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<p>Hi Eliot,</p>
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<p>> <span style="font-size: 12pt;">Or provide a vm flag which controls whether special selector #class (the bytecode for sending #class) is inlined or not. This flag can also control #== & #~~.</span></p>
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<div>Could you give me some (code) pointers about this inlining? :-)</div>
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<div>> <span style="font-size: 12pt;">There is a good solution to this. Low-level Inspectors *should not* use instVarAt: et al. Instead they should access object intervals via “the mirror protocol”, see the methods in Context, #object:instVarAt:[put:] #object:at:[put:]
#objectClass:</span>
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<div>I like and do agree to your vision for tidying up ProtoObject!</div>
<div>Maybe we could also consider some "MetaDecorator" that inherits from ProtoObject and implements #class, #instVarAt: etc. for arbitrary targets? This might simplify the access to the mirror protocol, and it would be a bit more object-oriented:</div>
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<div>(MetaDecorator on: aMorph) class "--> Morph"</div>
<div><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">(MetaDecorator on: aProxy</span><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">)
class "--> Proxy"</span><br>
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<div><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;"><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">(MetaDecorator
on: aProxy</span><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">) instVarsInclude: 'target' "--> true"</span><br>
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<div><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;"><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;"><br>
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<div><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;"><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">> </span></span>Hopefully
I’ve answered your questions and given you pointers. I’m interested in collaborating to make this a reality in the next major release.<span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;"><span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">
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<div>You did. I'm too.</div>
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<div>Best,</div>
<div>Christoph</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>Von:</b> Squeak-dev <squeak-dev-bounces@lists.squeakfoundation.org> im Auftrag von Eliot Miranda <eliot.miranda@gmail.com><br>
<b>Gesendet:</b> Freitag, 14. Februar 2020 18:27:14<br>
<b>An:</b> The general-purpose Squeak developers list<br>
<b>Betreff:</b> Re: [squeak-dev] [Bug & design issue] Messages understood or not understood by ProtoObject</font>
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<div>Hi Christoph,
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<blockquote type="cite">On Feb 14, 2020, at 5:18 AM, Thiede, Christoph <Christoph.Thiede@student.hpi.uni-potsdam.de> wrote:<br>
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<p>Hi all! :-)</p>
<p>When we were fiddling around with some proxy implementations today, Marcel and I discovered an interesting set of issues regarding ProtoObjects.</p>
<p>Let's start with a concrete bug:</p>
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<p><b>Code to reproduce <do it>:</b></p>
<p>ProtoObject new class.</p>
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<p><b>Expected behavior:</b></p>
<p>DNU exception, because #class is implemented on Object only.</p>
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<p><b>Actual behavior:</b></p>
<p>Outputs ProtoObject.</p>
<p>Even stranger: If you debug the method instead and simulate the #class send, you get a "simulated message class not understood" error.</p>
<p>If you subclass ProtoObject and override/forward #class, the results will deviate based on whether you are debugging or executing the code.</p>
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<p><b>Some thoughts:</b></p>
<p><span style="font-size: 12pt;">There are two options: Either to implement #class on ProtoObject (by </span><span style="font-size: 12pt;">moving the primitive definition up from Object</span><span style="font-size: 12pt;">), or to </span><span style="font-size: 12pt;">remove
it from the specialObjectsArray so that a send to #class is not compiled differently than a send to any other regular message.</span><br>
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Or provide a vm flag which controls whether special selector #class (the bytecode for sending #class) is inlined or not. This flag can also control #== & #~~.<br>
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<p>This leads us to the rather general question: <b>What special messages should an instance of ProtoObject understand?</b></p>
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IMO *only* #doesNotUnderstand:
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<div>See below on the mirror protocol</div>
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<p>We don't answer this question completely consistent at the moment: For example, #instVarAt: is implemented on Object, but #instVarsInclude: is implemented on ProtoObject. Again, it is a bit weird that the implementation of #instVarsInclude: calls #class
which is implemented on Object only. And so it goes on:</p>
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<ul style="margin-bottom: 0px; margin-top: 0px;">
<li>#someObject is implemented on Object, but #nextObject is available in ProtoObject.</li><li>#withArgs:executeMethod: is implemented on ProtoObject, whereas #perform:withArguments: is on Object.</li><li>#ifNotNil: is implemented on ProtoObject; however, #ifNotNilDo: is implemented on Object. (btw: does the latter still have any <span>raison d'être?)</span></li><li><span>#flag: exists on ProtoObject, and #yourself exists on Object only. Isn't #yourself rather a syntactical element we would like to behave exactly the same way for every possible message receiver? (btw: Is there a good reason not to speed up #yourself
via specialObjectsArray)?</span></li><li><span>(I don't claim for completeness)</span></li></ul>
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<div>And just some other problems regarding to ProtoObject (just collecting them here instead of forgetting them forever):</div>
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<li><span>ObjectTracer is broken due to several reasons. Will fix this soon.</span><br>
</li><li><span>Inspectors cannot inspect ProtoObjects correctly (for example: <span>Inspector openOn: (ObjectTracer on: Display)). This is because #instVarAt: is implemented on Object only so this message is forwarded.<br>
Maybe we should replace these critical calls from Inspector by something like #tryPrimitive:173withArgs:? But should we do so in Inspector or rather in a new subclass named ProtoInspector or similar? Hm, then we would need to implement #inspectorClass on ProtoObject,
which is bad either ... We will keep investigating this issue.</span></span></li></ul>
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There is a good solution to this. Low-level Inspectors *should not* use instVarAt: et al. Instead they should access object intervals via “the mirror protocol”, see the methods in Context, #object:instVarAt:[put:] #object:at:[put:] #objectClass:</div>
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<div>You see hacks everywhere (the xxxFoo messages in serialization) because we don’t have a well-defined notion of what is in a proxy. Having one that is as minimal as possible is IMNERHO the clearest definition possible, and the most minimal is just having
a #doesNotUnderstand: that prevents recursive does not understand vm exit and displays a stack trace without sending further messages to the proxy (because it uses the mirror primitives).</div>
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<div>Note that the mirror primitives are what the execution machinery in Context use to ensure that bytecodes do what they are expected to do. An I et car access should *not* send instVarAt: to a proxy; it should access the inst var of the proxy, etc.</div>
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<div>However, the crucial question is: What special messages should an instance of ProtoObject understand? On the one hand, the idea of ProtoObjects is to work as total proxies with a maximum amount of forwarding potential, which implies a minimum number of
methods. On the other hand, certain aspects such as accessing instvars or executing messages are really identity-related. <span style="font-family: Calibri, Helvetica, sans-serif, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; font-size: 16px;">Looking
forward to your comments :-)</span></div>
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Hopefully I’ve answered your questions and given you pointers. I’m interested in collaborating to make this a reality in the next major release.</div>
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<div>Best,</div>
<div>Christoph</div>
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