How can I modify a scalar reference passed to a subroutine reference?

StackOverflow https://stackoverflow.com/questions/2853506

Question

I have a function to convert documents into different formats, which then calls another function based on the type document. It's pretty straight forward for everything aside from HTML documents which require a bit of cleaning up, and that cleaning up is different based on where it's come from. So I had the idea that I could pass a reference to a subroutine to the convert function so the caller has the opportunity to modify the HTML, kinda like so (I'm not at work so this isn't copy-and-pasted):

package Converter;
...
sub convert
{
    my ($self, $filename, $coderef) = @_;

    if ($filename =~ /html?$/i) {
        $self->_convert_html($filename, $coderef);
    }
}

sub _convert_html
{
    my ($self, $filename, $coderef) = @_;

    my $html = $self->slurp($filename);
    $coderef->(\$html); #this modifies the html
    $self->save_to_file($filename, $html);
}

which is then called by:

Converter->new->convert("./whatever.html", sub { s/<html>/<xml>/i });

I've tried a couple of different things along these lines but I keep on getting 'Use of uninitialized value in substitution (s///)'. Is there any way of doing what I'm trying to do?

Thanks

Was it helpful?

Solution

If it were me, I would avoid modifying the scalar ref and just return the changed value:

sub _convert_html
{
    my ($self, $filename, $coderef) = @_;

    my $html = $self->slurp($filename);
    $html = $coderef->( $html ); #this modifies the html
    $self->save_to_file($filename, $html);
}

However, if you want to modify a sub's arguments, it is worth knowing that all sub arguments are pass-by-reference in Perl (the elements of @_ are aliased to the arguments of the sub call). So your conversion sub can look like:

sub { $_[0] =~ s/<html>/<xml>/ }

But if you really want to operate on $_, like you have in your desired code example, you need to make _convert_html() look like:

sub _convert_html
{
    my ($self, $filename, $coderef) = @_;

    my $html = $self->slurp($filename);

    $coderef->() for $html;

    $self->save_to_file($filename, $html);
}

The for is an easy way to properly localize $_. You can also do:

sub _convert_html
{
    my ($self, $filename, $coderef) = @_;

    local $_ = $self->slurp($filename);

    $coderef->();

    $self->save_to_file($filename, $_);
}

OTHER TIPS

Remember that an s/// by itself operates on $_, but your scalar reference is being passed into your callback sub as an argument, and is therefore in the @_ array.

So you can just change your callback sub to something like this:

sub { my ( $ref ) = @_; $$ref =~ s/<html>/<xml>/i }

Or, you could take advantage of the aliased nature of Perl subroutine arguments, and modify it directly:

sub _convert_html { 
    ...
    $coderef->( $html );
}

and then

sub { $_[0] =~ s/<html>/<xml>/i }

(This will actually modify the original string, as long as the argument is a scalar variable and not a literal string.)

Try this:

Converter->new->convert("./whatever.html", sub { ${$_[0]} =~ s/<html>/<xml>/i; });

You are getting an uninitialized value warning because the substitution isn't being given anything to operate on ($_ is undefined in its scope). You need to tell it where to find its value (in @_, as a reference).

If you want to be fancy you could make the coderef operate on all its args by default:

sub { map { $$_ =~ s/<html>/<xml>/i } @_ }
Licensed under: CC-BY-SA with attribution
Not affiliated with StackOverflow
scroll top