Re: [Pharo-users] Symbol equality method #= - weird condition in the Pharo sourcecode
On 1 Mar 2019, at 17:08, Petr Fischer via Pharo-users <pharo-users@lists.pharo.org> wrote:
From: Petr Fischer <petr.fischer@me.com> Subject: Symbol equality method #= - weird condition in the Pharo sourcecode Date: 1 March 2019 at 17:08:03 GMT+1 To: pharo-users@lists.pharo.org
Hello, this is Symbol equality method in Pharo:
1: = aSymbol 2: "Compare the receiver and aSymbol." 3: self == aSymbol ifTrue: [^ true]. 4: self class == aSymbol class ifTrue: [^ false]. 5: "Use String comparison otherwise" 6: ^ super = aSymbol
Look at line 4 - what does it mean? That's wrong, isn't it?
Typically, every symbol comparisons end up in line 3, but if you do some work with forward proxies for example, condition on line 3 is "false" and then weird things on line 4 happens.
If line 4 and further are correct, can someone explain a little?
Thanks! pf
Yes, that looks weird. Line 4 should probably be removed, unless I am missing something. Symbols are by definition always #== so in that sense, #= should not even be implemented (as #= on Object is defined as #==), but since its direct super class String already overwrote #=, it has to follow. The super call in line 6 is what allows Symbols and String to be compared. I would say line 4 is a kind of sanity check, but probably not needed.
Issue submitted: https://github.com/pharo-project/pharo/issues/2744 pf
On 1 Mar 2019, at 17:08, Petr Fischer via Pharo-users <pharo-users@lists.pharo.org> wrote:
From: Petr Fischer <petr.fischer@me.com> Subject: Symbol equality method #= - weird condition in the Pharo sourcecode Date: 1 March 2019 at 17:08:03 GMT+1 To: pharo-users@lists.pharo.org
Hello, this is Symbol equality method in Pharo:
1: = aSymbol 2: "Compare the receiver and aSymbol." 3: self == aSymbol ifTrue: [^ true]. 4: self class == aSymbol class ifTrue: [^ false]. 5: "Use String comparison otherwise" 6: ^ super = aSymbol
Look at line 4 - what does it mean? That's wrong, isn't it?
Typically, every symbol comparisons end up in line 3, but if you do some work with forward proxies for example, condition on line 3 is "false" and then weird things on line 4 happens.
If line 4 and further are correct, can someone explain a little?
Thanks! pf
Yes, that looks weird. Line 4 should probably be removed, unless I am missing something.
Symbols are by definition always #== so in that sense, #= should not even be implemented (as #= on Object is defined as #==), but since its direct super class String already overwrote #=, it has to follow.
The super call in line 6 is what allows Symbols and String to be compared.
I would say line 4 is a kind of sanity check, but probably not needed.
On Fri, Mar 01, 2019 at 05:18:27PM +0100, Sven Van Caekenberghe wrote:
On 1 Mar 2019, at 17:08, Petr Fischer via Pharo-users <pharo-users@lists.pharo.org> wrote:
From: Petr Fischer <petr.fischer@me.com> Subject: Symbol equality method #= - weird condition in the Pharo sourcecode Date: 1 March 2019 at 17:08:03 GMT+1 To: pharo-users@lists.pharo.org
Hello, this is Symbol equality method in Pharo:
1: = aSymbol 2: "Compare the receiver and aSymbol." 3: self == aSymbol ifTrue: [^ true]. 4: self class == aSymbol class ifTrue: [^ false]. 5: "Use String comparison otherwise" 6: ^ super = aSymbol
Look at line 4 - what does it mean? That's wrong, isn't it?
Typically, every symbol comparisons end up in line 3, but if you do some work with forward proxies for example, condition on line 3 is "false" and then weird things on line 4 happens.
If line 4 and further are correct, can someone explain a little?
Thanks! pf
Yes, that looks weird. Line 4 should probably be removed, unless I am missing something.
