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Issue: STANDARD-INPUT-INITIAL-BINDING (version 7)

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 Pierson 5/23/88, per comments by Moon
               Version 4 by Pierson 5/26/88, clean up
    	       Version 5 by Pierson 6/28/88, simple design per Masinter
    	       Version 6 by Pierson 7/ 5/88, clean up and split issue
	       Version 7 by Pierson 7/ 8/88, clean up per Pitman
Status:        For Internal Discussion

Problem description:

CLtL requires that *STANDARD-INPUT*, *STANDARD-OUTPUT*,
*ERROR-OUTPUT*, *TRACE-OUTPUT*, *QUERY-IO*, and *DEBUG-IO* are
initially bound to synonym streams to *TERMINAL-IO*.  This requirement
hampers the integration of Common Lisp with many existing and
potential operating environments.

For example, a Unix implementation is currently unable to legally
support Unix standard error output even though Common Lisp defines
*ERROR-OUTPUT* because *ERROR-OUTPUT* is required to start out bound
to the same stream as *STANDARD-OUTPUT*.  A workstation environnment
which provides stream access to windows as an extension is currently
forbidden to make trace output appear in a separate window by default
because *TRACE-OUTPUT* is required to start out bound to the same
stream as *STANDARD-OUTPUT*.

Proposal (STANDARD-INPUT-INITIAL-BINDING:DEFINED-CONTRACTS):

A Common Lisp implementation is required to provide the following
initial streams.  Each initial stream has a specific purpose as
defined in CLtL.  This proposal redefines the initial bindings of
the streams and leaves the rest of the CLtL description unchanged.

    *TERMINAL-IO*
    *STANDARD-INPUT*
    *STANDARD-OUTPUT*
    *ERROR-OUTPUT*
    *TRACE-OUTPUT*
    *QUERY-IO*
    *DEBUG-IO*

    	The initial bindings of these variables are undefined except
	that:
	    1. They are all initially bound to open streams.
	    2. The streams must support input and/or output as
	       indicated by the variable name.
    	    3. None of the standard streams (including *TERMINAL-IO*)
	       may be directed by synonym streams to another of these
	       stream variables (except *TERMINAL-IO*), whether
	       directly or by indirection via some composite stream
	       such as a two way stream with one of the arms being a
	       synonym stream.
	    4. Any or all of these streams may be synonyms for the a
	       common implementation dependent stream.  For example,
	       in an interactive Common Lisp invocation running on a
	       character terminal, all of the streams mentioned here
	       might be synonym streams (or two-way streams to synonym
	       streams) to a pair of hidden terminal input/output
	       streams maintained by the implementation.

	The intent of the above rules is to ensure that it is always
	safe to bind any of the above variables to another of the
	above variables without unduly restricting implementation
	flexibility.


Test Cases/Examples:

(PROGN
   (PRINT "Output" *STANDARD-OUTPUT*)
   (PRINT "Error" *ERROR-OUTPUT*))

In current Common Lisp will write:
    ------
    Output
    Error
    ------

With proposal *might* write:
    ------
    Output
    ------
    and "Error" appears somewhere else.


(LET ((*STANDARD-OUTPUT* *DEBUG-IO*))
  ...)

In current Common Lisp: 
    Might cause a circular stream reference if *DEBUG-IO* was
    bound to a two-way stream made up of synonym streams to
    *STANDARD-INPUT* and *STANDARD-OUTPUT*.

With this proposal:
    Would be guaranteed not to cause a circular stream reference.


(LET ((*STANDARD-INPUT*  *TERMINAL-IO*)
      (*STANDARD-OUTPUT* *TERMINAL-IO*))
  ...)

In current Common Lisp: 
    Might cause a circular stream reference because *TERMINAL-IO* was
    bound to a two-way stream made up of synonym streams to
    *STANDARD-INPUT* and *STANDARD-OUTPUT*.

With this proposal:
    Would be guaranteed not to cause a circular stream reference.


Rationale:

This proposal attempts to provide a balance between over-specifying
behavior to the point that Lisp programs can't behave like other
programs in conventional operating systems and providing enough
specification that Common Lisp programs can perform portable input and
output.

Current practice:

Lucid binds *TERMINAL-IO* to a special internal stream type.  Franz
binds *TERMINAL-IO* to a special internal stream type for terminal
streams which reads from Unix standard input and writes to Unix
standard output.  KCL binds *TERMINAL-IO* to a standard two-way-stream
with input from Unix standard input and output to Unix standard
output.  Symbolics Genera binds *TERMINAL-IO* as appropriate for each
process, usually to a window for interactive applications or to a
stream which will conjure an interaction window on demand for
background tasks.

Cost to Implementors:

All implementations will have to change to some degree but the changes
will probably be simple and localized.  All known implementations
already support the underlying streams required to implement this
proposal.

Cost to Users:

User code which depends on the strict binding hierarchy in CLtL may
have to change.  

Cost of non-Adoption:

It will continue to be difficult or impossible to integrate portable
Common Lisp progams in conventional operating system environments.
Many implementations will have to continue to choose between
conforming to the standard and providing a superior user environment.

Benefits:

Implementations will be more able to match their IO behavior to their
environment and their user's expectations.

Aesthetics:

Improved because this area becomes better defined.

Discussion:

Moon says that *TERMINAL-IO* (and, by extension, *QUERY-IO*, and
*DEBUG-IO*) should fail to work in a non-interactive environment where
nothing like a terminal exists.  This proposal fails to address this.

Masinter notes that:
    ``In many multi-processing multi-window environments,
      the "initial binding" for *STANDARD-INPUT*, *QUERY-INPUT*
      differs for each process.''  

Pierson supports STANDARD-OUTPUT-INITIAL-BINDING:DEFINED-CONTRACTS.