Issue: DECLARE-TYPE-FREE (Version 5)

[masinter.pa@Xerox.COM]

I added my reaction JonL's comment about nested types to the discussion
section. I also mentioned in the "Cost to Implementors" the possibility
that it might be necessary to remove error checks.

I think this is ready for release. Complain soon if you don't.

!
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)
		

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):
  
  Avoid the phrase "affects only variable bindings".  Clarify that a type
  declaration means that it is an error for the value of the variable not
  to be a member of the declared type, within the scope of the declaration.
  Clarify that the above programs are valid, and that this  kind of 
  declaration means the same thing as wrapping a THE form around every 
  reference to the variable, including modifying references by setq or
setf.
  Clarify that if nested type declarations refer to the same variable, then
  the value of the variable must be a member of the intersection of the 
  declared types.


Rationale:

  It 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 implements DECLARE-TYPE-FREE:ALLOW already; but 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:

  Another cleanup issue, DECLARATION-SCOPE, addresses the scope of 
  declarations. This proposal carefully uses the phrase "within the 
  scope of the declaration" to avoid confounding the two issues. 

  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.

  A style note might add that since nested type declarations intersect,
  it would be bad style to have inner declarations be subtypes of the outer
  ones. For example

    (locally (declare (type x fixnum))
       (locally (declare (type x (or bit package)))
         (setq x 1)))


   would be confusing. Such code might be generated by macros, however.