Hi Ron, Thank you very much. It makes perfect sense. Now i am trying to practise it. Thank you again..
Serdar Sahin -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Ron Teitelbaum Sent: Friday, August 04, 2006 2:01 AM To: 'A friendly place to get answers to even the most basic questionsaboutSqueak.' Subject: RE: [Newbies] Changing error message Hi Sedar, Welcome to the list. Before giving you and explanation let me say that if there is anything I mention which you do not understand please feel free to ask questions. There are a few things to understand about exception handling. First is that there is a class called Exception. Have a look at the hierarchy of the class. There are two parts to exception handling. There is the error itself, and then there is the exception handler. An error is signaled (sometimes called raised). The handler catches the error and handles it. Notice in the hierarchy that there is a class Error. This is usually where you want to start for errors in code because if you wrote a handler for Exception you would prevent errors, or handle errors yourself that you would rather the system handle. Things like halt are a good example. When an exception is signaled the system looks in the calling stack for a handler. A handler could that used the class of the exception or any of its superclasses can handle the error. For example if I have a: Error subclass: #MyGeneralError And MyGeneralError subclass: #MyNoDigitError Then if the code does MyNoDigitError signal: 'You should have more then one digit here. Thank you' Then if you have a handler for #MyNoDigitError or #MyGeneralError or both, it will execute the handler for which ever one comes first in the stack. Ok I'm glad you made it this far stick with me and then I'll give you an example which should help to make it clear. For a stack to have a handler the code that you are executing needs to have an exception wrapper. In other words you need to wrap the code that you think might signal an error. To wrap the code you put your code in a block and then add the Error class and the handler. [self myCode] on: MyNoDigitError do: [:ex | self myHandlerCode]. [A little aside. You can skip this part if this is already too complicated. What I like to do is code my handlers on my exception class so instead of self myHandlerCode I use ex myHandlerCode. The ex is an instance of your error class so you can put code right on that class to do what you want. Also you can use tag to add more information on your class like the instances you need to operate on. You can even add additional ivars to your exception class to hold what ever you want. The benefit of this is that all your exception handlers are in one place so it's easy to find and reusable!] Ok so examples: First something that just works. Create the classes above MyGeneralError and MyNoDigitError Notice that both of these work. [MyNoDigitError signal] on: MyNoDigitError do: [Transcript cr; show: 'Hey I hit an error']. [MyNoDigitError signal] on: MyGeneralError do: [Transcript cr; show: 'Hey I hit an error']. Now it's time to use your imagination. Because this is a very primitive example you have to actually imagine using it. Imagine a method. ATMMachine >>depositAmount: anAmoutToDeposit "return to the sender aBoolen which represents the acceptance or rejection of anAmountToDeposit. When accepted the receiver will forward anAmountToDeposit to the authenticated users selected bankAcount where it is placed on hold until anAmountToDeposit is verified" self userIsAuthenticated ifFalse: [ATMLoginError signal: 'User has not been authenticated'. ^false]. self selectedBankAccount ifNil: [ATMNoSelectedBankAccount signal: 'Please select an account to deposit too.'. ^false]. anAmountToDepost hasDigits ifFalse [^MyNoDigitError signal: 'I can only deposit numbers'. ^false]. self selectedBankAccount holdDepositForVerification: anAmountToDeposit. ^true Ok well that handles the error signaling the error. Now we need to wrap it. ATMMachine >> deposit "request amount to deposit and perform depositAmount" anAmountToDeposit := self getAmountFromUser: 'How much did ya want to deposit?'. [self depositAmount: anAmountToDeposit] on: ATMLoginError, AtMNoSelectedBankAccount, MyNoDigitError do: [:ex | self cutUpUsersAtmCardAndBlowRasberriesAtThem]. So now you can see that if the user does something wrong we handle the error. Notice that the errors are raised up the stack in depositAmount: but the handler is executed in deposit method. This is very useful for some very interesting but very hard to read hacks. If someone is interested please let me know. Ok so now to answer your question. Thought I'd never get here huh, if you want to change the way an exception is raised you need to insert your own handler before the default one catches it. Try this: Execute the following in a workspace: 10/0 Notice that the error class is ZeroDivide. You can intercept the handler by doing this. [10/0] on: ZeroDivide do: [:ex | ex signal: 'Dividing by zero could destroy the universe']. Since you provided your own handler in the stack before the regular handler could catch it you changed the message. Please feel free to ask questions. Happy coding! Ron Teitelbaum President / Principal Software Engineer US Medical Record Specialists [EMAIL PROTECTED] > From: Serdar Sahin > Sent: Thursday, August 03, 2006 5:38 AM > > Hi, > > Is it possible to change some error messages ? For example, if i get > "Error: At least one digit expected here" error, i want to show my own > error message. Is that possible ? > > Thanks, _______________________________________________ Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners _______________________________________________ Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners
