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Scheme uses ports for I/O. A port, which can be treated like
any other Scheme object, serves as a source or sink for data. A port
must be open before it can be read from or written to. The standard
I/O port, console-i/o-port, is opened automatically when you
start Scheme. When you use a file for input or output, you need to
explicitly open and close a port to the file (with procedures described
in this chapter). Additional procedures let you open ports to strings.
Many input procedures, such as read-char and read, read
data from the current input port by default, or from a port that you
specify. The current input port is initially console-i/o-port,
but Scheme provides procedures that let you change the current input
port to be a file or string.
Similarly, many output procedures, such as write-char and
display, write data to the current output port by default, or to
a port that you specify. The current output port is initially
console-i/o-port, but Scheme provides procedures that let you
change the current output port to be a file or string.
All ports read or write only ASCII characters.
Every port is either an input port, an output port, or both. The following predicates distinguish all of the possible cases.
Returns #t if object is a port, otherwise returns
#f.
Returns #t if object is an input port, otherwise returns
#f. Any object satisfying this predicate also satisfies
port?.
procedure: output-port? object
Returns #t if object is an output port, otherwise returns
#f. Any object satisfying this predicate also satisfies
port?.
Returns #t if object is both an input port and an output
port, otherwise returns #f. Any object satisfying this predicate
also satisfies port?, input-port?, and
output-port?.
procedure+: guarantee-input-port object
procedure+: guarantee-output-port object
procedure+: guarantee-i/o-port object
These procedures check the type of object, signalling an error of
type condition-type:wrong-type-argument if it is not an input
port, output port, or I/O port, respectively. Otherwise they
return object.
The next five procedures return the runtime system's standard ports. All of the standard ports are dynamically bound by the REP loop; this means that when a new REP loop is started, for example by an error, each of these ports is dynamically bound to the I/O port of the REP loop. When the REP loop exits, the ports revert to their original values.
Returns the current input port. This is the default port used by many
input procedures. Initially, current-input-port returns the
value of console-i/o-port.
procedure: current-output-port
Returns the current output port. This is the default port used by many
output procedures. Initially, current-output-port returns the
value of console-i/o-port.
procedure+: notification-output-port
Returns an output port suitable for generating "notifications", that
is, messages to the user that supply interesting information about the
execution of a program. For example, the load procedure writes
messages to this port informing the user that a file is being loaded.
Initially, notification-output-port returns the value of
console-i/o-port.
Returns an output port suitable for generating "tracing" information
about a program's execution. The output generated by the trace
procedure is sent to this port. Initially, trace-output-port
returns the value of console-i/o-port.
procedure+: interaction-i/o-port
Returns an I/O port suitable for querying or prompting the user.
The standard prompting procedures use this port by default
(see section Prompting). Initially, interaction-i/o-port returns the
value of console-i/o-port.
procedure+: with-input-from-port input-port thunk
procedure+: with-output-to-port output-port thunk
procedure+: with-notification-output-port output-port thunk
procedure+: with-trace-output-port output-port thunk
procedure+: with-interaction-i/o-port i/o-port thunk
Thunk must be a procedure of no arguments. Each of these procedures binds one of the standard ports to its first argument, calls thunk with no arguments, restores the port to its original value, and returns the result that was yielded by thunk. This temporary binding is performed the same way as dynamic binding of a variable, including the behavior in the presence of continuations (see section Dynamic Binding).
with-input-from-port binds the current input port,
with-output-to-port binds the current output port,
with-notification-output-port binds the "notification" output
port, with-trace-output-port binds the "trace" output port, and
with-interaction-i/o-port binds the "interaction" I/O port.
procedure+: set-current-input-port! input-port
procedure+: set-current-output-port! output-port
procedure+: set-notification-output-port! output-port
procedure+: set-trace-output-port! output-port
procedure+: set-interaction-i/o-port! i/o-port
Each of these procedures alters the binding of one of the standard ports and returns an unspecified value. The binding that is modified corresponds to the name of the procedure.
console-i/o-port is an I/O port that communicates with the
"console". Under unix, the console is the controlling terminal of the
Scheme process. Under Windows and OS/2, the console is the window that
is created when Scheme starts up.
This variable is rarely used; instead programs should use one of the standard ports defined above. This variable should not be modified.
For compatibility with old code, console-input-port and
console-output-port are synonyms for this variable.
procedure: close-input-port port
procedure: close-output-port port
Closes port and returns an unspecified value. If port is a
file port, the file is closed. close-input-port and
close-output-port are synonyms for close-port that are
defined for compatibility with standard Scheme.
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