Custom Ports

The make-input-port and make-output-port procedures create ports with arbitrary control procedures:

• (make-input-port read-char-proc char-ready?-proc close-proc [peek-char-proc]) creates an input port.

  The read-char-proc argument is a procedure that takes no arguments and returns the next input character from the port. When no more characters are available from the port, read-char-proc returns eof. (If a non-character and non-eof value is returned, the exn:i/o:port:user exception is raised.)

  The char-ready?-proc argument is a procedure that takes no arguments and returns a true value if a character (or eof) is ready to be read, #f otherwise.

  The close-proc argument is a procedure of no arguments to be called when the port is closed.

  The peek-char-proc argument is a procedure that takes no arguments and returns the next input character from the port, but also saves the character for the next read-char-proc or peek-char-proc call. If peek-char-proc is not provided, the default procedure uses read-char-proc. If peek-char-proc is provided, MzScheme does not check that calling read-char-proc obtains the value returned by a previous peek-char-proc call.

• (make-output-port write-proc close-proc) creates an output port.

  The write-proc argument is a procedure that takes a string and writes it. (The write-proc procedure can safely store or mutate this string.)

  The close-proc argument is a procedure of no arguments; it is called when the port is closed.

Ports created by make-input-port and make-output-port are immediately open for reading or writing. If the close procedure does not have any side-effects, then the custom port does not need to be explicitly closed.