singleton variables in prolog

Question

I was testing my new version of SWI prolog and keep coming across the error :singleton variable.

Example:

member(X,[X|T]).

member(X,[X|         


        

Answer 1

You can read about it on the official page of SWI-Prolog FAQ

The most common cases this warning appears are:

1. Spelling mistakes in variables
2. Forget to use/bind a variable

SWI suggest some ways to ignore it:

1. Use anonymous variable named _ for this purpose.
2. Use your variable starting with _ (like _T, _X), to avoid warning and document what you ignore.
3. If you are aware of what you are doing, you can use :- style_check(-singleton). and all warnings should go away.