However, you might wish to write to somewhere else during the execution of your Prolog program - for example, you might want to write to a file held on the computer on which the program is running, or on some local file server.

To change the current output stream, you use one of the Prolog built-in extra-logical predicates tell and append/1 *. If Prolog executes the goal tell('output.dat'), then output will subsequently go to the file output.dat, in the current working directory of the workstation* that is running Prolog.

If the specified file cannot be found in the current working directory, it will be created. If the file does exist, it will be overwritten. If you use append/1, subsequent write operations will add material to the end of the file, instead of overwriting the file. If you do not have permission to write files in the current directory, you will see an error message:

?- tell('/usr/bin/ztrash').  
ERROR: tell/1: No permission to open source_sink `/usr/bin/ztrash' (Permission denied)

This means that either you do not have permission to write files in the directory /usr/bin, or if the file ztrash already exists in this directory, that you do not have permission to write to that file.

If the file is able to be written, then subsequent write operations send their data to the file. The parameter to tell can be a path to the file that is wanted, as in the example above, or it could be just the file name, e.g. tell('output.dat'). Prolog will continue to issue prompts for more queries and print bindings while you are "telling" or "appending" a file, but any explicit write operations access the file. The built-in extra-logical predicate told (with no argument) allows you to revert to writing data to the original window. Example:

?- tell('info.dat'), write(thirsty(jack)), nl.
true.
?- told, write(drunk(jack)).
drunk(jack)
true.
?- halt.
% cat info.dat # - # is Unix comment char, cat lists info.dat
thirsty(jack)
% 

See also current input stream, input, output, files.

* append/1 is not related to append/3, which in turn has nothing to do with output streams.

# Strictly speaking, output.dat will be expected to be in the current working directory of the command interpreter that started Prolog. The command interpreter will be running on the workstation/computer, and sending output to a window on that workstation or computer.

Example: Suppose we have the following facts:

Then consider the following queries and their outputs:

?- teaches(dr_fred, Course), studies(Student, Course).

Course = english
Student = alice ;

Course = english
Student = angus ;

Course = drama
Student = amelia ;

false.

Backtracking is not inhibited here. Course is initially bound to history, but there are no students of history, so the second goals fails, backtracking occurs, Course is re-bound to english, the second goal is tried and two solutions found (alice and angus), then backtracking occurs again, and Course is bound to drama, and a final Student, amelia, is found.

?- teaches(dr_fred, Course), !, studies(Student, Course).

false.

This time Course is initially bound to history, then the cut goal is executed, and then studies goal is tried and fails (because nobody studieshistory). Because of the cut, we cannot backtrack to the teaches goal to find another binding for Course, so the whole query fails.

?- teaches(dr_fred, Course), studies(Student, Course), !.

Course = english
Student = alice ;

false.

Here the teaches goal is tried as usual, and Course is bound to history, again as usual. Next the studies goal is tried and fails, so we don't get to the cut at the end of the query at this point, and backtracking can occur. Thus the teaches goal is re-tried, and Course is bound to english. Then the studies goal is tried again, and succeeds, with Student = alice. After that, the cut goal is tried and of course succeeds, so no further backtracking is possible and only one solution is thus found.

?- !, teaches(dr_fred, Course), studies(Student, Course).

Course = english
Student = alice ;

Course = english
Student = angus ;

Course = drama
Student = amelia ;

false.
?- 

In this final example, the same solutions are found as if no cut was present, because it is never necessary to backtrack past the cut to find the next solution, so backtracking is never inhibited.

In practice, the cut is used in rules rather than in multi-goal queries, and some particular idioms apply in such cases. For example, consider the following code for max(X, Y, Max), which is supposed to bind Max to the larger of X and Y (which are assumed to be numbers).

max(X, Y, X) :- X > Y, !.
max(X, Y, Y).

max(X, Y, X) :- X > Y.
max(X, Y, Y) :- X =< Y.