Question

I have tried so many things but I could not found How I can implement following wish in prolog .

if list is empty
        call foo function
else
        do nothing

What I did:

list = [] -> foo(...) 
             ;
             fail.

But, it does not work

Answer 1

fail does not mean “do nothing”, but “fail (and backtrack)”.

You need to use true instead:

( List == [] -> foo(...) ; true ),

Also, List should be a variable, so use upper case.