Issue: DECLARATION-SCOPE

[Charles Hornig <Hornig@ALDERAAN.SCRC.Symbolics.COM>]

Issue:         DECLARATION-SCOPING

References:    Section 9.1 (pp. 153-157).

Category:      CHANGE

Edit history:  V1: Hornig@Symbolics.COM -- 5 January 1988

Problem description:

The description of the scope of declarations made with DECLARE is both
unclear (although unambiguous) and arguably a mistake.  The issue is
whether the scope of some or all of the declarations includes code
appearing in the non-body part of the special form containing the
declaration.


Proposal DECLARATION-SCOPING:LIKE-VARIABLE:

For the purposes of this proposal, we divide all declarations introduced
with DECLARE into two classes.  When a declaration of a variable appears
before the body of a special form or lambda-expression that binds that
variable, the declaration is called `normal'.  All other declarations
are called `free'.  This division replaces the division into pervasive,
nonpervasive, and SPECIAL declarations appearing in CLtL.

The scope of a `normal' declaration is exactly the scope of the
associated lexical variable or function.  If the declaration is
associated with a special variable, the scope is the scope the variable
would have had if it had not been special.  `Free' declarations are
scoped as if they appeared in a new LOCALLY form which surrounded the
entire special form of which the declaration appears at the beginning of
the body.

SPECIAL declarations may be either `normal', affecting both a binding
and references, or `free', affecting only references.  TYPE and IGNORE
declarations may only be `normal'.  (There has been some discussion in
X3J13 of permitting `free' TYPE declarations.)

If Common Lisp is extended to permit declarations in FLET and LABELS
forms, then declarations of functions (FTYPE, FUNCTION, INLINE, and
NOTINLINE) which appear before the body of a FLET or LABELS form which
defines that function are also `normal'.

Common Lisp is ambiguous about whether a variable may be bound several
times by a single form.  It has been proposed that multiple bindings be
permitted for LET*, DO*, PROG* forms and for &AUX variables in lambda
expressions.  If multiple bindings are permitted, `normal' declarations
are treated as if there were a separate `normal' declaration for each of
the bindings.


Examples:

;;; Some examples of `free' and `normal' declarations.

(let ((a 1))
  (declare (optimize speed))		;this is a `free' declaration
  (let ((b 2))
    (declare (type integer b))		;this is a `normal' declaration
    (declare (special a))		;this is a `free' declaration
    ()))

;;; This example applies if you believe that FLET may have declarations.
(flet ((foo (x) (1+ x)))
  (declare (notinline foo))		;this is a `normal' declaration
  (declare (notinline 1+))		;this is a `free' declaration
  ())

;;; The following code is from Pavel.
;;; It produces 7 in existing implementations.
;;; If the proposal is adopted, it will produce 8.
(let ((foo 6))			;a special binding of FOO
  (declare (special foo))	;`normal' declaration
  (let ((foo 7))		;a lexical binding of FOO
    (let ((foo (1+ foo)))	;is the second FOO special or not?
      (declare (special foo))	;`normal' declaration
      foo)))

;;; Treatment of LET* under the proposal if multiple bindings of the same name are allowed.
;;; This form produces the value 9.
(let ((foo 6))			;a special binding of FOO
  (declare (special foo))	;`normal' declaration
  (let ((foo 7))		;a lexical binding of FOO
    (let* ((foo (1+ foo))	;special binding, lexical reference
           (foo (1+ foo)))	;special binding, special reference
      (declare (special foo))	;`normal' declaration, applies to both bindings 
      foo))		        ;special reference


Rationale:

`Normal' declarations are made to control a particular binding of a
variable and should be scoped the same way as that binding.  This is as
true of `normal' declarations which were pervasive under the old rules
as it is of those that were not.


Current practice:

The `normal'/`free' division based on context replaces CLtL's static
pervasive/nonpervasive/SPECIAL division.  Most implementations implement
the rules in CLtL.  Symbolics currently implements rules based on
Zetalisp which are different from both this proposal and Common Lisp.
Symbolics plans to change to Common Lisp rules in the future.


Cost to Implementors:

The cost of implementing this change should be moderate.  The change
will be localized to a handful of places in the compiler and interpreter
which apply declarations to variables.  The other cost would be in
providing tools for users to find programs whose meaning will change.


Cost to Users:

The proposal changes only the treatment of `normal' declarations.  This
change will break very few existing production programs.

It is possible to mechanically examine a program to determine whether
its behavior would change under the new rules.  This permits an
implementation to provide a transition tool to ease conversion to the
new definition.


Cost of non-adoption:

The ability of a `normal' declaration to affect code outside the scope
of the variable which it appears to declare has led to endless confusion
and discussion at Symbolics, on the Common-Lisp mailing list, and
elsewhere.  It will continue to do so unless it is smoothed over somehow.


Benefits:

The costs of non-adoption will be avoided.


Aesthetics:

The distinction between `normal' and `free' declarations introduced by
this proposal is a natural one.


Discussion:

A proposal to forbid `free' declarations except in LOCALLY forms and a
proposal to have `free' declarations affect only the body were discarded
as being too incompatible.

The mapping from the existing pervasive/nonpervasive/SPECIAL division of
declarations and the one proposed here is complex.  In general,
nonpervasive declarations are `normal' and pervasive declarations are
`free'.  SPECIAL declarations are either `normal' or `free' based on
their context, and are no longer treated as a special case.

Some historical support for having `free' and `normal' declarations:

Date: Tue, 20 Dec 83 15:50 EST
 From: "David A. Moon" <Moon%SCRC-TENEX@MIT-MC.ARPA>
Subject: Declarations
To: Common-Lisp@SU-AI.ARPA
...
There are two disjoint classes of declaration: those that are attached
to a particular variable binding, and those that are not.  Note that I
am not discussing proclamations here; they have their own scoping rules
which are different from the rules for declarations.

The scoping rule for the first kind of declaration is that it applies to
precisely the text that is in the lexical scope of the variable binding
with which it is associated.  Such declarations are shadowed by a
variable binding for the same name inside their scope.  Since the
lexical scoping rules are very well and precisely defined, we already
understand everything about this kind of declaration.

The scoping rule for the second kind of declaration is that it is
pervasive.  The declaration is attached to a lambda-expression or to a
form (one of the special forms listed on page 125).  The declaration
applies to all text inside that expression or form; there are no special
cases such as init-forms of LET or DO.  Such declarations are shadowed
by a conflicting declaration inside their scope.

Possible names for the two kinds of declaration are "lexical" and
"pervasive" or "variable" and "non-variable."
...