It is wrong in a Spur image, because we now have subclasses of Symbol. But removing line 4 is not the right solution. See Nicolas' implementation in Squeak: Symbol>>= aSymbol "Compare the receiver and aSymbol." self == aSymbol ifTrue: [^ true]. aSymbol isSymbol ifTrue: [^ false]. "Use String comparison otherwise" ^ super = aSymbol Dave
Symbols are by definition always #== so in that sense, #= should not even be implemented (as #= on Object is defined as #==), but since its direct super class String already overwrote #=, it has to follow.
The super call in line 6 is what allows Symbols and String to be compared.
I would say line 4 is a kind of sanity check, but probably not needed.
Why ? Please explain ...
On 1 Mar 2019, at 18:02, David T. Lewis <lewis@mail.msen.com> wrote:
On Fri, Mar 01, 2019 at 05:18:27PM +0100, Sven Van Caekenberghe wrote:
On 1 Mar 2019, at 17:08, Petr Fischer via Pharo-users <pharo-users@lists.pharo.org> wrote:
From: Petr Fischer <petr.fischer@me.com> Subject: Symbol equality method #= - weird condition in the Pharo sourcecode Date: 1 March 2019 at 17:08:03 GMT+1 To: pharo-users@lists.pharo.org
Hello, this is Symbol equality method in Pharo:
1: = aSymbol 2: "Compare the receiver and aSymbol." 3: self == aSymbol ifTrue: [^ true]. 4: self class == aSymbol class ifTrue: [^ false]. 5: "Use String comparison otherwise" 6: ^ super = aSymbol
Look at line 4 - what does it mean? That's wrong, isn't it?
Typically, every symbol comparisons end up in line 3, but if you do some work with forward proxies for example, condition on line 3 is "false" and then weird things on line 4 happens.
If line 4 and further are correct, can someone explain a little?
Thanks! pf
Yes, that looks weird. Line 4 should probably be removed, unless I am missing something.
It is wrong in a Spur image, because we now have subclasses of Symbol. But removing line 4 is not the right solution. See Nicolas' implementation in Squeak:
Symbol>>= aSymbol "Compare the receiver and aSymbol." self == aSymbol ifTrue: [^ true]. aSymbol isSymbol ifTrue: [^ false]. "Use String comparison otherwise" ^ super = aSymbol
Dave
Symbols are by definition always #== so in that sense, #= should not even be implemented (as #= on Object is defined as #==), but since its direct super class String already overwrote #=, it has to follow.
The super call in line 6 is what allows Symbols and String to be compared.
I would say line 4 is a kind of sanity check, but probably not needed.
Is it consistent with the definition of #= in String such that 'aSymbol = aString' iff 'aString = aSymbol'? Sorry, I don't have an image at hand. Am 1. März 2019 18:40:11 MEZ schrieb Sven Van Caekenberghe <sven@stfx.eu>:
Why ? Please explain ...
On 1 Mar 2019, at 18:02, David T. Lewis <lewis@mail.msen.com> wrote:
On Fri, Mar 01, 2019 at 05:18:27PM +0100, Sven Van Caekenberghe wrote:
On 1 Mar 2019, at 17:08, Petr Fischer via Pharo-users
<pharo-users@lists.pharo.org> wrote:
From: Petr Fischer <petr.fischer@me.com> Subject: Symbol equality method #= - weird condition in the Pharo
sourcecode
Date: 1 March 2019 at 17:08:03 GMT+1 To: pharo-users@lists.pharo.org
Hello, this is Symbol equality method in Pharo:
1: = aSymbol 2: "Compare the receiver and aSymbol." 3: self == aSymbol ifTrue: [^ true]. 4: self class == aSymbol class ifTrue: [^ false]. 5: "Use String comparison otherwise" 6: ^ super = aSymbol
Look at line 4 - what does it mean? That's wrong, isn't it?
Typically, every symbol comparisons end up in line 3, but if you do some work with forward proxies for example, condition on line 3 is "false" and then weird things on line 4 happens.
If line 4 and further are correct, can someone explain a little?
Thanks! pf
Yes, that looks weird. Line 4 should probably be removed, unless I am missing something.
