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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.

Scheme Procedure: make-soft-port pv modes

Return a port capable of receiving or delivering characters as specified by the modes string (see open-file). pv must be a vector of length 5 or 6. Its components are as follows:

  1. procedure accepting one character for output
  2. procedure accepting a string for output
  3. thunk for flushing output
  4. thunk for getting one character
  5. thunk for closing port (not by garbage collection)
  6. (if present and not #f) thunk for computing the number of characters that can be read from the port without blocking.

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 (see eof-object? in The Revised^5 Report on Scheme) it indicates that the port has reached end-of-file. For example:

(define stdout (current-output-port))
(define p (make-soft-port
            (lambda (c) (write c stdout))
            (lambda (s) (display s stdout))
            (lambda () (display "." stdout))
            (lambda () (char-upcase (read-char)))
            (lambda () (display "@" stdout)))

(write p p) ⇒ #<input-output: soft 8081e20>