HTML::Template Interpolation Error?

[hbo]

Greets.

Some code I wrote came back to me today with the complaint that it was blowing up with an error:

HTML::Template : Attempt to set nonexistent parameter '<input type="ch
+eckbox" ...

The code in question looked like this:

    $html->param('rcadd_list' =>
         checkbox_group( ...

So it looks like the 'rcadd_list' parameter is getting swallowed. To make a long story short, changing the code like this:

    $html->param('rcadd_list' =>
         ''.checkbox_group( ...

Fixed it up.

I'm relying on interpolating the return value of a CGI call into the parameter list of an HTML::Template call throughout this code, not to mention throughout my career. Why does it fail in this case?

Update:

blokhead supplied my answer.

checkbox_group() returns a list in list context. HTML::Template::param takes a list of key/value pairs. The first pair was thus

 'rcadd_list' => <input type="checkbox ..>

where the second parameter is the first <input> tag returned by the checkbox_group call. But then the next <input> tag becomes a parameter tag for HTML::Template::param(), which is totally fubar.

Prepending an empty string as above forced scalar context, which made checkbox_group return a space-seperated list of input tags, which was just what I wanted. I ended up changing the code like so:

   # We need to force scalar context because
   # checkbox_group() returns a list, otherwise.
   $html->param(rcadd_list => 
     scalar ( checkbox_group( ...

Which makes the whole thing more explicit, in case anyone as thick as I comes along to work on this in the future. 8)

Thanks to blokhead for the correct answer!

[blokhead]

checkbox_group returns a list! So what HTML::Template gets is:

$html->param(
   rcadd_list                 => "<input type=checkbox ...",
   "<input type=checkbox ..." => "<input type=checkbox ...",
   "<input type=checkbox ..." => ...
);

This causes an HTML::Template error when die_on_bad_params is set. Either use join "" => checkbox_group(...) or force scalar context (with scalar or with concatenation as you did above) to get what you want.

[hbo]

Bingo, I think!

So, why does ''.checkbox_group(.. flatten the list? Is

"some string".('a','list','of','strings');

equivilant to

"some string".'a'.'list'.'of'.'strings';

?

[blokhead]

No, it's because the author of CGI.pm wrote it to do so:

sub checkbox_group {
    ...
    return wantarray ? @elements : join(' ',@elements)
    ...
}

A list in scalar context (as in your example directly above) returns the last element of the list (your expression evaluates to "some stringstrings"). For a function, if you want different behavior (as in checkbox_group), you check for scalar or list context with wantarray and return the appropriate thing manually.

The CGI docs could probably do a better job making this obvious, though. Perhaps there's a blanket statement somewhere about all HTML-generation functions having this behavior in scalar context?

[hardburn]

The problem is likely in your template, probably because of a missing quote or '>'.

[hbo]

None of those are a problem as far as I can tell. And note that the solution is to force string interpolation by prepending ''. to the checkbox_group paramater.

Here's the relevant section of the template:

<TMPL_IF name=rcadd_list>
<P>
Please select what additional packages you would like installed via Re
+d Carpet.<br>
You may select multiple packages.  All packages with a check mark  wil
+l be installed.
<p>
<TMPL_VAR name=rcadd_list>
</TMPL_IF>

[bradcathey]

If you feel it's not an issue, you can, of course, set die_on_bad_params => 0. Other wise I usually find an error like this showing up because of a misnamed-misspelled-missing bracket-mismatched quote in my HTML, or even an incorrect file name in my calling Perl. Just my .02.