It is wrong in a Spur image, because we now have subclasses of Symbol. But removing line 4 is not the right solution. See Nicolas' implementation in Squeak:
Symbol>>= aSymbol "Compare the receiver and aSymbol." self == aSymbol ifTrue: [^ true]. aSymbol isSymbol ifTrue: [^ false]. "Use String comparison otherwise" ^ super = aSymbol
Dave
Symbols are by definition always #== so in that sense, #= should not
even be implemented (as #= on Object is defined as #==), but since its direct super class String already overwrote #=, it has to follow.
The super call in line 6 is what allows Symbols and String to be
compared.
I would say line 4 is a kind of sanity check, but probably not
needed.
Ah, it is an optimisation: if the first #== fails, but the argument is also a Symbol, then that means the are different for sure, so false is returned early, instead of failing in super's #= after that. And with ByteSymbol and WideSymbol, although they are exclusive (can never be equal), they can be different and if they are both Symbols, then you can return early (not just when their classes differ).
On 1 Mar 2019, at 18:40, Sven Van Caekenberghe <sven@stfx.eu> wrote:
Why ? Please explain ...
On 1 Mar 2019, at 18:02, David T. Lewis <lewis@mail.msen.com> wrote:
On Fri, Mar 01, 2019 at 05:18:27PM +0100, Sven Van Caekenberghe wrote:
On 1 Mar 2019, at 17:08, Petr Fischer via Pharo-users <pharo-users@lists.pharo.org> wrote:
From: Petr Fischer <petr.fischer@me.com> Subject: Symbol equality method #= - weird condition in the Pharo sourcecode Date: 1 March 2019 at 17:08:03 GMT+1 To: pharo-users@lists.pharo.org
Hello, this is Symbol equality method in Pharo:
1: = aSymbol 2: "Compare the receiver and aSymbol." 3: self == aSymbol ifTrue: [^ true]. 4: self class == aSymbol class ifTrue: [^ false]. 5: "Use String comparison otherwise" 6: ^ super = aSymbol
Look at line 4 - what does it mean? That's wrong, isn't it?
Typically, every symbol comparisons end up in line 3, but if you do some work with forward proxies for example, condition on line 3 is "false" and then weird things on line 4 happens.
If line 4 and further are correct, can someone explain a little?
Thanks! pf
Yes, that looks weird. Line 4 should probably be removed, unless I am missing something.
It is wrong in a Spur image, because we now have subclasses of Symbol. But removing line 4 is not the right solution. See Nicolas' implementation in Squeak:
Symbol>>= aSymbol "Compare the receiver and aSymbol." self == aSymbol ifTrue: [^ true]. aSymbol isSymbol ifTrue: [^ false]. "Use String comparison otherwise" ^ super = aSymbol
Dave
Symbols are by definition always #== so in that sense, #= should not even be implemented (as #= on Object is defined as #==), but since its direct super class String already overwrote #=, it has to follow.
The super call in line 6 is what allows Symbols and String to be compared.
I would say line 4 is a kind of sanity check, but probably not needed.
On Fri, Mar 1, 2019 at 12:02 PM Sven Van Caekenberghe <sven@stfx.eu> wrote:
Ah, it is an optimisation: if the first #== fails, but the argument is also a Symbol, then that means the are different for sure, so false is returned early, instead of failing in super's #= after that.
And with ByteSymbol and WideSymbol, although they are exclusive (can never be equal), they can be different and if they are both Symbols, then you can return early (not just when their classes differ).
Thanks for explaining that! As an aside, this is the kind of information that should be in a method's comment. Rationale and explanation, rather than what it does.
On 1 Mar 2019, at 18:40, Sven Van Caekenberghe <sven@stfx.eu> wrote:
Why ? Please explain ...
On 1 Mar 2019, at 18:02, David T. Lewis <lewis@mail.msen.com> wrote:
On Fri, Mar 01, 2019 at 05:18:27PM +0100, Sven Van Caekenberghe wrote:
On 1 Mar 2019, at 17:08, Petr Fischer via Pharo-users <
pharo-users@lists.pharo.org> wrote:
From: Petr Fischer <petr.fischer@me.com> Subject: Symbol equality method #= - weird condition in the Pharo
sourcecode
Date: 1 March 2019 at 17:08:03 GMT+1 To: pharo-users@lists.pharo.org
Hello, this is Symbol equality method in Pharo:
1: = aSymbol 2: "Compare the receiver and aSymbol." 3: self == aSymbol ifTrue: [^ true]. 4: self class == aSymbol class ifTrue: [^ false]. 5: "Use String comparison otherwise" 6: ^ super = aSymbol
Look at line 4 - what does it mean? That's wrong, isn't it?
