Soft Ports
----------
A "soft-port" is a port based on a vector of procedures capable of
accepting or delivering characters. It allows emulation of I/O ports.
- primitive: make-soft-port pv modes
Returns a port capable of receiving or delivering characters as
specified by the MODES string (Note:open-file.).
VECTOR must be a vector of length 6. Its components are as
follows:
0. procedure accepting one character for output
1. procedure accepting a string for output
2. thunk for flushing output
3. thunk for getting one character
4. thunk for closing port (not by garbage collection)
For an output-only port only elements 0, 1, 2, and 4 need be
procedures. For an input-only port only elements 3 and 4 need be
procedures. Thunks 2 and 4 can instead be `#f' if there is no
useful operation for them to perform.
If thunk 3 returns `#f' or an `eof-object' (*note eof-object?:
(r4rs)Input.) it indicates that the port has reached end-of-file.
For example:
(define stdout (current-output-port))
(define p (make-soft-port
(vector
(lambda (c) (write c stdout))
(lambda (s) (display s stdout))
(lambda () (display "." stdout))
(lambda () (char-upcase (read-char)))
(lambda () (display "@" stdout)))
"rw"))
(write p p) => #<input-output: soft 8081e20>