Page 218 - DCAP506_ARTIFICIAL_INTELLIGENCE

Artificial Intelligence

Notes 14.10.1 Input/Output using Current Streams

Input/output predicates in which no stream is mentioned address the current streams. The
predicate listing/0/1 also sends its outputs to the existing output stream. The Prolog system
concerns error messages on the existing error stream.
When the Prolog system is begin, the corresponding standard streams are first opened, i.e.
Stream Alias Default
Current input current_input user_input (standard input)

Current output current_output user_output (standard output)
Current error output current_error user_error (standard error output)
You can point to the current streams by their alias names in input/output predicates which need
a stream to be mentioned.
You can query or modify the settings for the current streams with the following predicates:
Predicate Purpose

current_input(?Stream) Query current input stream
set_input(@Stream) Set current input stream
see(@Stream) Set and if required, open current input stream
seeing(?Stream) Query current input stream
seen Reset current input stream to user_input and if required, close the first
stream
current_output(?Stream) Query current output stream
set_output(@Stream) Set current output stream

tell(@Stream) Set and if required, open current output stream
telling(?Stream) Query current output stream
told Reset current output stream to user_output and if required, close the
first stream
current_error(?Stream) Query current error stream
set_error(@Stream) Set current error stream

error_tell(@Stream) Set and if required, open current error stream
error_telling(?Stream) Query current error stream
error_told Reset current error stream to user_error and if required, close the first
stream
When you announce a file as the current input/output stream, you should primarily open it
with the predicate open/3/4 and then describe it as the current input/output stream with one of
the above predicates. If you desire to state a pipe as the current input/output stream, you should
first produce it with unix_make_pipe/1 and open it with open/3/4. Then you define the pipe as
the existing input/output stream.
With the following predicates, input is utilizing the current input stream and output is utilizing
the current output stream.

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