Issue: DECLARE-TYPE-FREE (Version 6)

[CL-cleanup@Sail.stanford.edu]

Here's a first pass at this writeup. I've convinced myself today that
the issue for type declarations is their extent, not their scope. 
Only name bindings have scope; type declarations don't introduce
name bindings, they only introduce constraints on those bindings, 
and the constraints are dynamic.

I better stop before I dig myself a bigger hole. This writeup
also attempts to fix some of the terminology of the "proposal"
part.

If you like this, the writeup needs to be extended with JAR's 
examples of assignments that violate the declarations outside
of the scope but within the extent of the declaration, and should
say that they are "in error".

!
Issue:         DECLARE-TYPE-FREE
References:    CLtL p.158
	       DECLARATION-SCOPE
Category:      CLARIFICATION/ADDITION
Edit history:  Version 1, 18-Sep-88, Moon
               Version 2, 22-Sep-88, Moon
                (small edits to reflect mail discussion)
               Version 3, 22-Sep-88, Masinter
               Version 4, 27-Sep-88, JonL 
	       Version 5, 30-Sep-88, Masinter (cost to implementors)
	       Version 6, 06-Oct-88, Pitman (minor edits in Discussion)
	       Version 7,  5-Dec-88, Masinter (scope->extent)

Problem description:

  Section 9.2 of CLtL, p158, says that a declaration specifier like
  (TYPE type var1 var2 ...) "... affects only variable bindings".  
  Since declarations can occur in contexts other than establishing 
  "variable bindings", most people interpret this statement to mean 
  that type declarations not in such context are either (1) completely 
  to be ignored, or (2) invalid CL  syntax.  Thus both of the following 
  forms would be suspect in that the type declarations could not have 
  any effect:

    (if (and (typep x 'fixnum) (typep y 'fixnum))
	(locally (declare (fixnum x y))		    ;LOCALLY does not bind
	  ...algorithm using x and y...)	    ; any variables.
	...similar algorithm using x and y...)

    (let ((y 'foo))
      (setq y 10)
      (let ((x 5))				    ;'y' is not being bound in
        (declare (fixnum y))			    ; this particular context.
        (incf y)
         ...random algorithm...))


Proposal (DECLARE-TYPE-FREE:ALLOW-DYNAMIC):
  
  Specify that a type declaration does not only "affect variable bindings";
  rather, type declarations are legal in all declarations; the interpretation
  of a type declaration is that, within the extent of the form containing
  the declaration, it is an error for the value of the declared variable not
  to be of the declared type.

  This is stronger constraint than what would be implied by wrapping
  a THE form around every reference to the variable, including modifying
 references by SETQ or SETF, and, for type declarations that also affect a 
 variable binding, around the computation of the initial value.

Rationale:

  This proposal enables optimizing compilers to make use of the otherwise
  ignored type information.  Many people have often asked  for it, and
  there is no strong reason to forbid it.
  
Current practice:

  Lucid Common Lisp allows "free" type declarations;  under some 
  circumstances the compiler issues a warning message that such usage 
  is an extension to Common Lisp.

Cost to Implementors:

  Implementations that might currently warn about such declarations
  would have to remove the warning; otherwise, it is valid to ignore 
  type declarations.

Cost to Users:

  None, this is a compatible addition.

Cost of non-adoption:

  Common Lisp will be less self-consistent.

Benefits:

  Programmers will be able to use type declaration to express their
  intent, rather than having to manually insert THE wrappers around 
  every reference.


Esthetics:

  It is a simpler interpretation for type declaration specifiers, with
  fewer special cases; hence reduces the number of exceptions in the
  language.

Discussion:

  This issue has been discussed at the Fort Collins X3J13 meeting in
  November 1987, and at length on the various electronic mailing lists.

  At least one current implementation is able to generate more efficient
  code when declarations are associated with a particular binding, since
  it then has the option to choose type-specific specialized storage for 
  the runtime value of the variable.  So, for example, 

      (let ((x v)) (declare (type float x)) (+ x x))

  is sometimes more efficient than

      (let ((x v)) (locally (declare (type float x)) (+ x x)))

  However, the local type declarations allowed by this proposal do
  provide some useful information, even if it is not the *most* useful.
  It is possible for a sufficiently "smart" compiler to infer the 
  equivalent of a "binding declaration" when it can ascertain that the 
  type of the binding value -- 'v' above -- is commensurate with the 
  type locally declared over the scope of usage of the variable.

  It may be useful for a compiler to issue a warning whenever it finds
  nested type declarations referring to the same variable and the
  intersection of the declared types is null.

  Documentation might want to discuss the style implications of
  nested declarations intersecting. The interesting cases are:
   - An inner declaration could be a subtype of an outer one.
     This is the most useful case and probably the only one to
     be encouraged in code written by humans. e.g.,
       (locally (declare (type number x))
         (locally (declare (type integer x))
	   ...use X as integer...))
   - An outer declaration could be a subtype of an inner one.
     This is useless but harmless. It might happen as the result
     of certain macro situations. e.g.,
       (locally (declare (type integer x))
	 (locally (declare (type number x))
	   ...use X as integer...))
   - Two types may only partially overlap. This would presumably
     happen only as the result of a macro expansion.
       (locally (declare (type fixnum x))
         (locally (declare (type (or bit package) x))
           ...use X as BIT...))



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