conditional based on parameter undefined/defined fail when value is parameter

[jandrieu]

If I reference a value that is undefined in the current context, I get an unwelcome warning:
e.g., with parent context

{
  data: [{
    item : "exampleA",
    blah : "This example"
  },{ 
    item : "exampleB"
  }]
}

And a template "boo.dust"

{blah} is good

Invoked as a section:

{#data}
 {> boo/}
{/data}

The second entry, exampleB, gives a warning:

[DUST:INFO] Cannot find reference '{blah}' in template 'boo'

What I need to do is conditionally render a partial like this:

{@exists key='blah'}
{blah} is good
{/exists}

I wrote a helper that works fine in the above example, but it fails if instead of passing the key in the context, I send it as a parameter, e.g.,
FAIL

{
  data: [{
    item : "exampleA",
  },{ 
    item : "exampleB"
  }]
}

And a template "boo.dust"

{blah} is good

Invoked as a section:

{#data}
 {> boo blah="THIS EXAMPLE"/}
{/data}

I tried using {?exists/} as described in http://www.dustjs.com/docs/syntax/ but there isn't any actual example code, so it's not clear how to do it and I couldn't get it working.

Any pointers welcome.

[damondine]

Just opened an issue, so thought I would help with a question as well.

Using your example above, your partial should look like this:

{?blah}  
    {blah} is good  
{/blah}

The full output of the template would look like the following:

{#data}
  {?blah}
    {blah} is good
  {/blah}
{/data}