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telling/1
telling(-FileOrStream)
Unifies FileOrStream with the current output stream or file.
file_spec or stream_object
Exactly the same as current_output(FileOrStream)
, except
that FileOrStream will be unified with a filename if the current
output stream was opened by tell/1
(ref-iou-sfh).
Can be used to verify that FileNameOrStream
is still the
current output stream as follows:
% nonvar(FileNameOrStream), tell(FileNameOrStream), … telling(FileNameOrStream)
If the current output stream has not been changed (or if changed, then
restored), then the above sequence will succeed for all file names and all
stream objects opened by open/[3,4]
. However, it will fail for
all stream objects opened by tell/1
(since only filename access
to streams opened by tell/1
is supported). This includes the
stream object user_output
(since the standard output stream is
assumed to be opened by tell/1
, and so telling/1
would
return user
in this case).
If FileOrStream is instantiated to a value that is not the
identifier of the current output stream, then telling(FileOrStream)
simply fails.
Can be followed by tell/1
to ensure that a
section of code leaves the current output unchanged:
% var(OldFileNameOrStream), telling(OldFileNameOrStream), … tell(OldFileNameOrStream)
The above is analogous to its stream-object-based counterpart,
% var(OldStream), current_output(OldStream), … set_output(OldStream)
Both of these sequences will always succeed regardless of whether the current
output stream was opened by tell/1
or open/[3,4]
.
None.
tell/1
, open/[3,4]
, current_input/1
, ref-iou-sfh.