[Catalyst] Template::Toolkit + Empty Lists

Mark Blythe list at markblythe.com
Tue Aug 1 07:37:58 CEST 2006

This is slightly off-topic, but since I'm running into this problem
while using Catalyst, I'm hoping that others on the list may have
already solved it.

I was writing a simple template today, using TT + DBIC.  I had an
object and was exercising a has_many relationship.  I simply wanted to
see if any rows had been returned.  It seemed simple enough:

[% SET children = parent.children %]
[% IF children.list.size %]
[% END %]

Well, it turns out that the size will always be >= 1, because TT can't
seem to handle an empty list.  If the method called (parent.children)
returns an empty list, TT will substitute the return value with an
empty string: '''.

I tried to think of a general idiom to check for a non-empty list
being returned:

[% IF children.defined %] # no, because the empty string is defined
[% IF children %]  # breaks if you return a single-element list with a
zero: ( 0 )
[% IF children != '' %] # breaks if you return a single-element list
with an empty string: ( '' )

Of those three, the last one seems to be the least-common valid use
case, so I'm thinking I'll use that.  It's painfully stupid looking,
so I may hide it (actually, the reverse) in an "is_empty" vmethod.

I know I can get around this specific DBIx use case by using a
resultset instead.  However, I just know this will bite me again under
other circumstances, and I want to have a workable solution for
dealing with empty lists under TT.

Has anybody solved this reasonably well?  Am I missing something basic?


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