ranson@crcge1.UUCP (D. Ranson CNET) (04/07/88)
What is the difference between the ?- and :- operators in front of a
goal? I have seen both used in several Prolog books or manuals, with
no discernable pattern, and without any explanation.
Daniel Ranson
...!mcvax!inria!crcge1!ransonok@quintus.UUCP (Richard A. O'Keefe) (04/09/88)
In article <3379@crcge1.UUCP>, ranson@crcge1.UUCP (D. Ranson CNET) writes: > > What is the difference between the ?- and :- operators in front of a > goal? I have seen both used in several Prolog books or manuals, with > no discernable pattern, and without any explanation. > > Daniel Ranson > ...!mcvax!inria!crcge1!ranson What the difference is depends on the Prolog system. In DEC-10 Prolog, the rule is that ":-" begins "commands" and "directives". When one of these things is encountered in a file, it is processed and no output is produced unless something goes wrong. "?-" begins "queries", and the bindings produced by such queries are printed exactly as if you had typed the query at top level. That's why the top level prompt includes "?- ". If you want to give a command at top level and not have any bindings printed, you can use :-. For example | ?- :- source_file(p(_), File), compile(File). ===== ------------------------------------------ prompt command will not print the binding of File. PDP-11 Prolog, in order to save space (it ran on PDP-11s without separate I&D space, and 64kbytes is not a large address space) didn't allow an operator to be both infix and prefix, so in PDP-11 Prolog you had to use ?- for commands and directives, and couldn't put queries in files. Quintus Prolog doesn't distinguish between ?- and :- in files. I'd never actually noticed this, because I've never wanted to put a query in a file and have the bindings printed on the terminal. However, the trick of putting a :- in front of a top-level query to suppress the printing of the bindings is supported. I suggest that you forget about ?- and stick to :- . If you come across a Prolog system where you have to use ?- you will probably have to make a lot of other changes as well. Check your books again, though. They may be writing the ?- to show you the top-level prompt, as I did in the example above.