Typically, every symbol comparisons end up in line 3, but if you do some work with forward proxies for example, condition on line 3 is "false" and then weird things on line 4 happens.
If line 4 and further are correct, can someone explain a little?
Thanks! pf
Yes, that looks weird. Line 4 should probably be removed, unless I am missing something.
It is wrong in a Spur image, because we now have subclasses of Symbol. But removing line 4 is not the right solution. See Nicolas' implementation in Squeak:
Symbol>>= aSymbol "Compare the receiver and aSymbol." self == aSymbol ifTrue: [^ true]. aSymbol isSymbol ifTrue: [^ false]. "Use String comparison otherwise" ^ super = aSymbol
Dave
Symbols are by definition always #== so in that sense, #= should not
even be implemented (as #= on Object is defined as #==), but since its direct super class String already overwrote #=, it has to follow.
The super call in line 6 is what allows Symbols and String to be
compared.
I would say line 4 is a kind of sanity check, but probably not needed.
Richard Sargent wrote
As an aside, this is the kind of information that should be in a method's comment. Rationale and explanation, rather than what it does.
Yes!!! ----- Cheers, Sean -- Sent from: http://forum.world.st/Pharo-Smalltalk-Users-f1310670.html
Surely a Symbol should never be #= to anything that is not a Symbol? At least in VW, GST, Dolphin, and VAST, #a = 'a' 'a' = #a both answer false, and I'd be rather upset if a symbol ever compared equal to a string. The ANSI Smalltalk standard says in several places that "Symbol objects are identity objects", which is defined as "An object defined such that a=b implies a==b". So the definition I am expecting to see is Symbol>> = other ^self == other Anything else is inconsistent with the standard and common practice. Symbol hash and = are supposed to be fast all the or what's the point? On Sat, 2 Mar 2019 at 09:02, Sven Van Caekenberghe <sven@stfx.eu> wrote:
Ah, it is an optimisation: if the first #== fails, but the argument is also a Symbol, then that means the are different for sure, so false is returned early, instead of failing in super's #= after that.
And with ByteSymbol and WideSymbol, although they are exclusive (can never be equal), they can be different and if they are both Symbols, then you can return early (not just when their classes differ).
On 1 Mar 2019, at 18:40, Sven Van Caekenberghe <sven@stfx.eu> wrote:
Why ? Please explain ...
On 1 Mar 2019, at 18:02, David T. Lewis <lewis@mail.msen.com> wrote:
On Fri, Mar 01, 2019 at 05:18:27PM +0100, Sven Van Caekenberghe wrote:
On 1 Mar 2019, at 17:08, Petr Fischer via Pharo-users <
pharo-users@lists.pharo.org> wrote:
From: Petr Fischer <petr.fischer@me.com> Subject: Symbol equality method #= - weird condition in the Pharo
sourcecode
Date: 1 March 2019 at 17:08:03 GMT+1 To: pharo-users@lists.pharo.org
Hello, this is Symbol equality method in Pharo:
1: = aSymbol 2: "Compare the receiver and aSymbol." 3: self == aSymbol ifTrue: [^ true]. 4: self class == aSymbol class ifTrue: [^ false]. 5: "Use String comparison otherwise" 6: ^ super = aSymbol
Look at line 4 - what does it mean? That's wrong, isn't it?
Typically, every symbol comparisons end up in line 3, but if you do some work with forward proxies for example, condition on line 3 is "false" and then weird things on line 4 happens.
If line 4 and further are correct, can someone explain a little?
Thanks! pf
Yes, that looks weird. Line 4 should probably be removed, unless I am missing something.
