SWI-Prolog Reference Manual: Section 3.13.1

3.13.1 Input and Output Using Implicit Source and Destination

The package for implicit input and output destination is upwards compatible to DEC-10 and C-Prolog. The reading and writing predicates refer to resp. the current input- and output stream. Initially these streams are connected to the terminal. The current output stream is changed using tell/1 or append/1. The current input stream is changed using see/1. The streams current value can be obtained using telling/1 for output- and seeing/1 for input streams. The table below shows the valid stream specifications. The reserved names user_input, user_output and user_error are for neat integration with the explicit streams.

`user`	This reserved name refers to the terminal
`user_input`	Input from the terminal
`user_output`	Output to the terminal
`user_error`	Unix error stream (output only)
<`Atom`>	Name of a Unix file
`pipe(<Atom>`	Name of a Unix command

Source and destination are either a file, one of the reserved words above, or a term `pipe(Command)'. In the predicate descriptions below we will call the source/destination argument `SrcDest'. Below are some examples of source/destination specifications.

`?- see(data).`	%Start reading from file `data'.
`?- tell(stderr).`	%Start writing on the error stream.
`?- tell(pipe(lpr)).`	%Start writing to the printer.

Another example of using the pipe/1 construct is shown below. Note that the pipe/1 construct is not part of Prolog's standard I/O repertoire.

getwd(Wd) :- seeing(Old), see(pipe(pwd)), collect_wd(String), seen, see(Old), atom_chars(Wd, String). collect_wd([C|R]) :- get0(C), C \== -1, !, collect_wd(R). collect_wd([]).

see(+SrcDest): Make SrcDest the current input stream. If SrcDest was already opened for reading with see/1 and has not been closed since, reading will be resumed. Otherwise SrcDest will be opened and the file pointer is positioned at the start of the file.
tell(+SrcDest): Make SrcDest the current output stream. If SrcDest was already opened for writing with tell/1 or append/1 and has not been closed since, writing will be resumed. Otherwise the file is created or---when existing---truncated. See also append/1.
append(+File): Similar to tell/1, but positions the file pointer at the end of File rather than truncating an existing file. The pipe construct is not accepted by this predicate.
seeing(?SrcDest): Unify the name of the current input stream with SrcDest.
telling(?SrcDest): Unify the name of the current output stream with SrcDest.
seen: Close the current input stream. The new input stream becomes user.
told: Close the current output stream. The new output stream becomes user.