How can I modify a scalar reference passed to a subroutine reference?
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, $html);
}
which is then called by:
Converter->new->convert("./whatever.html", sub { s/<html>/<xml>/i });
I've tried a couple of different things along these lines but I keep on getting 'Use of uninitialized value in substitution (s///)'. Is there any way of doing what I'm trying to do?
Thanks
3 answers
solution1
5 ACCPTED 2010-05-17 23:15:11
If it were me, I would avoid modifying the scalar ref and just return the changed value:
sub _convert_html
{
my ($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, $_);
}
solution2
3 2010-05-17 23:11:26
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 ) = @_; $$ref =~ s/<html>/<xml>/i }
Or, you could take advantage of the aliased nature of Perl subroutine arguments, and modify it directly:
sub _convert_html {
...
$coderef->( $html );
}
and then
sub { $_[0] =~ s/<html>/<xml>/i }
(This will actually modify the original string, as long as the argument is a scalar variable and not a literal string.)
solution3
2 2010-05-17 23:10:27
Try this:
Converter->new->convert("./whatever.html", sub { ${$_[0]} =~ s/<html>/<xml>/i; });
You are getting an uninitialized value warning because the substitution isn't being given anything to operate on ( $_ is undefined in its scope). You need to tell it where to find its value (in @_ , as a reference).
If you want to be fancy you could make the coderef operate on all its args by default:
sub { map { $$_ =~ s/<html>/<xml>/i } @_ }
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