It is wrong in a Spur image, because we now have subclasses of Symbol. But removing line 4 is not the right solution. See Nicolas' implementation in Squeak:
Symbol>>= aSymbol "Compare the receiver and aSymbol." self == aSymbol ifTrue: [^ true]. aSymbol isSymbol ifTrue: [^ false]. "Use String comparison otherwise" ^ super = aSymbol
Dave
Symbols are by definition always #== so in that sense, #= should not
even be implemented (as #= on Object is defined as #==), but since its direct super class String already overwrote #=, it has to follow.
The super call in line 6 is what allows Symbols and String to be
compared.
I would say line 4 is a kind of sanity check, but probably not needed.
On 6 Mar 2019, at 15:15, Richard O'Keefe <raoknz@gmail.com> wrote:
Surely a Symbol should never be #= to anything that is not a Symbol? At least in VW, GST, Dolphin, and VAST, #a = 'a' 'a' = #a both answer false, and I'd be rather upset if a symbol ever compared equal to a string. The ANSI Smalltalk standard says in several places that "Symbol objects are identity objects", which is defined as "An object defined such that a=b implies a==b".
So the definition I am expecting to see is
Symbol>> = other ^self == other
Anything else is inconsistent with the standard and common practice. Symbol hash and = are supposed to be fast all the or what's the point?
Because it is very convenient. It has also been like this in Pharo (and Squeak) for a long time. With the correct implementation, #= and #~= are just as fast when both arguments are Symbols. (We need a little fix there, as recently uncovered). We do not feel bound to the ANSI standard (we won't break it just for fun, it is a guide, but it can't stop us evolving). There are many points of view in the world, yours might be different than mine.
On Sat, 2 Mar 2019 at 09:02, Sven Van Caekenberghe <sven@stfx.eu> wrote: Ah, it is an optimisation: if the first #== fails, but the argument is also a Symbol, then that means the are different for sure, so false is returned early, instead of failing in super's #= after that.
And with ByteSymbol and WideSymbol, although they are exclusive (can never be equal), they can be different and if they are both Symbols, then you can return early (not just when their classes differ).
On 1 Mar 2019, at 18:40, Sven Van Caekenberghe <sven@stfx.eu> wrote:
Why ? Please explain ...
On 1 Mar 2019, at 18:02, David T. Lewis <lewis@mail.msen.com> wrote:
On Fri, Mar 01, 2019 at 05:18:27PM +0100, Sven Van Caekenberghe wrote:
On 1 Mar 2019, at 17:08, Petr Fischer via Pharo-users <pharo-users@lists.pharo.org> wrote:
From: Petr Fischer <petr.fischer@me.com> Subject: Symbol equality method #= - weird condition in the Pharo sourcecode Date: 1 March 2019 at 17:08:03 GMT+1 To: pharo-users@lists.pharo.org
Hello, this is Symbol equality method in Pharo:
1: = aSymbol 2: "Compare the receiver and aSymbol." 3: self == aSymbol ifTrue: [^ true]. 4: self class == aSymbol class ifTrue: [^ false]. 5: "Use String comparison otherwise" 6: ^ super = aSymbol
Look at line 4 - what does it mean? That's wrong, isn't it?
Typically, every symbol comparisons end up in line 3, but if you do some work with forward proxies for example, condition on line 3 is "false" and then weird things on line 4 happens.
If line 4 and further are correct, can someone explain a little?
Thanks! pf
Yes, that looks weird. Line 4 should probably be removed, unless I am missing something.
It is wrong in a Spur image, because we now have subclasses of Symbol. But removing line 4 is not the right solution. See Nicolas' implementation in Squeak:
Symbol>>= aSymbol "Compare the receiver and aSymbol." self == aSymbol ifTrue: [^ true]. aSymbol isSymbol ifTrue: [^ false]. "Use String comparison otherwise" ^ super = aSymbol
Dave
Symbols are by definition always #== so in that sense, #= should not even be implemented (as #= on Object is defined as #==), but since its direct super class String already overwrote #=, it has to follow.
The super call in line 6 is what allows Symbols and String to be compared.
I would say line 4 is a kind of sanity check, but probably not needed.
participants (7)
-
David T. Lewis -
Petr Fischer -
Richard O'Keefe -
Richard Sargent -
Sean P. DeNigris -
Steffen Märcker -
Sven Van Caekenberghe