Why is ModificationForbidden not an Error?

Hi Christoph,

On Apr 9, 2020, at 7:01 AM, Thiede, Christoph <[hidden email]> wrote:



Hi all,

please take a short look at this behavior:

TestRunner openForSuite: MorphicUIManagerTest suite. "Run Selected"

At the moment (#19541), a bug in FileList2 class >> #endingSpecs (which I'm going to fix ASAP) breaks the test #testShowAllBinParts. But because ModificationForbidden is not an Error, it is not caught by the TestRunner so the whole suite run blows up, too.

I could make another example by triggering a ModificationForbidden in a #drawOn: method to destroy the whole image, too.
(Morph newSubclass compile: 'drawOn: x #(1) at: 1 put: 2'; new) openInHand)

So my question is: Why doesn't ModificationForbidden derive from Error? Isn't it actually an error? Similar illegal operations such as "#() at: 0" or "{} at: 1 put: #foo" raise some kind of Error, too. Everything I learned so far about Squeak's exception framework tells me that you should have a very good reason if you design an exception neither to derive from Error, nor from Notification. At the moment, we only do have 9 exceptions (bad pun ...) to this rule (and I'm not even sure whether MCNoChangesException couldn't be a notification, and Abort does not even have senders in the Trunk).

I'd be happy if you could give me some pointers on why ModificationForbidden does not follow this rule. How can we deal with this in order to fix the bugs/unexpected behaviors mentioned above?

I think this is an oversight in my part.  I agree that ModificationForbidden is an error.  I took the code from Pharo and didn’t notice ModificationForbidden was a Notification.

Please feel free to make it an Error.

Possibly related stuff:

• http://forum.world.st/Squeak-s-AssertionFailure-vs-SUnit-s-TestFailure-td5106818.html
• http://forum.world.st/The-Trunk-Kernel-eem-1294-mcz-td5112196.html
• http://forum.world.st/The-Trunk-Kernel-eem-1317-mcz-tp5113273p5113433.html
  

Best,

Christoph

Thanks for the fast feedback, I am going to commit this to the Inbox!

> Making it an error might also make smalltalkCI (through SUnit) catch it to continue. Failing the tests rather than halting the run.

Exactly, that was also my original motivation to ask this question :-)

Best,

Christoph

---

Von: Squeak-dev <[hidden email]> im Auftrag von Eliot Miranda <[hidden email]>
Gesendet: Donnerstag, 9. April 2020 17:03:59
An: The general-purpose Squeak developers list
Betreff: Re: [squeak-dev] Why is ModificationForbidden not an Error?

 

Hi Christoph,

On Apr 9, 2020, at 7:01 AM, Thiede, Christoph <[hidden email]> wrote:



Hi all,

please take a short look at this behavior:

TestRunner openForSuite: MorphicUIManagerTest suite. "Run Selected"

At the moment (#19541), a bug in FileList2 class >> #endingSpecs (which I'm going to fix ASAP) breaks the test #testShowAllBinParts. But because ModificationForbidden is not an Error, it is not caught by the TestRunner so the whole suite run blows up, too.

I could make another example by triggering a ModificationForbidden in a #drawOn: method to destroy the whole image, too.
(Morph newSubclass compile: 'drawOn: x #(1) at: 1 put: 2'; new) openInHand)

So my question is: Why doesn't ModificationForbidden derive from Error? Isn't it actually an error? Similar illegal operations such as "#() at: 0" or "{} at: 1 put: #foo" raise some kind of Error, too. Everything I learned so far about Squeak's exception framework tells me that you should have a very good reason if you design an exception neither to derive from Error, nor from Notification. At the moment, we only do have 9 exceptions (bad pun ...) to this rule (and I'm not even sure whether MCNoChangesException couldn't be a notification, and Abort does not even have senders in the Trunk).

I'd be happy if you could give me some pointers on why ModificationForbidden does not follow this rule. How can we deal with this in order to fix the bugs/unexpected behaviors mentioned above?

I think this is an oversight in my part.  I agree that ModificationForbidden is an error.  I took the code from Pharo and didn’t notice ModificationForbidden was a Notification.

Please feel free to make it an Error.

Possibly related stuff:

• http://forum.world.st/Squeak-s-AssertionFailure-vs-SUnit-s-TestFailure-td5106818.html
• http://forum.world.st/The-Trunk-Kernel-eem-1294-mcz-td5112196.html
• http://forum.world.st/The-Trunk-Kernel-eem-1317-mcz-tp5113273p5113433.html
  

Best,

Christoph

Hi Chris,

On Apr 9, 2020, at 3:16 PM, Chris Muller <[hidden email]> wrote:



ModificationForbidden is resumable like a Warning, and unlike most Errors.  Perhaps it should be a Warning.

