underscore prototype imposing scalar context

Aaronrp has asked for the wisdom of the Perl Monks concerning the following question:

So, one of my favorite pieces of perl is Text::Trim, which uses contexts to do Just The Right Thing. Here is the trim() function, with some comments that I wrote:

sub trim {
    @_ = @_ ? @_ : $_ if defined wantarray;
    # When the caller is looking for a return value 
    # (not void context), make a copy of the arguments, 
    # so the original value is not changed.
    # If there are no arguments passed, 
    # copy the value of $_ instead.

    for (@_ ? @_ : $_) { next unless defined; s/\A\s+//; s/\s+\z// }
    # If @_ contains any values, perform the 
    # substitution on all defined values. If @_ contains 
    # no values, perform the substitution on $_ instead.

    return @_ if wantarray || !defined wantarray;
    # return the changed values in array context 
    # or (for a reason which escapes me) void context.

    if (my @def = grep defined, @_) { 
       return "@def" 
    } else { return }
    # In scalar context, return the results of 
    # the substitution on all
    # defined values, if any. Otherwise just return.

}
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This code is pretty hard to read, mostly because of the punctuation variables. But it allows the caller to do

@trimmed = trim(@not_trimmed);
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foreach (@untrimmed) {
   say trim;
}
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trim(@data);

And the results make sense.

But, as with so many things, it doesn't work with lexical $_. I thought maybe I could get this to work with the new underscore _ prototype, but unfortunately that imposes scalar context on the argument. In other words,

sub trim (_;@) { 
   # ... as above
}

my @data = (' a', ' b ' , 'c ');

say trim (@data);
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displays not "abc" but "3", since that's the value of @data in scalar context (the number of elements).

I can't think of any way around this that allows the flexibility of the Text::Trim calling conventions and also the use of lexical $_.

Any ideas?

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Re: underscore prototype imposing scalar context by BrowserUk (Patriarch) on Mar 10, 2012 at 01:31 UTC
Does this help? `sub x(@;_) { print @_ ? @_ : $_ };; $_ = 'fred'; x();; fred $_ = 'fred'; x( 1 .. 5 );; 1 2 3 4 5 $_ = 'fred'; x( 1 );; 1` [download] With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. The start of some sanity?	[reply] [d/l]
Re^2: underscore prototype imposing scalar context by Aaronrp (Scribe) on Mar 10, 2012 at 20:21 UTC
No, that doesn't work. The unbackslashed @ eats up all the prototype characters behind it. This: `#!/ActivePerl/bin/perl use 5.014; use warnings; sub sayit (@;_); sayit (1, 2, 3); my @list = (4, 5, 6); sayit (@list); local $_ = 7; sayit; my $_ = 8; sayit; sub sayit (@;_) { my @list = @_; if (@list) { say "@list" } else { say "no list" } }` [download] yields this: `Prototype after '@' for main::sayit : @;_ at q.pl line 6. Prototype after '@' for main::sayit : @;_ at q.pl line 21. 1 2 3 4 5 6 no list no list` [download] The first two lines are due to the warnings pragma.	[reply] [d/l] [select]


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