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I often want to write short utility functions that do things like running a simple but frequently needed substitution on strings or the like. For these functions, I often want shortcutting behaviour: operate on $_ if not given a parameter, and operate destructively in-place if not asked to return values.

Writing functions that work like this is a real pain in the neck: there are multiple cases to be considered, you have to be careful to preserve the aliasing nature of @_ and $_, etc. It’s way too much work to keep doing it over and over.

So here’s a function maker that lets you build such functions, carefully written to rely entirely on aliasing, rather than jumping through any reference-taking hoops. It expects to be given a reference to a function which always operates destructively on its parameters (ie modifies the elements of @_), and from this builds a function that will default to $_ as its input and will either return modified copies or modify its operands in-place.

Eg.:

BEGIN { *basename = shortcutted { s!.*/!! for @_ }; }

for( '/path/to/foo', '/some/path/to/bar' ) {
    print "Munging " . basename . "\n";
    open my $fh, '<', $_ or die $!;
    # note how $_ still contains the full pathname
    # ...
}
[download]

sub shortcutted(&) {
    my $sub = shift;
    sub {
        my @byval;
        my $nondestructive = defined wantarray;
        $sub->( $nondestructive
            ? ( @byval = @_ ? @_ : $_ )
            : (          @_ ? @_ : $_ )
        );
        return $nondestructive ? @byval[ 0 .. $#byval ] : ();
    };
}
[download]

use Test::More;

sub original() { 'original' }
sub modified() { 'modified' }

my $test = shortcutted { $_ = modified for @_ };

plan tests => my $num_tests;

{
    local $_ = original;
    $test->();
    is( $_, modified, 'in-place on $_' );
    BEGIN { $num_tests += 1 }
}

{
    local $_ = original;
    my $res = $test->();
    is( $_,   original, 'nondestructive from $_' );
    is( $res, modified, '...returned correctly' );
    BEGIN { $num_tests += 2 }
}

{
    my $num = 10;
    my @original = ( original ) x $num;
    my @modified = ( modified ) x $num;
    $test->( my @data = @original );
    is_deeply( \@data, \@modified, 'in-place on params' );
    BEGIN { $num_tests += 1 }
}

{
    my $num = 10;
    my @original = ( original ) x $num;
    my @modified = ( modified ) x $num;
    my @res = $test->( my @data = @original );
    is_deeply( \@data, \@original, 'non-destructive from params' );
    is_deeply( \@res,  \@modified, '...returned correctly' );
    BEGIN { $num_tests += 2 }
}
[download]

In reply to Intelligently shortcutting $_-or-params, non-destructive-or-in-place function maker by Aristotle

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