One can override isResumable. There’s no invariant that’s a subclass of Error can’t be resumable.  IMO many more Error subclasses that aren’t should be isResumable.

Proper signaling and handling are independent of each other.  Please evaluate your decision from the handling side too -- whether it'd be better for TestRunner's handling to include ModificationForbidden.

 - Chris

On Thu, Apr 9, 2020 at 12:44 PM Thiede, Christoph <[hidden email]> wrote:

Thanks for the fast feedback, I am going to commit this to the Inbox!

> Making it an error might also make smalltalkCI (through SUnit) catch it to continue. Failing the tests rather than halting the run.

Exactly, that was also my original motivation to ask this question :-)

Best,

Christoph

---

Von: Squeak-dev <[hidden email]> im Auftrag von Eliot Miranda <[hidden email]>
Gesendet: Donnerstag, 9. April 2020 17:03:59
An: The general-purpose Squeak developers list
Betreff: Re: [squeak-dev] Why is ModificationForbidden not an Error?

 

Hi Christoph,

On Apr 9, 2020, at 7:01 AM, Thiede, Christoph <[hidden email]> wrote:



Hi all,

please take a short look at this behavior:

TestRunner openForSuite: MorphicUIManagerTest suite. "Run Selected"

At the moment (#19541), a bug in FileList2 class >> #endingSpecs (which I'm going to fix ASAP) breaks the test #testShowAllBinParts. But because ModificationForbidden is not an Error, it is not caught by the TestRunner so the whole suite run blows up, too.

I could make another example by triggering a ModificationForbidden in a #drawOn: method to destroy the whole image, too.
(Morph newSubclass compile: 'drawOn: x #(1) at: 1 put: 2'; new) openInHand)

So my question is: Why doesn't ModificationForbidden derive from Error? Isn't it actually an error? Similar illegal operations such as "#() at: 0" or "{} at: 1 put: #foo" raise some kind of Error, too. Everything I learned so far about Squeak's exception framework tells me that you should have a very good reason if you design an exception neither to derive from Error, nor from Notification. At the moment, we only do have 9 exceptions (bad pun ...) to this rule (and I'm not even sure whether MCNoChangesException couldn't be a notification, and Abort does not even have senders in the Trunk).

I'd be happy if you could give me some pointers on why ModificationForbidden does not follow this rule. How can we deal with this in order to fix the bugs/unexpected behaviors mentioned above?

I think this is an oversight in my part.  I agree that ModificationForbidden is an error.  I took the code from Pharo and didn’t notice ModificationForbidden was a Notification.

Please feel free to make it an Error.

Possibly related stuff:

• http://forum.world.st/Squeak-s-AssertionFailure-vs-SUnit-s-TestFailure-td5106818.html
• http://forum.world.st/The-Trunk-Kernel-eem-1294-mcz-td5112196.html
• http://forum.world.st/The-Trunk-Kernel-eem-1317-mcz-tp5113273p5113433.html
  

Best,

Christoph

Hi Christoph,

I having trouble figuring out whether your points are primarily about semantics or mechanics..

 

> ModificationForbidden is resumable like a Warning, and unlike most Errors.  Perhaps it should be a Warning.

-1. :-) Warnings are Notifications,

As are Errors.  Right?

and Notifications are characterized by their property not to have any side-effects on the core operation unless handled by any caller.

Mmmm, I'm not sure what point you're making here.  Mechanically, they're all basically the same.  See?

     Notification signal. 'resumed'.         "resumed"
     Warning signal.  'resumed'.             "(click Proceed) resumed"
     Error signal. 'resumed'                    "(click Proceed) resumed"

The fact that Errors can be resumed is a flat out bug, IMO, but they can...

If a ModificationForbidden exception is resumed, the object keeps unmodified, so this is a kinda dangerous resumption.

"Dangerous" how so?  There's no escaping that ModificationForbidden introduces new complexity if one isn't aware of it and how it works -- whether they use it or not, even -- could be in for some unexpected surprises in any case..

 

That's also why the defaultAction of a MF is not to resume the execution but to raise an UnhandledError instead, which, IMHO, is the (only) right way to handle the event that a low-level operation could not be executed as you would have expected.

And exactly how a Warning behaves...

    Warning signal    "unhandled, debugger appears"

    Notification signal    "unhandled, debugger does not appear"

    [ Warning signal ] on: Warning do: [ : warn | warn resume ]    "handled, debugger does not appear" 

In every mechanical way so far, we've seen that Warning is closer to Error than it is to Notification.

I would never expect the following to evaluate to 0 because any far-away caller could resume every possible MF:

#(0) at: 1 put: 1; first

Which has no bearing on whether its categorized as a Warning or Error... 

