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Predefined stream aliases

4.17.1 Predefined stream aliases

Each thread has five stream aliases: user_input, user_output, user_error, current_input, and current_output. Newly created threads inherit these stream aliases from their parent. The user_input, user_output and user_error aliases of the main thread are initially bound to the standard operating system I/O streams (stdin, stdout and stderr, normally bound to the POSIX file handles 0, 1 and 2). These aliases may be re-bound, for example if standard I/O refers to a window such as in the swipl-win.exe GUI executable for Windows. They can be re-bound by the user using set_prolog_IO/3 and set_stream/2 by setting the alias of a stream (e.g, set_stream(S, alias(user_output))). An example of rebinding can be found in library library(prolog_server), providing a telnet service. The aliases current_input and current_output define the source and destination for predicates that do not take a stream argument (e.g., read/1, write/1, get_code/1, ... ). Initially, these are bound to the same stream as user_input and user_error. They are re-bound by see/1, tell/1, set_input/1 and set_output/1. The current_output stream is also temporary re-bound by with_output_to/2 or format/3 using e.g., format(atom(A), .... Note that code which explicitly writes to the streams user_output and user_error will not be redirected by with_output_to/2.

Compatibility

Note that the ISO standard only defines the user_* streams. The‘current’streams can be accessed using current_input/1 and current_output/1. For example, an ISO compatible implementation of write/1 is 

write(Term) :- current_output(Out), write_term(Out, Term).

while SWI-Prolog additionally allows for 

write(Term) :- write(current_output, Term).

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LogicalCaptain said (2020-10-25T11:48:33):0 upvotes 0 0 downvotes
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Doc needs help

"Initially, these are bound to the same stream as user_input and user_error."

Should be "...and user_output".

format("Hello") prints to stdout, not stderr....

How to print to stderr

To print to stderr, just call

format(user_error,"Hello world!",[]).

In interactive mode, printing to stderr is not distinguishable from printing to stdout (both outputs go to the terminal).

However at the level of the shell, if we redirects stderr to /dev/null, nothing is shown (the swipl command below just runs a format/3 goal and then halts):

$ swipl -g 'format(user_error,"Hello world!~n",[]).' -t halt 2>/dev/null

On the other hand, this command properly emits text to stderr:

$ swipl -g 'format(user_error,"Hello world!~n",[]).' -t halt
Hello world!

Alias

Apparently user_ouput and user are aliases. One finds a mention of user in the documentation from time to time. Maybe it's an old alias?

?- write(user_output, "Hello").
Hello
true.

?- write(user, "Hello").
Hello
true.