Re: Can comparisons be limited to true/false?

[Jonathan Revusky]

Tim Colson wrote:
Howdy folks -
  I'm wondering how I might limit the comparisons in velocity templates
to only boolean conditions.
For example:
#if ($value > 20) HIGHLIGHTED BUTTON #else stdbutton #end
would throw an exception and would need to be written with
a context containing "IS_OVER_LIMIT", new Boolean() and this template:
#if ($IS_OVER_LIMIT) HIGHLIGHTED BUTTON #else stdbutton #end
Another example:
#if ($widget.size == "LARGE")  large #else not large #end
would throw an exception and would need to be written as:
#if ($widget.size.isLarge())  large #else not large #end
The question here is academic at this point. I do not care to argue
about why this might be good/bad/weird, nor do I want to hear how it
could be done in another engine. 
<LOL>

Yeah, you anticipated it. The template engine that cannot be named does 
work the way you want. The condition in an if must be a bona fide 
boolean. It doesn't coerce non-empty strings and whatnot into true 
etcetera. If the thing's not a boolean, it throws.

I'm just curious to know how one might go about messing with Velocity to
alter it's behaviour as described. Maybe I'll learn something in the
process of looking. :-)
You're in luck, Tim. I have actually been in the Velocity code to write 
the Velocity->FM template converter. The converter is actually a patched 
version of Velocity that builds up the tree using Velocity code and then 
dumps it in FreeMarker syntax.

So I know my around a bit.

The key API to look at is:

boolean 
org.apache.velocity.runtime.parser.Node.evaluate(InternalContextAdapter 
context);

This is the hook that is used to evaluate a node in the boolean if context.

To get the stricter behavior you want, you would do 2 things:

1. Patch the evaluate() method on line 334 of 
org/apache/velocity/runtime/parser/ASTReference.java so that it only 
accepts boolean values. As follows:

    public boolean evaluate( InternalContextAdapter context)
        throws MethodInvocationException
    {
        Object value = execute(null, context);
        if (value == null)
        {
            throw new MethodInvocationException("Invalid reference");
        }
        else if (value instanceof Boolean)
        {
            if (((Boolean) value).booleanValue())
                return true;
            else
                return false;
        }
        else
            return throw new MethodInvocationException("Cannot do a 
boolean evaluation of a non-boolean expression");
    }

2. Modify the evaluate() method on line 253 of 
org/apache/velocity/runtime/parser/SimpleNode.java
so that, instead of returning false there, it throws and exception. I 
guess it should throw MethodInvocationException, as in:

public boolean evaluate(InternalContextAdapter context)
{
//   return false; // Comment out.
   throw new MethodInvocationException("Cannot do a boolean evaluation 
of a non-boolean expression");
}

Then recompile and you should have the semantics you want.

Thanks!
You're welcome. You've got to love it when the best available authority 
on the Velocity code base is...

Yours truly,

Jonathan Revusky
--
lead developer, FreeMarker project, http://freemarker.org/
Velocity->FreeMarker template conversion tool, 
http.//freemarker.org/usCavalry.html

P.S. Or just use the unnameable template engine. It already works the 
way you want. :-)

Timo


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