[Jim McDonald: Issue: STANDARD-INPUT-INITIAL-BINDING (version 2)]

[Dan L. Pierson <pierson%mist@multimax.ARPA>]

Jim asked me to forward this to the cleanup committee a while ago.  I
think that it has some interesting ideas, but delves too much into
specifying implmentation for standards proposal.

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Date: Tue, 1 Mar 88 11:53:43 PST
 From: Jim McDonald <edsel!jlm@labrea.Stanford.EDU>
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To: pierson@multimax.arpa
In-Reply-To: Dan L. Pierson's message of Mon, 29 Feb 88 18:55:14 EST <8802292355.AA19855@mist.UUCP>
Subject: Issue: STANDARD-INPUT-INITIAL-BINDING (version 2)
Cc: edsel!eb@labrea.Stanford.EDU, edsel!jonl@labrea.Stanford.EDU,
        edsel!lnz@labrea.Stanford.EDU, edsel!kdo@labrea.Stanford.EDU


Dan -- you might want to consider these comments I drafted a few weeks
ago.  They're not quite in proposal format (the header is just for
your reference), but if you like the sense of this I can clean it up.
I think this is basically a generalization of your proposal (or at
least is more open-ended):

- -----

Issue:         STANDARD-INPUT-INITIAL-BINDING
References:    Standard streams (pp. 327-329)
Category:      CHANGE
Edit history:  Version 1 by Pierson and Haflich 1/19/87
    	       Version 2 by Pierson 2/29/88
    	       Version 3 by McDonald 3/01/88
Status:        For Internal Discussion


A number of primitive streams are instantiated when lisp starts
(via the function primitive-streams), and the results of that 
function are then used to build the higher level streams seen by
users.

- ------

primitive-streams &optional create-streams-p [function]

  If create-streams-p is true, creates and opens primitive streams in
  an implementation-dependant manner.  Data associated with any
  previously existing streams may be lost, and the effects of previous
  calls to add-primitive-stream and delete-primitive-stream may be
  lost.  Normally, create-streams-p will be true for just a single 
  call made when lisp starts.
  
  Always returns a list of triples of the form:

    (<keyword> <input-stream or nil> <output-stream or nil>).

  The following triples must be included in this list, but more are
  allowed, and various of the stream objects may be eq:

   ((:terminal-input   #<input-stream>  nil)
    (:terminal-output  nil              #<output-stream>)
    (:standard-input   #<input-stream>  nil)
    (:standard-output  nil              #<output-stream>)
    (:error-output     nil              #<output-stream>)
    (:query-input      #<input-stream>  nil)
    (:query-output     nil              #<output-stream>))

  It is an error to destructively modify this list or any of the
  triples within it.  Different calls to primitive-streams are free to
  return eq lists or not, as convenient.

- ------

  add-primitive-stream (keyword input-stream-or-nil output-stream-or-nil)

  Causes a three element list of the arguments to be prepended to the 
  list of triples returned by subsequent calls to (primitive-streams).
  
  Signals an error if the first argument is not a keyword, if the
  second argument is not nil or an input-stream, or if the third
  argument is not nil or an output-stream.

  Returns NIL.

  Note that this can be used to "shadow" :terminal-input, etc.

- ------

  delete-primitive-stream (keyword)

  The first triple starting with keyword which would otherwise have
  been returned by (primitive-streams) will no longer appear in the
  list of triples returned by (primitive-streams).
  
  Signals an error if the argument is not a keyword.

  Returns the triple which is deleted, or NIL if none existed, or
  :ERROR if the triple selected may not be removed.

  Note that this can be used to remove shadowing triples for 
  :terminal-input, e.g., but not all triples for it.

- ------

During lisp startup, two forms are executed:

 (primitive-streams t) 
 (initialize-stream-variables)

Initialize-stream-variables is defined by default as follows, but
implementations and/or users, applications, etc., can redefine it or
simply reset or rebind the stream variables as they choose. 

(defun initialize-stream-variables ()
  (let ((primitive-streams (primitive-streams)))
    (setq *terminal-io* 
	  (make-two-way-stream
	    (second (assoc :terminal-input  primitive-streams))
	    (third  (assoc :terminal-output primitive-streams))))
    (setq *standard-input*  
	  (second (assoc :standard-input  primitive-streams)))
    (setq *standard-output* 
	  (third  (assoc :standard-output primitive-streams)))
    (setq *error-output*    
	  (third  (assoc :error-output    primitive-streams)))
    (setq *query-io*        
	  (make-two-way-stream 
	    (second (assoc :query-input  primitive-streams))
	    (third  (assoc :query-output primitive-streams))))
    ))

- ------

Rather than mandating portability via some least common denominator,
this proposal makes stream initialization more of an open system, easy
to tailor to individual situations.

The rules are also intended to enable user code to be able to restore 
*terminal-io*, *standard-input*, *standard-output*, *error-output*,
and *query-io* to reasonable values, even if all of them have been
set to NIL or other undesirable values.

The functional interface to the triples is designed to discourage 
random tampering with sensitive data.




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