Custom Ports

  Ports with arbitrary control procedures are created with make-input-port and make-output-port:

• (make-input-port getc char-ready? close) creates an input port. The getc argument must be a procedure that takes no arguments and returns the next input character from the port. (If a non-character and non-eof value is returned, the  exn:i/o:user-port exception is raised.) The char-ready? argument must be a procedure that takes no arguments and returns a true value if a character is ready to be read or #f otherwise. The close argument must a procedure of no arguments; it is called when the port is closed.
• (make-output-port write close) creates an output port. Write must be a procedure that takes a string and writes it. Close must be 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-affects, then the custom port does not need to be explicitly closed.