Template Toolkit 2: still making me crazy

Template Toolkit 2, aka TT2, has long been a thorn in my side. Once upon a time, I really liked it, but the more I used it, the more it frustrated me. In almost every case, my real frustrations stem from the following set of facts:

  • TT2 is a templating system for Perl.
  • TT2 provides a language for use when adding logic to the templates.
  • The language is inferior to Perl. It may be useful to be inferior in some ways, to encourage programmers to move complex logic out of templates, but…
  • The language has significant conceptual mismatches with Perl.

I’ll start with this object:

package Thing {
  sub new      { bless { state => 0 } }
  sub name     { my $self = shift; $self->{name} // 'Default' }
  sub set_name { my $self = shift; $self->{name} = shift }

  sub next  { my $self = shift; return $self->{state}++ }
  sub error { my $self = shift; $self->{error} }
}

…and I’ll write this code…

my $tt2   = Template->new;
my $thing = Thing->new;

my $template = <<'END';
Got  : [% thing.next  %]
Error: [% thing.error %]
State: [% thing.state %]
END

$tt2->process(\$template, { thing => Thing->new })
  or die $tt2->error;

The output I get is:

Got  : 0
Error:
State: 1

We’ve got one problem, already: I was able to look at the object’s guts, and not because I obviously dereferenced the reference as a hash, but because I forgot that state was not a method. There is, as far as I can tell, no way to prohibit fallback from methods to dereference by configuring TT2.

There’s another problem: we stringified an undef, where we might have wanted some kind of default to display. In Perl we’d get a warning, but we don’t here. We probably wanted to write:

Error: [% thing.error.defined ? thing.error : "(none)" %]

That works. That also calls error twice, so maybe:

Error: [% SET error = thing.error; error.defined ? error : "(none)" %]

…but that won’t work, because it sets error to an empty string, which is defined. Why? Because TT2 doesn’t really have a concept of an undefined value. This can really screw you up if you need to pass undef to an object API that was designed for use by Perl code.

This should be obvious:

Name : [% thing.name %]

You get “Default” as the name.

Name : [% CALL thing.set_name("Bob"); thing.name %]

…and we get Bob. If we ever needed to clear it again, though,

Name : [% CALL thing.set_name("Bob"); CALL thing.set_name(UNDEF); thing.name %]

Well, this won’t work, because UNDEF isn’t really a thing. It isn’t declared, so it defaults to meaning an empty string. I thought you could, once upon a time, do something like this:

[% CALL thing.set_name( thing.error ) %]

…and that the undef returned by error would be passed as an argument. I may be mistaken. It doesn’t work now, anyway.

We need to detect these errors, anyway, right? In Perl, we’d have use warnings 'uninitialized' to tell us that we did print $undef. In TT2, there’s STRICT. We update our $tt2 to look like:

my $tt2   = Template->new(STRICT => 1);

Now, undefs in our templates are fatal. It’s important to note that the error isn’t stringifying undef, but evaluating something that results in undef. Our original template:

my $template = <<'END';
Got  : [% thing.next  %]
Error: [% thing.error %]
State: [% thing.state %]
END

…now fails to process. The error is: var.undef error - undefined variable: thing.error. In other words, thing.error is undefined, so we can’t use it. If we try to use our earlier solution:

my $template = <<'END';
Got  : [% thing.next  %]
Error: [% thing.error.defined ? thing.error : "(none)" %]
State: [% thing.state %]
END

We still get an error:

var.undef error - undefined variable: thing.error.defined

So, we can’t check whether anything is defined, because if it isn’t, it would’ve been illegal to evaluate it that far. You can always pass in a helper:

my $undef_or = sub {
  my ($obj, $method, $default) = @_;
  $obj->$method // $default;
};

my $template = <<'END';
Got  : [% thing.next  %]
Error: [% undef_or(thing, "error", "(none)") %]
State: [% thing.state %]
END

$tt2->process(\$template, { thing => Thing->new, undef_or => $undef_or })
  or die $tt2->error;

This, of course, still doesn’t solve the inability to pass an undefined value to a Perl interface. In fact, it doesn’t deal with any kind of variable passing.

I like the idea of discouraging templates from including too much program logic. On the other hand, I loathe the idea of providing a large and complex language in the templates that can still be used to put too much logic in there, but without making as much sense as Perl or working well with existing Perl interfaces.

I’ll take Text::Template or HTML::Mason any day of the week, instead.

Written on July 3, 2013
perl   programming