Issue DECLARE-ARRAY-TYPE-ELEMENT-REFERENCES Writeup

Status:	Passed, Jan 89 X3J13


Issue:         DECLARE-ARRAY-TYPE-ELEMENT-REFERENCES


References:    Array type specifiers, pp. 45-46


Related issues: ARRAY-TYPE-ELEMENT-TYPE-SEMANTICS, DECLARE-TYPE-FREE,

                SYMBOL-MACROLET-DECLARE


Category:      CHANGE


Edit history:  Version 1,  7-Oct-88, Pierson

               Version 2, 13-Jan-89, Pierson (Moon and JonL comments)

    	       Version 3, 13-Jan-89, Pierson (Pitman comments)


Problem description:


In principle, array type specifiers could be used both for declaring

the storage format of the array and for implicitly declaring the types

of the elements held by those arrays. Unfortunately, the current 

definition of the meaning of array type specifiers does not explicitly

specify that the latter use of these declarations is legitimate.


Proposal (DECLARE-ARRAY-TYPE-ELEMENT-REFERENCES:RESTRICTIVE):


Within the lexical scope of an array type declaration, all references

to array elements are assumed to satisfy the exact declared element

type.  It is an error if this is ever violated.  A compiler may treat

the code within the scope of the array type declaration as if each

access of an array element was surrounded by an appropriate THE form.


Examples:


(DEFVAR *ONE-ARRAY* (MAKE-ARRAY 10 :ELEMENT-TYPE '(SIGNED-BYTE 5)))

(DEFVAR *ANOTHER-ARRAY* (MAKE-ARRAY 10 :ELEMENT-TYPE '(SIGNED-BYTE 8)))


(DEFUN FROB (AN-ARRAY)

  (DECLARE (TYPE (ARRAY (SIGNED-BYTE 5) 1) AN-ARRAY))

  (SETF (AREF AN-ARRAY 1) 31)		; OK

  (SETF (AREF AN-ARRAY 2) 127)		; Should signal an error

  (SETF (AREF AN-ARRAY 3) (* 2 (AREF AN-ARRAY 3))) ; Run-time decision needed

  (LET ((FOO 0))

    (DECLARE (TYPE (SIGNED-BYTE 5) FOO))

    (SETF FOO (AREF AN-ARRAY 0))))	; Declared to be safe


(FROB *ONE-ARRAY*)			; Legal call, should signal an error

(FROM *ANOTHER-ARRAY*)			; Is probably an undetectable error


Note that the above definition of FROB is equivalent to:


(DEFUN FROB (AN-ARRAY)

  (DECLARE (TYPE (ARRAY (SIGNED-BYTE 5) 1) AN-ARRAY))

  (SETF (THE (SIGNED-BYTE 5) (AREF AN-ARRAY 1) 31))

  (SETF (THE (SIGNED-BYTE 5) (AREF AN-ARRAY 2) 127))

  (SETF (THE (SIGNED-BYTE 5) (AREF AN-ARRAY 3))

	(* 2 (THE (SIGNED-BYTE 5) (AREF AN-ARRAY 3))))

  (LET ((FOO 0))

    (DECLARE (TYPE (SIGNED-BYTE 5) FOO))

    (SETF FOO (THE (SIGNED-BYTE 5) (AREF AN-ARRAY 0)))))


Given an implementation in which fixnums are 29 bits but fixnum arrays

are upgraded to signed 32-bit arrays, the following should be compiled

with all fixnum arithmetic:


(DEFUN BUMP-COUNTERS (COUNTERS)

  (DECLARE (TYPE (ARRAY FIXNUM *) BUMP-COUNTERS))

  (DOTIMES (I (LENGTH COUNTERS))

    (INCF (AREF COUNTERS I))))


Test Cases:


TBS


Rationale:


This mandates a useful and commonly expected behavior.  It complements

proposal ARRAY-TYPE-ELEMENT-TYPE-SEMANTICS, which deals with array

type specifiers as they refer to arrays as a whole.  


This proposal is consistent with SYMBOL-MACROLET-DECLARE:ALLOW and

DECLARATION-SCOPE:LEXICAL.


Current practice:


???


Cost to Implementors:


TBS


Cost to Users:


Probably none; while this is technically a change, code that declares

an array to contain one thing and depends on it containing something

else is blatantly buggy.


Cost of non-adoption:


Users will continue to expect declaration syntax to be more useful

than it really is.


Performance impact:


None.


Benefits:


Array type declarations will behave in a more useful and intuitive way.


Aesthetics:


Improved because the meaning of type declarations will coincide more

clearly with their appearance.


Discussion:


Pierson and Pitman support this proposal.