Files and I/O

• By default, the users terminal is the general input/output stream - i.e. all input is read from the terminal and all output directed to the terminal
• For instance, using the read and write commands, all interaction is via the user terminal

do :-
 write('command: '),
 read(Command),
 perform(Command).

perform( stop) :- !.
perform( Command) :- 
 execute( Command),
 do.

Until stop is encountered, the program executes user commands.

Possible to write multiple commands to the output stream (i.e. the terminal - unless specified otherwise).

write_colour(X) :-
 colour(X, Colour),
 write('The colour of'),
 write(X),
 write('is'),
 write(Colour),
 write('.'),
 nl.

• Where colour is a already present in the program, and associates colours with objects
• nl is a built-in predicate - which provides a new line

Once a result to any input and output operation has been obtained, all further results are ignored. Therefore, I/O operations are deterministic in that (while backtracking) they cannot be re-satisfied. Therefore, writing :

write('hello'), fail.

will only produce one output hello and stop.

Possible to read and write a term by doing :

read(X), write(X), fail.

Only one term may be read from the current input stream - the conjunction of goals, i.e. read and write, will then fail because backtracking cannot take place.

 
• Changing I/O streams
• Reading from a file
• Writing to file
• Reading Characters