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Re^3: Why doesn't Perl provide %_ as the hash equivalent of @_ in subs?

by smls (Friar)
on Sep 26, 2013 at 11:19 UTC ( #1055818=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Why doesn't Perl provide %_ as the hash equivalent of @_ in subs?
in thread Why doesn't Perl provide %_ as the hash equivalent of @_ in subs?

"or will it be a reference (and how would that work)?"

I don't know enough about how Perl implements arrays and hashes and about how tied hashes work, to answer that.

But even if there's no good way to meaningfully allow modification of %_, that doesn't mean the magic variable should not be provided at all, it just means it should be read-only.


Comment on Re^3: Why doesn't Perl provide %_ as the hash equivalent of @_ in subs?
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Re^4: Why doesn't Perl provide %_ as the hash equivalent of @_ in subs?
by vsespb (Hermit) on Sep 26, 2013 at 11:45 UTC
    that doesn't mean the magic variable should not be provided at all, it just means it should be read-only.
    That looks like inconsistency to me.
    sub f1 { my ($x, $y) = @_; z($x, $y); }
    and
    sub f2 { z($_[0], $_[1]); }

    Above two examples are not identical. Later one is something low-level, and it's just asking for trouble (for example in cases like f2($1), f2($.) or f2($x, $x) when z() modifies arguments ).
    (NOTE: another common point of view is that caller of such functions is asking for trouble)

    Example:
    sub z { $_[0]++; print $_[1] } sub f1 { my ($x, $y) = @_; z($x, $y); } my $x = 4; f1($x, $x);
    prints 4
    sub z { $_[0]++; print $_[1] } sub f2 { z($_[0], $_[1]); } my $x = 4; f2($x, $x);
    prints 5

    So IMHO you should not use @_ just because it's shortest. Use it when you know what you're doing.

    And now you suggest add another %_ which behaves completely different way.

    Also making %_ readonly won't help much in cases when you pass magic variable as argument (example: RT#54728).
Re^4: Why doesn't Perl provide %_ as the hash equivalent of @_ in subs?
by ikegami (Pope) on Sep 26, 2013 at 14:16 UTC
    It's possible to alias hash values.
    $ perl -MData::Alias -E' @_ = ( a=>4 ); alias %_ = @_; $_[1]=5; say $_{a}; $_{a}=6; say $_[1]; ' 5 6

    However, copying args is a good thing. That's why we don't access @_ directly.

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