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Re: Issue: SYNTACTIC-ENVIRONMENT-ACCESS (Version 1)
- To: Eric Benson <firstname.lastname@example.org>
- Subject: Re: Issue: SYNTACTIC-ENVIRONMENT-ACCESS (Version 1)
- From: masinter.pa@Xerox.COM
- Date: 2 Oct 88 11:57 PDT
- Cc: email@example.com, firstname.lastname@example.org
- In-reply-to: Eric Benson <email@example.com>'s message of Sun, 2 Oct 88 11:30:37 pdt
would you say this supercedes:
References: Declaring Global Variables and Named Constants (pp68-69),
Declaration Specifiers (p157)
Edit history: 07-Mar-88, Version 1 by Pitman
21-May-88, Version 2 by Pitman (correct test case, add discussion)
Status: For Internal Discussion
CLtL does not define a way to test to see if a variable has been
proclaimed special (for the purposes of either binding or reference).
Programs such as macros, code-walkers, and program-generating programs
may need such information from time to time in order to do certain kinds
of reasoning about code-motion, unused variables, etc.
Add a function SPECIAL-VARIABLE-P by analogy with SPECIAL-FORM-P
which is defined as:
SPECIAL-VARIABLE-P symbol &optional environment [Function]
Returns T iff -symbol- names a variable which is SPECIAL in the
indicated lexical -environment-. Otherwise, it returns NIL.
It is an error if -symbol- is not a symbol. If not supplied, the
-environment- defaults to NIL, meaning the null lexical environment.
This function will be useful in determining whether a reference to
the variable named by SYMBOL in the indicated ENVIRONMENT will be
a special reference.
Note: Since special variable proclamations are pervasive and
declarations are not, the technique for determining whether binding
the variable named by SYMBOL is not dependent on the surrounding
lexical environment. It is instead dependent only on the global
environment and on the declarations of the form which would accomplish
the binding. Whether the variable has been globally proclaimed special
can be determined by doing (SPECIAL-VARIABLE-P 'symbol). Whether the
variable is locally declared SPECIAL can be checked only by parsing
the declarations looking for (DECLARE ... (SPECIAL ... symbol ...)).
(PROCLAIM '(SPECIAL SPECIAL-FOO))
(MACROLET ((TEST (NAME &ENVIRONMENT ENV)
`'(,NAME ,(SPECIAL-VARIABLE-P NAME ENV)) ))
(LIST* (TEST SPECIAL-FOO) ;0
(LET ((SPECIAL-FOO 1) (FOO 1))
(LIST* (TEST SPECIAL-FOO) ;1
(LET ((SPECIAL-FOO 2) (FOO 2))
(DECLARE (SPECIAL FOO))
(LIST* (TEST SPECIAL-FOO) ;2
(LET ((SPECIAL-FOO 3) (FOO 3))
(LIST (TEST SPECIAL-FOO) ;3
=> ((SPECIAL-FOO T) (FOO NIL) ;0
(SPECIAL-FOO T) (FOO NIL) ;1
(SPECIAL-FOO T) (FOO T) ;2
(SPECIAL-FOO T) (FOO NIL)) ;3
This would allow programs that reason about other programs to obtain
important information about SPECIAL declarations and proclamations.
Interpreters and compilers must, of necessity, have a way to do this
In some implementations, information about special variable proclamations
is kept on a symbol's plist, and users eventually "figure out" how to take
advantage of that.
In most implementations, getting information about special declarations
is neither documented nor easy to "figure out".
Symbolics Genera has undocumented internal function which does this.
Cost to Implementors:
By necessity, compilers and interpreters must have a way to get the
information returned by this facility. In general, it should just be
a matter of providing a program interface to that facility.
Cost to Users:
None. This is an upward-compatible extension.
Cost of Non-Adoption:
Some code-walkers, macros, etc. would continue be hard to write in a
The cost of non-adoption would be avoided.
Although SPECIAL variables provide some benefit to Common Lisp, that
benefit has not been without price. It's difficult to do proper code
analysis if lexical and special variables look the same. The presence
of this operator makes it easier to write code which reasons clearly
and correctly about other programs, and so will probably tend to
improve the aesthetics of such programs.
This proposal came to the Cleanup committee from the Japanese community.
Pitman wrote it up formally.
Pitman and Moon support SPECIAL-VARIABLE-TEST:SPECIAL-VARIABLE-P.