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Issue STREAM-ACCESS Writeup

Proposal ADD-TYPES-ACCESSORS was accepted at the January 1989 meeting.

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Issue: STREAM-ACCESS

References: streams (Chapter 21 of CLtL)

Category: ADDITION

Edit History: 17-Jun-88, version 1 by Walter van Roggen

30-Nov-88, version 2 by Masinter

Problem Description:

There are many components of streams which are specified upon creation

but are not accessible afterwards. Furthermore there is no way in

Common Lisp to determine the type of a stream to see if it has particular

components, or even if it is OPEN.

The accessors wanted are those associated with broadcast streams,

concatenated streams, echo streams, file streams, string streams,

synonym streams, two way streams.

There are three proposals, which differ only by the whether

they include types, type predicates, or both, in addition to

the stream component acessors. Ballots can be either for

one of the proposals or none. (Other combinations of, say,

accessors without either predicates or types, or types without

accessors, do not seem reasonable and are not being proposed

at this time.)

Proposal STREAM-ACCESS:ADD-TYPES-PREDICATES-ACCESSORS

First, add a function to determine whether a stream is "OPEN":

OPEN-STREAM-P stream [Function]

Returns T if a stream is open, NIL if it is closed. It is an error

if the argument is not a stream.

Streams are "open" until they have been closed with

CLOSE, or, the dynamic context of the creating/accessing

macros of WITH-OUTPUT-TO-STRING, WITH-OPEN-FILE,

WITH-INPUT-FROM-STRING, WITH-OPEN-STREAM,

have been exited.

There are three kinds of things to add associated with each kind of

stream: data types, predicates, accessors.

Stream data types:

BROADCAST-STREAM (returned by MAKE-BROADCAST-STREAM)

CONCATENATED-STREAM (returned by MAKE-CONCATENATED-STREAM)

ECHO-STREAM (returned by MAKE-ECHO-STREAM)

FILE-STREAM (returned by OPEN or created by WITH-OPEN-FILE)

STRING-STREAM (returned by MAKE-STRING-INPUT-STREAM,

MAKE-STRING-OUTPUT-STREAM, and created by WITH-INPUT-FROM-STRING

and WITH-OUTPUT-TO-STRING and FORMAT with second argument NIL)

SYNONYM-STREAM (created by MAKE-SYNONYM-STREAM)

TWO-WAY-STREAM (created by MAKE-TWO-WAY-STREAM)

The stream data types are all subtypes of type STREAM and are mutually

exclusive. (In particular, a synonym stream is only of type SYNONYM-STREAM.)

Stream Predicates:

Each of these returns T if the object is of the corresponding type,

and NIL otherwise.

BROADCAST-STREAM-P, CONCATENATED-STREAM-P,

ECHO-STREAM-P, FILE-STREAM-P, STRING-STREAM-P,

SYNONYM-STREAM-P, TWO-WAY-STREAM-P

Note that the predicates do not "follow the link" of a synonym

stream.

Stream Informational Functions:

BROADCAST-STREAM-STREAMS broadcast-stream ==> list of streams

This function returns a list of output streams that constitute

all the streams the broadcast stream is broadcasting to. It is

an error if the argument is not of type BROADCAST-STREAM.

CONCATENATED-STREAM-STREAMS concatenated-stream ==> list of streams

This function returns a list of input streams that constitute

the ordered set of streams the concatenated stream still has to

to read from, starting with the current one it is reading from.

The list may be () if no more streams remain to be read.

It is an error if the argument is not of type CONCATENATED-STREAM.

ECHO-STREAM-INPUT-STREAM echo-stream ==> input-stream

ECHO-STREAM-OUTPUT-STREAM echo-stream ==> output-stream

These functions return the corresponding component stream. It is

an error if the argument is not of type ECHO-STREAM.

SYNONYM-STREAM-SYMBOL synonym-stream ==> symbol

This function returns the symbol whose SYMBOL-VALUE the

synonym stream is using. It is

an error if the argument is not of type SYNONYM-STREAM.

TWO-WAY-STREAM-INPUT-STREAM two-way-stream ==> input-stream

TWO-WAY-STREAM-OUTPUT-STREAM two-way-stream ==> output-stream

These functions return the corresponding component stream. It is

an error if the argument is not of type TWO-WAY-STREAM.

Proposal: STREAM-ACCESS:ADD-TYPES-ACCESSORS

Identical to ADD-TYPES-PREDICATES-ACCESSORS except to leave out the

stream type predicates.

Proposal: STREAM-ACCESS:ADD-PREDICATES-ACCESSORS

Identical to ADD-TYPES-PREDICATES-ACCESSORS except to not

identify new data types. The accessors act as if the types were specified

(i.e., are mutually excusive).

Current Practice:

VAX LISP implements ADD-TYPES-PREDICATES-ACCESSORS.

We have not surveyed other implementations.

Cost to Implementors:

All of the proposals are reasonably simple to implement, since the information

must be present for nearly all types.

Cost to Users:

The proposals are upward-compatible, and should have little impact.

Cost of Non-Adoption:

The benefits would not be available in a portable fashion.

Benefits:

Programs would be able to access useful information otherwise hidden.

Discussion:

This issue has come up frequently, particularly dealing with SYNONYM-STREAMs.

The behavior of OPEN-STREAM-P on, for example, broadcast streams, might

be specified in a variety of alternative ways. This specification seems the simplest.

There are three proposals for voting because there was no agreement at the

October X3J13 on the issue of whether types, predicates, or both should be

added.

There was a proposal at one time to add a new function FOLLOW-SYNONYM-STREAM

which could be written as

(defun follow-synonym-stream (x)

(if (synonym-stream-p x)

(follow-synonym-stream (symbol-value (synonym-stream-symbol x)))

x))

i.e., which chases through zero or more synonym stream indirections.


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