4.6.7.4 Opening a Stream

Before I/O operations can take place on a stream, the stream must be opened, and it must be set to be current input or current output. As illustrated in the figure “Categorization of Stream Handling Predicates”, the operations of opening and setting are separate with respect to the stream predicates, and combined in the File Specification Predicates.

• open(File, Mode, Stream) attempts to open the file File in the mode specified (read,write or append). If the open/3 request is successful, a stream object, which can be subsequently used for input or output to the given file, is unified with Stream.

  The read mode is used for input. The write and append modes are used for output. The write option causes a new file to be created for output. If the file already exists, it is set to empty and its previous contents are lost. The append option opens an already-existing file and adds output to the end of it. The append option will create the file if it does not already exist.

  Options can be specified by calling open/4.

• set_input(Stream) makes Stream the current input stream. Subsequent input predicates such as read/1 and get_code/1 will henceforth use this stream.

• set_output(Stream) makes Stream the current output stream. Subsequent output predicates such as write/1 and put/1 will henceforth use this stream.

Opening a stream and making it current are combined in see and tell:

• see(S) makes file S the current input stream. If S is an atom, it is taken to be a file specification, and
  • if there is an open input stream associated with the filename, and that stream was opened by see/1, it is made the current input stream;
  • Otherwise, the specified file is opened for input and made the current input stream. If it is not possible to open the file, see/1 fails. In addition, if the fileerrors flag is set (as it is by default), see/1 sends an error message to the standard error stream and calls abort/0, returning to the top level.
• tell(S) makes S the current output stream.
  • If there is an open output stream currently associated with the filename, and that stream was opened by tell/1, it is made the current output stream;
  • Otherwise, the specified file is opened for output and made the current output stream. If the file does not exist, it is created. If it is not possible to open the file (because of protections, for example), tell/1 fails. In addition, if the fileerrors flag is set (which it is by default), tell/1 sends an error message to the standard error stream and calls abort/0, returning to the top level.

It is important to remember to close streams when you have finished with them. Use seen/0 or close/1 for input files, and told/0 or close/1 for output files.

• open_null_stream(Stream) opens a text output stream that is not connected to any file and unifies its stream object with Stream. Characters or terms that are sent to this stream are thrown away. This predicate is useful because various pieces of local state are kept for null streams: the predicates character_count/2, line_count/2 and line_position/2 can be used on these streams (see ref-iou-sos).