 

How could you write reliable code if you had to make such assumptions? I believe that this would lead us to crazy shambles where you had to question every obvious fact

I'm sorry Christoph.  I must've missed your point.  MF needs to be able to be resumed.  Squeak allows any Error to be resumed, but this fails your semantic definition, below...

 

Warnings say: You shouldn't do this, or maybe you don't actually want to do this, but it is possible to continue doing so, so we won't stop you from doing so until you decide otherwise.

Which is EXACTLY what ModificationForbidden is..!

 

Errors say: You can't do this, and we won't permit that attempt, so you can either decide to actively ignore this error and skip to the next statement, or we'll blow up the execution (and almost ever open a Debugger).

And why it's precisely wrong for ModificationForbidden.  See above.  If you "can't do this", why would you want to allow it to be resumed?   This is why only the most exceptional Errors are declared resumable (even though it's functionally meaningless, just "documentation").  The vast majority of Errors are not resumable.

 - Chris 

Hi Christoph,

On Apr 9, 2020, at 7:01 AM, Thiede, Christoph <[hidden email]> wrote:



Hi all,

please take a short look at this behavior:

TestRunner openForSuite: MorphicUIManagerTest suite. "Run Selected"

At the moment (#19541), a bug in FileList2 class >> #endingSpecs (which I'm going to fix ASAP) breaks the test #testShowAllBinParts. But because ModificationForbidden is not an Error, it is not caught by the TestRunner so the whole suite run blows up, too.

I could make another example by triggering a ModificationForbidden in a #drawOn: method to destroy the whole image, too.
(Morph newSubclass compile: 'drawOn: x #(1) at: 1 put: 2'; new) openInHand)

So my question is: Why doesn't ModificationForbidden derive from Error? Isn't it actually an error? Similar illegal operations such as "#() at: 0" or "{} at: 1 put: #foo" raise some kind of Error, too. Everything I learned so far about Squeak's exception framework tells me that you should have a very good reason if you design an exception neither to derive from Error, nor from Notification. At the moment, we only do have 9 exceptions (bad pun ...) to this rule (and I'm not even sure whether MCNoChangesException couldn't be a notification, and Abort does not even have senders in the Trunk).

I'd be happy if you could give me some pointers on why ModificationForbidden does not follow this rule. How can we deal with this in order to fix the bugs/unexpected behaviors mentioned above?

I think this is an oversight in my part.  I agree that ModificationForbidden is an error.  I took the code from Pharo and didn’t notice ModificationForbidden was a Notification.

Please feel free to make it an Error.

Possibly related stuff:

• http://forum.world.st/Squeak-s-AssertionFailure-vs-SUnit-s-TestFailure-td5106818.html
• http://forum.world.st/The-Trunk-Kernel-eem-1294-mcz-td5112196.html
• http://forum.world.st/The-Trunk-Kernel-eem-1317-mcz-tp5113273p5113433.html
  

Best,

Christoph

> On 2020-04-09, at 6:55 PM, Chris Muller <[hidden email]> wrote:
>
> The two main use cases of ModificationForbidden present opposite perspectives onto it.

In that case perhaps we actually need two different signals?

tim
--
tim Rowledge; [hidden email]; http://www.rowledge.org/tim
Useful random insult:- Has an inferiority complex, but not a very good one.

Just another thought: I see some confusion around the Proceed button in the debugger, and I felt the same confusion sometimes ago. Please forgive me for the sacrilege, but should we maybe question its general existence?

Depending on the domain, it actually performs a mix of #retry (made possible by the quite confusing recursive implementation of Object >> #at:, Object >> #doesNotUnderstand: and others, I did not yet found any other motivation than the Proceed button to use recursion here) and ignore (#resumeUnchecked:, especially useful for UnhandledWarnings and Halts). Would it be a reasonable goal to eliminate this button in the pre-debugger window and replace it with two buttons, Restart and Ignore? We could hide even hide the restart button if not appropriate (i.e., the exception is not an error).

What do you think?

And it would be great if anyone here could explain why Object >> #at: & Co. are recursive! :-)

Best,

Christoph

---

Von: Squeak-dev <[hidden email]> im Auftrag von Thiede, Christoph
Gesendet: Samstag, 11. April 2020 16:32 Uhr
An: Chris Muller; The general-purpose Squeak developers list
Betreff: Re: [squeak-dev] Why is ModificationForbidden not an Error?

 

Hi all! Thank you very much for having this interesting discussion :-)

@Chris:

> > -1. :-) Warnings are Notifications,

> As are Errors.  Right?

