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 27.9.3 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.
 
  -- Scheme Procedure: make-soft-port pv modes
  -- C Function: scm_make_soft_port (pv, modes)
      Return a port capable of receiving or delivering characters as
      specified by the MODES string ( open-file File Ports.).  PV
      must be a vector of length 5.  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' ( eof-object?
      (r5rs)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>
 

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