To: J3                                                     J3/19-153
From: Robert Corbett
Subject: Categories of elemental procedures
Date: 2019-February-14
References: 18-007r1

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NUMBER: F18/011
TITLE: Categories of elemental procedures
KEYWORDS: elemental
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTIONS:

The list of categories of elemental procedures at the start of
Subclause 15.8 "Elemental procedures" [325:11-12] is incomplete.
Procedures that are defined to be elemental in intrinsic modules
are not included.  Type-bound procedures are not included.
Dummy procedures and procedure pointers are not included, but
there is evidence that those omissions were intentional.

The standard intrinsic modules specify several procedures to be
elemental.  The omission of module procedures declared in
intrinsic modules from the list in Subclause 15.8 creates a
contradiction.

Subclause 7.5.6.2 [80:5-8] and Subclause 7.5.7.3 [82:16] both
assume that type-bound procedures may be elemental.

Nonintrinsic elemental procedures cannot be used as actual
arguments [301: 29,  15.5.1 "Syntax of a procedure reference"
p1].  Elemental procedures specified in the standard inrinsic
modules are generic, and so they cannot be passed as actual
arguments.  Elemental intrinsic procedures can be passed as
actual arguments [309: 2-5, 15.5.2.9 "Actual arguments
associated with dummy procedure entities" p1], but the text
cited indicates that dummy procedures cannot elemental.

Procedure pointers pose a bit of a problem.  A nonintrinsic
elemental procedure cannot be the target of a pointer
assignment [165:34, 10.2.2.2 "Syntax of the pointer
assignment statement" ].  However, paragraph 3 of
Subclause 10.2.2.4 "Pointer procedue assignment" states

      If the pointer object has an explicit interface,
      its characteristics shall be the same as the
      pointer target except that the pointer target
      may be pure even if the pointer object is not
      pure and the pointer target may be an elemental
      intrinsic procedure even if the pointer object
      is not elemental.

The final phrase suggests that a pointer object may be
elemental, but it does not say that it can be elemental.

Q1. Is a procedure in an intrinsic module that is specified
    to be elemental an elemental procedure?

Q2. Can a type-bound procedure be elemental?

Q3. Can a dummy procedure be specified to be elemental?

Q4. Can a procedure pointer be specified to be elemental?


ANSWERS:

A1. Procedures in intrinsic modules that are specified to
    be elemental are elemental.  An edit is provided to
    correct the omission in Subclause 15.8.

A2. A type-bound procedure that is bound to an elemental
    procedure is elemental.  An edit is provided to correct
    the omission in Subclause 15.8.

A3. A dummy procedure cannot be specified to be elemental.
    An edit is provided to make this restriction explicit.

A4. A procedure pointer cannot be specified to be elemental.
    An edit is provided to make this restriction explicit.

The edits provided are intended to make the impact on the
existing standard small.  More extensive changes are probably
desirable, and could be made in a revision of the standard.
In particular, the restrictions on dummy arguments and
procedure pointers could be made constraints.



EDITS to 18-007r1:

[167:8, 10.2.4.4 Procedure pointer assignment, p3] Replace
"elemental intrinsic procedure even if the pointer object
 is not elemental."

with

"elemental intrinsic procedure, even though the pointer
 object is not elemental."

{ Remove the suggestion that a procedure pointer might
  be elemental. }


[325:11-12, 15.8.1 Elemental procedure declaration and
 interface, p1] Replace the sentence

"An elemental procedure is ... an elemental subprogram."

with

"An elemental procedure is

  * an elemental intrinsic procedure (16.1),

  * a module procedure in an intrinsic module, if it is
    specified to be elemental,

  * a procedure that is defined by an elemental subprogram,
    or

  * a type-bound procedure that is bound to an elemental
    procedure.

 A dummy procedure or procedure pointer shall not be
 specified to be ELEMENTAL."

{ Add missing items the list in Subclause 15.8.  Add a
  missing restriction to Subclause 15.8. }



SUBMITTED BY:  Robert Corbett

HISTORY: 19-nnn   m218  Submitted