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Cleanup Issue `DECLARE-ARRAY-TYPE-ELEMENT-REFERENCES`

Status: Passed, Jan 89 X3J13
Category: CHANGE
References: Array type specifiers, pp. 45-46
Related issues: ARRAY-TYPE-ELEMENT-TYPE-SEMANTICS, DECLARE-TYPE-FREE, SYMBOL-MACROLET-DECLARE

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 (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.

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)