To: J3                                                     J3/19-136r1
From:      Daniel Chen & Malcolm Cohen
Subject:   F2018 interp: In array element order execution should
           apply to IMPURE elemental function only
Date: 2019-February-12


0. Response from /INTERP

0.1 Summary

There is no problem, therefore this does not warrant processing as a
defect report.

0.2 Discussion

The quoted sentence is
 "In the array case, the values of the elements, if any, of the result are
  the same AS WOULD HAVE BEEN OBTAINED IF the scalar function had been
  applied separately, in array element order..."
(emphasis mine).

That is, this sentence makes no requirement on the actual evaluation
order, only that the result is the same AS IF it were evaluated in array
element order.  Because the rules for PURE guarantee order-independence,
this statement makes no requirement on the evaluation order for a PURE
procedure, and indeed allows for parallel evaluation.

Thus, there is no problem with this sentence, which has been carefully
constructed to work for both PURE and IMPURE procedures.

Thus, the recommendation is that this not be processed as a defect.

0.3 Further discussion

Even in the case of IMPURE, the sentence requires sequential ordered
evaluation only if the result or side effects would differ.

For example, an IMPURE procedure that assigned the value .TRUE. to a
global variable if an argument were zero, made no assignment to the
variable if the argument were non-zero, and had no other "impure"
effects, could potentially be processed in any order or even in parallel.

0.4 The original paper

The original paper is reproduced below for reference.

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1. Introduction

In F2003, the standard says (12.7.2),
"In the array case, the values of the
elements, if any, of the result are the same as would have been
obtained if the scalar function had been applied separately,
in any order, to corresponding elements of each array actual
argument."

In F2008, we added the following in the Introduction section[xvi]
"An impure elemental procedure processes array arguments in array
element order."

However, we changed
p1 of 12.8.2 Elemental function actual arguments and results
from "in any order" to "in array element order" without saying this
should apply to IMPURE elemental function only.

In F2018, we kept the same wording as F2008 (15.8.2)


2. Interpretation Request

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NUMBER: F18/xxxx
TITLE:  PURE Elemental function should be executed in any order.
KEYWORDS: ELEMENTAL, PURE
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

If the argument of an pure elemental function is an array, should
the result be the same as it would have been obtained if the scalar
function had been applied, in any order, to corresponding elements of
each array actual argument.

ANSWER:

Yes. This is an oversight.

EDITS to 18-007r1:

[326: 22] In 15.8.2 Elemental function actual arguments and results
Change "in array element order" to "in array element order if it has
the PURE prefix or in any order otherwise"

It makes the whole sentence read,

"In the array case, the values of the elements, if any, of the result
are the same as would have been obtained if the scalar function had
been applied separately, in array element order if it has the PURE
prefix or in any order otherwise, to corresponding elements of each
array actual argument."

SUBMITTED BY: Daniel

HISTORY: 19-xxx    m218  submitted


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