> cat testloop.p 

% File:  testloop.p
%        show how to read and loop; and get exit gracefully

testloop :- 
  read(X),
  X \== 'quit',
  write(X),
  nl,
  testloop.

testloop.

 >
 >
 > pl

Welcome to SWI-Prolog (Version 2.7.16)
Copyright (c) 1993-1996 University of Amsterdam.  All rights reserved.

For help, use ?- help(Topic). or ?- apropos(Word).

1 ?- ['testloop.p'].
testloop.p compiled, 0.00 sec, 1,304 bytes.

Yes
2 ?- testloop.
|: duck.
duck
|: chicken.
chicken
|: quit.

Yes
3 ?- halt.

 >