CLHS: Issue STANDARD-INPUT-INITIAL-BINDING Writeup

Issue STANDARD-INPUT-INITIAL-BINDING Writeup

Status: Passed, Jan 89 X3J13

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

Version 8 by Pierson 7/ 8/88, yet more clean up

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*

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

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.

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

unless the initial value of *DEBUG-IO* had been changed to a value

that did not conform the restrictions in this proposal. While no

Common Lisp implementation should do this, a user program might.

(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 and Pitman support STANDARD-INPUT-INITIAL-BINDING:DEFINED-CONTRACTS.