feature {NONE} -- Initialization r:REAL make do io.put_string ("Input a number ") io.read_real r:=io.last_real io.put_string ("Inverse of ") io.put_real (r) io.put_string (" is ") io.put_real(quasi_inverse (r)) rescue retry end ---------------------------- quasi_inverse (x: REAL): REAL is -- 1/x if possible, otherwise 0 require x/=0 local division_tried: BOOLEAN do if not division_tried then Result := 1/x else io.put_string ("Divide by zero") end end end

        Can you help me? I am just a beginner.
  • Colin LeMahieu (7 years ago 2/11/2009)

    Contract violations aren't exceptions, they aren't supposed to be caught in a retry clause.

    The require clause: require i <= 2

    Doesn't mean an exception will be raised if i is greater than 2, it means you have a program bug that needs to be fixed because someone called find while i was bigger than 2.

    Before you call `find' you should have a function that checks to make sure i <= 2 and take an appropriate measure if it is not.

  • Colin Adams (7 years ago 2/11/2009)

    A precondition violation DOES raise an exception (assuming that the OP has precondition monitoring turned on). The point is (as Bernd pointed out) that precondition violations are raised in the client, not the supplier. Colin Adams

    • Phung Dinh Vu (7 years ago 2/11/2009)

      I don't know much about violation and exception.

      When is an exception occured. that is when pre or postcondition is violated. And how can we control the exception or violation. In java, we can do it by try and proper catch block, and then we can prcess it in our way with proper exception. In eiffel, i don't know how can we do this? Any more, i know when we use condition we often use this: positive : account > 0

      But the word positive use for what purpose ?

      • Colin LeMahieu (7 years ago 2/11/2009)

        Try these functions for an example of the difference in exceptions: make do one two end one -- Exception in `proc' require clause is raised in *calling* function, `one' local retried: BOOLEAN value: INTEGER do if not retried then value := proc (3) end rescue retried := True retry end two -- Exception in `proc' ensure clause is raised in *called* function, `proc' local value: INTEGER do value := proc (2) end proc (i: INTEGER): INTEGER require i <= 2 local retried: BOOLEAN do if not retried then Result := 2 * i end ensure Result <= 3 rescue retried := True Result := 3 retry end

        When an exception is in the require' clause, the exception is raised in the *calling* function. When an exception is in the ensure' clause, the exception is raised in the *called* function.

        • Phung Dinh Vu (7 years ago 3/11/2009)

          Thanks

          Phung Dinh Vu- Hut Now I understand little. I'm coding for check. Thank "Colin LeMahieu" very much. ohm, Can I ask you a question ? LeMahieu is likely a vietnames name. Are you VietNames?

  • Colin Adams (7 years ago 3/11/2009)

    Tags appear in error messages

    The "positive:" part is the assertion tag.

    When you are monitoring assertions, if an aseertion fails, the tag will be printed as part of the error message. This helps you track down which assertion actually failed.

    Colin Adams

  • Phung Dinh Vu (7 years ago 3/11/2009)

    Can't control exception or violation.

    Thank LeMahieu. I have fixed your code like this: class EXCEPTION2 create make feature {ANY} make do io.put_string ("Entry make%N") one two io.put_string ("Exit make") end one -- Exception in 'proc' require clause is raised in *calling* function, 'one' local retried: BOOLEAN value: INTEGER do io.put_string ("you call one method%N") if not retried then value := proc (3) end rescue io.put_string ("You call one's rescue%N") retried := True retry end -- one two -- Exception in 'proc' ensure clause is raised in *called* function, 'proc' local value: INTEGER do io.put_string ("you call two method%N") value := proc (2) end -- two proc (i: INTEGER): INTEGER require i <= 2 local retried: BOOLEAN do io.put_string ("you call proc method%N") if not retried then Result := 2 * i end ensure Result <= 3 rescue io.put_string ("You call proc's rescue%N") retried := True Result := 3 retry end -- proc end
    When i run this class by press F5, the screen only view like this:

    http://svkstn.co.cc/picture1/screen_F5.jpg

    And in the feature, cussor stop at instruction : i <= 2 When I debug each instruction by press F10. The screen is likely:

    http://svkstn.co.cc/picture1/screen_F10.jpg

    I meant when I debug the screen view like I hope it would be. My question is: Why is there diffrent from two that way, when I use F5 to run, and when I use F10 to bug ? Any more, I know that use F5 to run file, when the violation or exception is occured the programme will stop and not excute any more, but I don't hope such that. How can I fix this? I mean in Java I can catch the exception and process it in proper way, and when I press F5 to run Java file, and when an exception occur and catch, the programme won't stop and continue excute. See this example: public class Except { public Except(){ int a = 0; System.out.println ("Enter construction method"); try{ System.out.println (1/a); } catch(Exception e){ System.out.println (e.toString ()); } System.out.println ("Exit construction method"); } public static void main(String[] args) { new Except(); } } And the out put screen is: Enter construction method java.lang.ArithmeticException: / by zero Exit construction method Process completed. As you see "Exit construction method" is still viewed in the screen. In eiffel I don't see it. How can you explain this? Best wishes !

    • Colin LeMahieu (7 years ago 3/11/2009)

      By default when debugging Eiffel Studio will stop when an exception happens.

      You can change this by going to menu -> Execution -> Exception handling -> POSTCONDITION_VIOLATION (ensure) -> Disable / Catch / *Ignore*.

      This is how to get it to do what you want.

      I would recommend against using postcondition failures and precondition failures as normal exceptions because this isn't their intent. "Assertion Violation rule: A run-time assertion violation is the manifestation of a bug."

      Read http://docs.eiffel.com/eiffelstudio/general/guided_tour/language/tutorial-09.html

      • Creative Bachkhoa (7 years ago 4/11/2009)

        Thanks LemaHieu very much!

           I've tried running your example, it run well. I also take some other tests based on your example. But there is still things I don't understand.
          In the Exception Handling mode, the PRECONDITION and POSTCONDITION have there modes: disable, ignore, catch . Can you explain to me what is the difference between disable and catch?
         I hope your reply based on contract view. Actually, I'd like to explore the semantics and philosophy in Eiffel. 
        I think you are very good at eiffel programing. Thanks you again!