Issue DECLARE-TYPE-FREE Writeup

Status:		Proposal LEXICAL passed Jan 89 X3J13

Forum:         Cleanup

Issue:         DECLARE-TYPE-FREE

References:    CLtL p.158

               DECLARATION-SCOPE

Related issues: FUNCTION-TYPE-ARGUMENT-TYPE-SEMANTICS

               DECLARATION-SCOPE

               SPECIAL-TYPE-SHADOWING

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)

               Version 8,  7-Dec-88, Masinter (back to scope)

               Version 9,  2-Jan-89, Moon (2 proposals, to clarify discussion)

               Version 10, 12-Jan-89, Masinter (add back lost v.6 phrase 

                                                re nested declarations)



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):

  

  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, during the execution of any expression 

  within the scope of the declaration,  it is an error for the value of

  the declared variable not to be of the declared type. For declarations

  that are associated with variable bindings, the type declaration also

  applies to the initial binding of the variable. In the special case

  of a declaration for which there are no executable expressions

  within the scope of the declaration (e.g., (locally (declare (integer x)))),

  the result is as if there were executable expressions.


  In this proposal, a type declaration affects not only variable

  references within its scope, but also affects variable references that

  are outside the scope of the declaration but dynamically inside the

  execution of a form that is itself inside the scope of the

  declaration.  Such references can exist when the variable is SPECIAL

  or when the declaration is not attached to the variable's binding, so

  that the scope of the declaration does not include the entire scope

  of the variable.


  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.



Proposal (DECLARE-TYPE-FREE:LEXICAL):


  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, during the execution of any reference to the

  declared variable within the scope of the declaration, it is an error for

  the value of the declared variable not to be of the declared type; and

  during the execution of any SETQ of the declared variable within the scope

  of the declaration, it is an error for the newly assigned value of the

  declared variable not to be of the declared type; and at the moment the

  scope of the declaration is entered, it is an error for the value of the

  declared variable not to be of the declared type.


  In this proposal, a type declaration affects only variable references within

  its scope, and the meaning of "free" and "variable-binding-associated" type

  declarations can be described identically.


  This proposal is equivalent to saying that the meaning of a type declaration

  is equivalent to changing each reference to <var> within the scope of the

  declaration to (THE <type> <var>), changing each expression assigned to the

  variable within the scope of the declaration to (THE <type> <new-value>),

  and executing (THE <type> <var>) at the moment the scope of the declaration

  is entered.


  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.


Examples:


;; this is an error under DECLARE-TYPE-FREE:ALLOW:

;; the assertion that x is a fixnum is violated between the two 

;; calls to (zap)

;; this is a valid program under DECLARE-TYPE-FREE:LEXICAL


        (let ((x 12) (y 'foo))

          (flet ((zap () (rotatef x y)))

            (locally (declare (fixnum x))

              (zap)

              (zap)

              x)))


;; this is an error under both proposals


        (let ((x 12) (y 'foo))

          (flet ((zap () (rotatef x y)))

            (locally (declare (fixnum x))

              (zap)

              (print x)

	      (zap)

              x)))


;; this is an error under DECLARE-TYPE-FREE:ALLOW, because

;; the assertion that x is a fixnum

;; is violated during the call to zap, even though few 

;; implementations will be able to check:

;; this is a valid program under DECLARE-TYPE-FREE:LEXICAL


        (let ((x 12) (y 'foo))

          (flet ((zap ()

                   (rotatef x y)

                   (rotatef x y)))

            (locally (declare (fixnum x))

              (zap)

              x)))


;; this is an error under both proposals, even though the

;; violation of the type constraint happens after the form

;; with the declaration is exited.


   (let ((f (let ((x 3))

              (declare (fixnum x))

              #'(lambda (z) (incf x z)))))

     (funcall f 4.3))



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.

  

  DECLARE-TYPE-FREE:ALLOW is more restrictive on programs and hence allows

  more freedom for optimizing compilers.  DECLARE-TYPE-FREE:LEXICAL is easier

  to understand but allows a specialized representation only where the scope

  of the variable is the same as the scope of the declaration or the compiler

  can prove that there are no relevant other references to the variable.


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:


  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.


  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...))