Nope, sorry ;-)

 

> The two main use cases of ModificationForbidden present opposite perspectives onto it.  For "protection from accidentally modifying a literal in a CompiledMethod", I understand the inclination to make it an Error.  However, for the context of db handling, [#markDirty + #resume] is normal processing, not an "error", and not even exceptional, either, so perhaps this is just a semantic irritation sensitivity on my part.. sorry. 

> But if they want to use ModificationForbidden for a Write Barrier, they'll probably want to extricate it from Error in the handler:

> 

>    [ myDbApp doStuff ]

>      on: ModificationForbidden

>      do:

>           [ : forbidden |

>           forbidden object beWritableObject.

>           forbidden resumptionValue: forbidden retryModificationNoResume.

>           forbidden resume: forbidden resumptionValue ]

>      on: Error

>      do: [ : err | myDbApp logErrorAndNotifyUser ]

This clarification was very helpful for me. I was not aware of your second use case before.

In my opinion, we are talking about two completely different use cases for the same exception.

Literal protection is a low-level error, comparable to out-of-bounds things and primitive failures. You almost never do want to ignore or fix them.

Write barriers for #markDirty, on the other hand, sound like a domain-specific topic to me that should neither raise an error, nor eventually signal an UnhandledError, but be resumed automatically (unless handled differently) by a #defaultAction that removes the barrier (and additional handlers could, if desired, turn on some cache or so).

I have never dealt with Magma or any other DB framework for Smalltalk, but why do you (or would you) use these VM-based mechanisms for a high-level feature? Without knowing any details about your application, personally I would probably design an own exception (DirtyModification?) for that purpose. This would allow you to clearly distinguish between low-level errors ("whoops, I just made an attempt to change a read-only code literal") and higher-level exceptions (DirtyModification). If we would follow my proposal to raise ModificationForbidden from Object instead of Context, you could also consider to override #modificationForbiddenFor:at:put: in your database objects to raise DirtyModification instead of ModificationForbidden (in the same way as some classes override #error or #primitiveError).

However, focusing again on the literal protection aspect, I still don't see when the current default #resume behavior of ModificationForbidden would be ever helpful. MF >> #resume calls Exception >> #resume: which does nothing more than to return resumptionValue! When do we actually need this resumptionValue?

Why can't we design MF like the following:

#resume - not possible, will signal IllegalResumeAttempt

#retryModification - retries the modification and returns nothing special

#forceModification: - makes the object to modify writable, retries the modification and, optionally, makes the object read-only again

> If we don't want to break things (that are somehow wrong according to contemporary notion), we could make it a Warning in the Squeak 5.x release stream and turn it into an error with Squeak 6.0. That is: make it an Error today in Trunk, but if we would create a 5.4 release, we would have to remember changing it back... :-/

IIRC ModificationForbidden was introduced in 6.0alpha the first time?

Best,

Christoph

---

Von: Squeak-dev <[hidden email]> im Auftrag von Nicolas Cellier <[hidden email]>
Gesendet: Samstag, 11. April 2020 11:16:00
An: Chris Muller; The general-purpose Squeak developers list
Betreff: Re: [squeak-dev] Why is ModificationForbidden not an Error?

 

In this case we might want specialized subclasses...

Le sam. 11 avr. 2020 à 03:45, Chris Muller <[hidden email]> a écrit :

I think so, because if you try to use it for something else, it'll get entangled with your application's standard error handling.  Regular apps have error handling like:

   [ myApp doStuff ] on: Error do: [ : err | myApp logErrorAndNotifyUser ]

and so for the use-case, Protect CompiledMethod Literals from Accidental Modification, inheriting from Error will allow the best backward compatibility with existing handlers.

But if they want to use ModificationForbidden for a Write Barrier, they'll probably want to extricate it from Error in the handler:

   [ myDbApp doStuff ]

     on: ModificationForbidden

     do:

          [ : forbidden |

          forbidden object beWritableObject.

          forbidden resumptionValue: forbidden retryModificationNoResume.

          forbidden resume: forbidden resumptionValue ]

     on: Error

     do: [ : err | myDbApp logErrorAndNotifyUser ]

But now that I'm handling ModificationForbidden separately, what if I want Protect CompiledMethod Literals from Accidental Modification, too?  I'm no longer handling correctly for that, because the handler has to assume they're signaled in the context of the DB use case and not the Protection use case.

 - Chris

On Fri, Apr 10, 2020 at 12:06 PM tim Rowledge <[hidden email]> wrote:

> On 2020-04-09, at 6:55 PM, Chris Muller <[hidden email]> wrote:
>
> The two main use cases of ModificationForbidden present opposite perspectives onto it.

In that case perhaps we actually need two different signals?

tim
--
tim Rowledge; [hidden email]; http://www.rowledge.org/tim
Useful random insult:- Has an inferiority complex, but not a very good one.