Method invocation arrow (LPORM)

[Jan Eden]

Hi all,

I just work my way through "Learning Perl Objects, References & Modules". Now at one point, I am stuck: Randal introduces classes and methods in Chapter 8.

He gives the following example for overriding methods:

{ package Mouse
@ISA = qw{Animal};
...
sub speak {
my $class = shift;
...
Animal::speak($class);
...
}
}

Since there is a method speak in Mouse, it would override the parent's classes method Animal::speak if the latter were not called explicitly.

But, as Randal points out, this forces Perl to look for speak in Animal andnowhere else - without the method invocation arrow, it cannot check @ISA for ancestor classes.

So far, I get the point. But then he introduces the following solution:

$class->Animal::speak(@_);

Apart from the fact that @_ should be unnecessary here (or did I get something wrong), this should expand to:

Mouse::Animal::speak("Mouse");

And when Perl does not find Animal::speak in Mouse, to:

Animal::Animal::speak("Mouse";

and to higher classes if speak is not found there. Now my question:

Does Perl reduce Animal::Animal::speak("Mouse") automatically to Animal::speak("Mouse"). And, if speak is not in Animal, how does it handle an expression like:

LivingCreature::Animal::speak("Mouse");

It seems to me that the hard-coded package name we got rid of by using method calls just got back into our syntax. At the same time, I am sure the code works (have not tried it yet) and Perl does as it should.

But how does this work?

Thanks for any explanations,

Jan
--
There's no place like ~/

[Jeff 'Japhy' Pinyan]

On Feb 7, Jan Eden said:

>I just work my way through "Learning Perl Objects, References & Modules".
>Now at one point, I am stuck: Randal introduces classes and methods in
>Chapter 8.

Hopefully I'll answer before Randal. ;)

>{ package Mouse
> @ISA = qw{Animal};
> ...
> sub speak {
> my $class = shift;
> ...
> Animal::speak($class);
> ...
> }
>}

I was about to ask why it's written this way, but upon checking the
source, I see that this is the way the first example on overriding a
method looks.

>Since there is a method speak in Mouse, it would override the parent's
>classes method Animal::speak if the latter were not called explicitly.
>
>But, as Randal points out, this forces Perl to look for speak in Animal
>and nowhere else - without the method invocation arrow, it cannot check
>@ISA for ancestor classes.

Right. I was about to say "why is it written this way?" but then I saw
the rest of the examples (and read the rest of your email).

>So far, I get the point. But then he introduces the following solution:
>
>$class->Animal::speak(@_);
>
>Apart from the fact that @_ should be unnecessary here (or did I get
>something wrong), this should expand to:
>
>Mouse::Animal::speak("Mouse");

No, it does not. If $class is 'Mouse', then

$class->method(@_);

will try looking for Mouse::method(), and if not, it will look through
Mouse's @ISA for a class that DOES supply method(). BUT HERE, we're using

$class->OtherClass::method(@_);

which says explicitly to start looking for method() in OtherClass (and if
it fails there, look in OtherClass's @ISA). So

$class->Animal::speak();

in your case becomes

Animal::speak($class);

except that it becomes that DYNAMICALLY.

>And when Perl does not find Animal::speak in Mouse, to:

Ah, here's the confusion. This isn't looking for a method named
'Animal::speak' in 'Mouse'; it's looking for a method named 'speak' in
'Animal'.

Only the right-most part of a Thing::Like::this denotes the name of a
method.

--
Jeff "japhy" Pinyan [email]japhy@pobox.com[/email] [url]http://www.pobox.com/~japhy/[/url]
RPI Acacia brother #734 [url]http://www.perlmonks.org/[/url] [url]http://www.cpan.org/[/url]
<stu> what does y/// stand for? <tenderpuss> why, yansliterate of course.
[ I'm looking for programming work. If you like my work, let me know. ]

[Jan Eden]

Jeff 'japhy' Pinyan wrote:

>BUT HERE, we're using
>
>$class->OtherClass::method(@_);
>
>which says explicitly to start looking for method() in OtherClass
>(and if it fails there, look in OtherClass's @ISA). So
>
>$class->Animal::speak();
>
>in your case becomes
>
>Animal::speak($class);
>
>except that it becomes that DYNAMICALLY.

I see. So Animal just marks the starting point for the search, and it mightbecome LivingCreature::speak($class) later on if speak is not found in Animal.

>

>>And when Perl does not find Animal::speak in Mouse, to:

>
>Ah, here's the confusion. This isn't looking for a method named
>'Animal::speak' in 'Mouse'; it's looking for a method named 'speak'
>in 'Animal'.
>
>Only the right-most part of a Thing::Like::this denotes the name of
>a method.

That's exactly the information I was looking for. Thank you, Japhy.

- Jan
--
These are my principles and if you don't like them... well, I have others. - Groucho Marx

[James Edward Gray II]

On Feb 7, 2004, at 10:02 AM, Jeff 'japhy' Pinyan wrote:

> On Feb 7, Jan Eden said:
>

>> { package Mouse
>> @ISA = qw{Animal};
>> ...
>> sub speak {
>> my $class = shift;
>> ...
>> Animal::speak($class);
>> ...
>> }
>> }

>
> I was about to ask why it's written this way, but upon checking the
> source, I see that this is the way the first example on overriding a
> method looks.

I was going to ask this too. I'm glad I finished reading the email.

I'm not familiar with the book though, so I will ask this? Does it
eventually go on to introduce SUPER::? That's what I would prefer here
and I just want to make sure you're aware of the option.