*start*

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Date: Fri, 7 Apr 89 17:18 EDT

From: David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>

Subject: Did you blow it?

To: Guy Steele <gls@Think.COM>, rpg@lucid.com

cc: x3j13@sail.stanford.edu

In-Reply-To: <8904071720.AA18178@verdi.think.com>

Message-ID: <19890407211851.9.MOON@EUPHRATES.SCRC.Symbolics.COM>


    Date: Fri, 7 Apr 89 13:20:57 EDT

    From: Guy Steele <gls@Think.COM>


       Date: Thu, 6 Apr 89 15:19:42 PDT

       From: Richard P. Gabriel <rpg@lucid.com>


       When I read the effect of Issue: DECLARE-TYPE-FREE on page 223, I

       completely flipped. I think you might have gotten it wrong.


He did.


       You say that in this code:


       (defun f (x)

	 (declare (type float x))

	 (let ((x 'a)) ...)

	  ...)


       The declaration affects both bindings of x. This cannot make any sense

       at all.  I don't have marked which version of this issue passed, but I

       think neither implies this. The most that is implied is that if you

       say this:


       (defun f (x)

	 ...

	 (let ((y 'a))

	  (declare (type float x))

	  ...)

	 ...)


       then the declaration applies to variable references within the let-y

       and not to within some larger scope.


       I hope you are wedged, because otherwise the proposal is wedged.


    I also hope I am wedged.  I am taking the liberty of cc'ing this

    to X3J13 so that others can let me know what they think.


    I believe I was confused by this paragraph from proposal

    DECLARE-TYPE-FREE:LEXICAL, passed January 1989:


      This proposal is equivalent to saying that the meaning of a type declaration

      is equivalent to changing each reference to <var> within the scope of the

      declaration to (THE <type> <var>), changing each expression assigned to the

      variable within the scope of the declaration to (THE <type> <new-value>),

      and executing (THE <type> <var>) at the moment the scope of the declaration

      is entered.


    The ambiguity concerns whether in


       (defun f (x)

	 (declare (type float x))

	 x     ;reference 1

	 (let ((x 'a)) ...)

	   x   ;reference 2

	   ...)


    the two references are construed to be to the same *variable*.

    (I readily grant that they refer to different *bindings*.)

    I assumed that they were contrued to be the same variable,

    in which case I believe that what I wrote on page 223 of the

    CLtL II draft is a correct conclusion.


The phrase "within the scope of the declaration" quoted above is

supposed to be a precisely defined phrase.  The passed cleanup

issue DECLARATION-SCOPE was supposed to define that phrase.

Unfortunately, there is a problem: the precise language in version

2 of the proposal was replaced with much less precise language 

in version 4, which was the version that was voted upon.

The version 2 language was:

  

  The scope of a `bound' 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 at the beginning of whose body

  the declaration appears.  This is the same as what CLtL p.155 defines to

  be the scope of `pervasive' declarations.


This answers your question about special variables.  I think that

for declarations that concern variable or function bindings, but are

not actually attached to a binding (i.e. are used free), the correct

scope is the same as the scope of a non-special binding of that name

surrounding the form to which the binding is attached; the language

about LOCALLY quoted above is out of date.


Gurg.  Your example above is misindented.  If you meant


       (defun f (x)

	 (declare (type float x))

	 x     ;reference 1

	 (let ((x 'a))

	   x   ;reference 2

	   ...)


then the scope of the declaration does not include (let ((x 'a)) x ...)

because type declarations are not "pervasive".  Thus what you wrote

on p.223 of CLtL is wrong.


But if you meant


       (defun f (x)

	 (declare (type float x))

	 x     ;reference 1

	 (let ((x 'a)) ...)

	 x   ;reference 2

	 ...)


then the example does not address the question since reference 1

and reference 2 are clearly both in the scope of the type declaration.


    Now that you have pointed it out, I agree that a more likely

    and more desirable interpretation is that the references are

    considered to be to different variables.  Question: what if x

    had been proclaimed SPECIAL?  Then what would the interpretation be?


A special declaration should not affect the scope of a type declaration.

Now, the version of DECLARATION-SCOPING that actually passed does not

actually say that.  But I don't think any other position is arguable.