Input/Output

The easiest way of doing input/output in Prolog is using the so-called DEC-10 predicates. They are based on the idea of having a current input and output, which can be redirected to write to and read from files. The basic DEC-10 I/O predicates are shown in Table 4.4.

Predicate	Explanation
`write(X)`	Write the term `X` on the current output stream.
`nl`	Start a new line on the current output stream.
`read(X)`	Read a term (finished by a full stop) from the current input stream and unify it with `X`.
`put(N)`	Write the ASCII character code `N`. `N` can be a string of length one.
`get(N)`	Read the next character code and unify its ASCII code with `N`.
`see(File)`	`File` becomes the current input stream.
`seeing(File)`	The current input stream is `File`.
`seen`	Close the current input stream.
`tell(File)`	`File` becomes the current output stream.
`telling(File)`	The current output stream is `File`.
`told`	Close the current output stream.

There are more sophisticated I/O predicates based on opening and closing streams explicitly: handles to the files are returned, which can be passed to the I/O predicates. The interface is similar to what is provided by most operating systems, and available in many programming languages.

All I/O predicates perform side-effects: they change the state of the world (changing the contents of the screen or a disk file, in this case; broadcasting messages over the net, if writing / reading is made on a socket stream) in such a way that persists even after backtracking. Side-effects predicates are not easily formalized from a logical point of view, because the state of the whole world has to be taken into account.