Karaf shell - while condition expression

[Daniel McGreal]

The shell is great!
May I ask for a pointer in how to use an expression (function) to drive a while 
loop.

For example, I’ve tried:

while $(scope:function) { … }

and various variations but typically run into:

Error executing command shell:while: unable to convert argument condition with 
value 'true' to type org.apache.karaf.shell.api.console.Function

Any hint appreciated!
Thanks, Dan.

P.S. My function returns a boolean.