James

[Jan Eden]

James Edward Gray II wrote:

>On Feb 7, 2004, at 10:02 AM, Jeff 'japhy' Pinyan wrote:
>

>> On Feb 7, Jan Eden said:
>>

>>> { package Mouse
>>> @ISA = qw{Animal};
>>> ...
>>> sub speak {
>>> my $class = shift;
>>> ...
>>> Animal::speak($class);
>>> ...
>>> }
>>> }

>>
>> I was about to ask why it's written this way, but upon checking the
>> source, I see that this is the way the first example on overriding a
>> method looks.

>
>I was going to ask this too. I'm glad I finished reading the email.
>
>I'm not familiar with the book though, so I will ask this? Does it
>eventually go on to introduce SUPER::? That's what I would prefer here
>and I just want to make sure you're aware of the option.

Yes, SUPER:: is introduced just two paragraphs later. I read on, but the uncertainty about $class->Animal::speak kept bugging me. ;)

- Jan
--
These are my principles and if you don't like them... well, I have others. - Groucho Marx

[Jan Eden]

James Edward Gray II wrote:

>On Feb 7, 2004, at 10:02 AM, Jeff 'japhy' Pinyan wrote:
>

>> On Feb 7, Jan Eden said:
>>

>>> { package Mouse
>>> @ISA = qw{Animal};
>>> ...
>>> sub speak {
>>> my $class = shift;
>>> ...
>>> Animal::speak($class);
>>> ...
>>> }
>>> }

>>
>> I was about to ask why it's written this way, but upon checking the
>> source, I see that this is the way the first example on overriding a
>> method looks.

>
>I was going to ask this too. I'm glad I finished reading the email.
>
>I'm not familiar with the book though, so I will ask this? Does it
>eventually go on to introduce SUPER::? That's what I would prefer here
>and I just want to make sure you're aware of the option.

Yes, SUPER:: is introduced just two paragraphs later. I read on, but the uncertainty about $class->Animal::speak kept bugging me. ;)

- Jan
--
These are my principles and if you don't like them... well, I have others. - Groucho Marx

[James Edward Gray II]

On Feb 7, 2004, at 2:20 PM, Jan Eden wrote:

> Yes, SUPER:: is introduced just two paragraphs later. I read on, but
> the uncertainty about $class->Animal::speak kept bugging me. ;)

Excellent. I figured that was the case, but I was just making sure.

James

[Randy W. Sims]

On 2/7/2004 1:23 PM, James Edward Gray II wrote:

> On Feb 7, 2004, at 10:02 AM, Jeff 'japhy' Pinyan wrote:
>

>> On Feb 7, Jan Eden said:
>>

>>> { package Mouse
>>> @ISA = qw{Animal};
>>> ...
>>> sub speak {
>>> my $class = shift;
>>> ...
>>> Animal::speak($class);
>>> ...
>>> }
>>> }

>>
>>
>> I was about to ask why it's written this way, but upon checking the
>> source, I see that this is the way the first example on overriding a
>> method looks.

>
>
> I was going to ask this too. I'm glad I finished reading the email.
>
> I'm not familiar with the book though, so I will ask this? Does it
> eventually go on to introduce SUPER::? That's what I would prefer here
> and I just want to make sure you're aware of the option.

Hmm, I think a good exercise would be to write code snippets to
demonstrate the differences between the four method calls:

Package::method()
$class->method()
$class->Package::method()
$class->SUPER::method()

or five if you count:

method Package ...

[James Edward Gray II]

On Feb 8, 2004, at 6:51 AM, Randy W. Sims wrote:

> Hmm, I think a good exercise would be to write code snippets to
> demonstrate the differences between the four method calls:
>
> Package::method()

Is this really a "method call"? I think of it as a package qualified
subroutine call.

> $class->method()
> $class->Package::method()
> $class->SUPER::method()

This one should only be used inside an overriden method, I think.

> or five if you count:
>
> method Package ...

Let's not. <laughs>

James

[Randal L. Schwartz]

>>>>> "Jan" == Jan Eden <lists@jan-eden.de> writes:

Jan> Yes, SUPER:: is introduced just two paragraphs later. I read on, but the uncertainty about $class->Animal::speak kept bugging me. ;)

Thank you for the feedback. I'll note that for a future edition of the book.
That's also very similar to the way I have it in "perlboot", but nobody
noted it there yet. :(

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095
<merlyn@stonehenge.com> <URL:http://www.stonehenge.com/merlyn/>
Perl/Unix/security consulting, Technical writing, Comedy, etc. etc.
See PerlTraining.Stonehenge.com for onsite and open-enrollment Perl training!

[Jeff 'Japhy' Pinyan]

On Feb 9, Randal L. Schwartz said:

>>>>>> "Jan" == Jan Eden <lists@jan-eden.de> writes:

>
>Jan> Yes, SUPER:: is introduced just two paragraphs later. I read on, but the uncertainty about $class->Animal::speak kept bugging me. ;)
>
>Thank you for the feedback. I'll note that for a future edition of the book.
>That's also very similar to the way I have it in "perlboot", but nobody
>noted it there yet. :(

I would have, but I didn't think it necessary. When the question was
asked, I went to the perlboot doc since I figured that was where the
chapter originated.

--
Jeff "japhy" Pinyan [email]japhy@pobox.com[/email] [url]http://www.pobox.com/~japhy/[/url]
RPI Acacia brother #734 [url]http://www.perlmonks.org/[/url] [url]http://www.cpan.org/[/url]
<stu> what does y/// stand for? <tenderpuss> why, yansliterate of course.
[ I'm looking for programming work. If you like my work, let me know. ]