How to use the "Function Probe" block?

Hi guys,

First of all I was wondering if there was a documentation for blocks
such as the Selector and Function Probe. The Selector block was easy
enough that I eventually figured out how to use it.

I am having trouble using the Function probe block. I want to use
Function probe with the Selector to dynamically change the input index
of the Selector Block.
I set
ID: message (going to be used as input index)
Block ID: gr_sig_source_x_0 (I set it as ID block of the signal
source, a square wave)
Function Name: Sample_Rate(), frequency(), … (confused)
Function Args: samp_rate, 1000, … (confused)
Poll Rate:samp_rate

What I should put for the function name and function args?