bad code to good golf

Uri Guttman wrote:
> is there any guarantee of evaluation order in arg lists? will the
> bless/splice always be executed before the pop?

I believe it is undefined, so it's not recommended that you do anything orde
r
dependent in the argument list.  But it is stable.

As implemented, "complex" expressions are evaluated first, then simple ones.
This leads to classic oddities like this:

perl -wle 'sub foo { print join " ", @_ }  $i = 1;  foo($i, $i++, $i++)'
3 1 2

This is not specific to Perl, but I believe a C thing as well.


> my longer but amusing answer is this:
>
> sub new {
> 	bless{reverse%{{reverse@_}}}
> }
>
> it is not warning safe and it leaves '' => CLASS in the hash. but it has
> no temp vars, calls to splice or shift.

You fell into the trap!  That's an improper constructor, can you tell us why
?


--
Robrt:   People can't win
Schwern: No, but they can riot after the game.

>>>>> "MGS" == Michael G Schwern <schwern@pobox.com> writes:

MGS> Uri Guttman wrote: 

MGS> I believe it is undefined, so it's not recommended that you do
MGS> anything order dependent in the argument list.  But it is stable.

so that breaks the pop solutions IMO. it may not always work in all perl
versions.
 

MGS> You fell into the trap!  That's an improper constructor, can you tell u
s why?

i forgot the shift of the class arg which i had on my test code. it was
a paste error. and it makes it even longer.

uri

--
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]
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* shmem <georg.moritz@millenux.com> [2007-12-10 12:55]:
> Well, if we were to return an array reference instead of a hash
> ref,
>
> sub new{bless\@_,shift}
>
> works. Why does the shift get executed before an array
> reference is constructed - but not if a hashref is constructed
> - from an array?

It probably doesn’t. Either way, evaluation order is a red
herring: your construction will work regardless of which
subexpression is evaluated first!

You managed to confuse yourself. Congratulations. :-)  (Or you
are underhandedly trying to confuse us. In that case, sorry bub,
better luck next time. :-) )

What happens is that `\` takes a *reference* to the array. And
of course when you do that, any modifications of the referent,
including *subsequent* modifications, will be seen by anyone who
holds a reference to it.

In contrast, the hash constructors make a *copy* of the array,
and that copy will not be affected but subsequent modifications
to the source array.

Anyone who has trouble following should consider the difference
between the following two:

sub new { bless \@_, shift }
# vs
sub new { bless [@_], shift }

Consider how many arrays are involved in either case, and which
one is affected by the `shift`.

--
*AUTOLOAD=*_;sub _{s/(.*)::(.*)/print$2,(",$\/"," ")[defined wantarray]/e;$1
}
&Just->another->Perl->hack;
#Aristotle Pagaltzis // <http://plasmasturm.org/>

>>>>> "AP" == A Pagaltzis <pagaltzis@gmx.de> writes:

AP> * shmem <georg.moritz@millenux.com> [2007-12-10 12:55]: 

AP> In contrast, the hash constructors make a *copy* of the array,
AP> and that copy will not be affected but subsequent modifications
AP> to the source array.

and that is beside that point that it totally is broken. the specified
API is a list of key/value pairs. how would an array of those be used as
an object? i don't feel like doing a linear search to get arg values. if
i wanted that i would use lisp assoc lists! :)

uri

--
Uri Guttman  ------  uri@stemsystems.com  -------- [url]http://www.stemsystems.com[/url
]
--Perl Consulting, Stem Development, Systems Architecture, Design and Coding
-
Search or Offer Perl Jobs  ----------------------------  [url]http://jobs.perl.org[/url
]

On Dec 10, 2007 3:53 AM, shmem <georg.moritz@millenux.com> wrote:
> Well, if we were to return an array reference instead of a hash ref,
>
> sub new{bless\@_,shift}
>
> works. Why does the shift get executed before an array reference is
> constructed - but not if a hashref is constructed - from an array?

No it doesn't. It breaks in various and subtle ways, at least on a
couple versions of Perl that I used.

I actually did this once. (The class was representing vectors to
overload various operators, so it made sense to use a blessed array
ref.) The problem is that perl doesn't really refcount @_, so \@_
gives you a very unstable object whose contents are apt to change
underneath you for no apparent reason. I think you're probably fine as
long as you're within the call that the @_ belongs to, but if you ever
return it, weird stuff happens.

I can't come up with a simple test case for it. But it seems
reasonable that "fixing" the problem would slow everything down, and
in this case I'd rather have the speed. I'm not convinced that it's
really a bug anyway, as long as it doesn't crash. Making a copy is
what I really intended with that code; I didn't *want* a reference to
an argument list. I'm not even sure what that would mean.

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