A port is an abstraction of an input or output device:
(input-port? obj )
(output-port? obj )
A file is, logically, a sequence of characters. A port can be created for a file by opening the file, either for input or output.
(open-input-file filename )
(open-output-file filename )
The filename is a string; these functions return a port that may then be used for I/O. Each of the I/O functions in Scheme allows an optional port argument.
When a program is finished with a file, the file must be closed.
(close-input-port port )
(close-output-port port )
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