The package for implicit input and output destinations is (almost)
compatible with Edinburgh DEC-10 and C-Prolog. The reading and writing
predicates refer to, resp., the current input and output
streams. 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 stream's current value can be obtained using telling/1
for output and seeing/1
for input.
Source and destination are either a file, user, or a
term‘pipe(Command)’. The reserved stream name user
refers to the terminal.104The ISO
I/O layer uses user_input, user_output and user_error.
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(user). | % Start writing to the
terminal. |
?- tell(pipe(lpr)). | % Start writing to the
printer. |
Another example of using the pipe/1 construct is shown
below.105As of version 5.3.15, the
pipe construct is supported in the MS-Windows version, both for swipl.exe
and swipl-win.exe. The implementation uses code from the LUA
programming language (http://www.lua.org).
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_codes(Wd, String).
collect_wd([C|R]) :-
get0(C), C \== -1, !,
collect_wd(R).
collect_wd([]).
The effect of tell/1
is not undone on backtracking, and since the stream handle is not
specified explicitly in further I/O operations when using
Edinburgh-style I/O, you may write to unintended streams more easily
than when using ISO compliant I/O. For example, the following query
writes both "a" and "b" into the file‘out’:
?- (tell(out), write(a), false ; write(b)), told.
Compatibility notes
Unlike Edinburgh Prolog systems, telling/1
and seeing/1
do not return the filename of the current input/output but rather the
stream identifier, to ensure the design pattern below works under all
circumstances:106Filenames can be
ambiguous and SWI-Prolog streams can refer to much more than just files.
...,
telling(Old), tell(x),
...,
told, tell(Old),
...,
The predicates tell/1
and see/1
first check for user, the
pipe(command) and a stream handle. Otherwise, if the
argument is an atom it is first compared to open streams associated to a
file with exactly the same name. If such a stream exists,
created using
tell/1
or see/1,
output (input) is switched to the open stream. Otherwise a file with the
specified name is opened.
The behaviour is compatible with Edinburgh Prolog. This is not
without problems. Changing directory, non-file streams, and multiple
names referring to the same file easily lead to unexpected behaviour.
New code, especially when managing multiple I/O channels, should
consider using the ISO I/O predicates defined in section
4.17.2.
- see(+SrcDest)
- Open SrcDest for reading and make it the current input (see
set_input/1).
If SrcDest is a stream handle, just make this stream the
current input. See the introduction of section
4.17.3 for details.
- tell(+SrcDest)
- Open SrcDest for writing and make it the current output (see
set_output/1).
If SrcDest is a stream handle, just make this stream the
current output. See the introduction of section
4.17.3 for details.
- 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)
- Same as current_input/1,
except that
user is returned if the current input is the
stream user_input to improve compatibility with traditional
Edinburgh I/O. See the introduction of
section 4.17.3 for details.
- telling(?SrcDest)
- Same as current_output/1,
except that
user is returned if the current output is the
stream user_output to improve compatibility with
traditional Edinburgh I/O. See the introduction of
section 4.17.3 for details.
- seen
- Close the current input stream. The new input stream becomes
user_input.
- told
- Close the current output stream. The new output stream becomes
user_output.
