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Scheme's I/O procedures do not need a port argument if the
port happens to be standard input or standard output.
However, if you need these ports explicitly, the
zero-argument procedures current-input-port and
current-output-port furnish them. Thus,
(display 9 (current-output-port))
have the same behavior.
A port is associated with a file by opening the file.
The procedure open-input-file takes a filename argument
and returns a new input port associated with it. The
procedure open-output-file takes a filename argument and
returns a new output port associated with it. It is an
error to open an input file that doesn't exist, or to open
an output file that already exists.
After you have performed I/O on a port, you should close it
with close-input-port or close-output-port.
In the following, assume the file
hello.txt contains the
single word hello.
(define i (open-input-file "hello.txt"))
(define j (read i))
Assume the file
greeting.txt does not exist before the
following programs are fed to the listener:
(define o (open-output-file "greeting.txt"))
(display "hello" o)
(write-char #\space o)
(display 'world o)
greeting.txt will now contain the
7.3.1 Automatic opening and closing of file ports
Scheme supplies the procedures call-with-input-file
that will take care of opening a
port and closing it after you're done with it.
The procedure call-with-input-file takes a filename
argument and a procedure. The procedure is applied to an
input port opened on the file. When the procedure
completes, its result is returned after ensuring that the
port is closed.
(let* ((a (read-char i))
(b (read-char i))
(c (read-char i)))
(list a b c))))
=> (#\h #\e #\l)
The procedure call-with-output-file does the analogous
services for an output file.
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