J3/10-231
To: J3
From: Malcolm Cohen
Subject: Interp: ELEMENTAL type parameters
Date: 2010 October 08

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NUMBER: F08/?
TITLE: ELEMENTAL functions with nonconstant type parameters
KEYWORDS: ELEMENTAL, type parameter
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

Interpretation F03/0119, which became part of the Fortran 2008
standard, added as constraint the requirement of an elemental function
that its result variable
  "shall not have a type parameter that is defined by an expression
   that is not a constant expression".

This makes a number of valid Fortran 95 programs invalid, for no
apparent technical reason.  Moreover, no text was added to clause 1 to
indicate this incompatibility with Fortran 95.

Here are several examples of non-problematic Fortran 95 elemental
procedures that are apparently not valid Fortran 2008.

  MODULE ex1
    INTEGER :: mylen
  CONTAINS
    ELEMENTAL CHARACTER(mylen) FUNCTION f(a)
      INTEGER,INTENT(IN) :: a
      f = REPEAT(CHAR(a),mylen)
    END FUNCTION
  END MODULE

  ELEMENTAL FUNCTION ex2(a,b)
    CHARACTER(*),INTENT(IN) :: a,b
    CHARACTER(LEN(a)+LEN(b)) :: ex2
    ex2 = a//b
  END FUNCTION

  MODULE ex3
    INTEGER,ALLOCATABLE :: x(:)
  CONTAINS
    PURE FUNCTION gcd(a)
      INTEGER,INTENT(IN) :: a(:)
      gcd = ... code to calculate this omitted ...
    END FUNCTION
    ELEMENTAL FUNCTION f(y,z)
      CHARACTER(*),INTENT(IN) :: y,z
      CHARACTER(gcd([x,LEN(y),LEN(z)])) :: f
      f = ...
    END FUNCTION
  END MODULE

Was this incompatibility with Fortran 95 an oversight?

ANSWER:

Yes, this was an oversight; requiring the type parameter values to be
constant is unnecessarily onerous.  An edit is supplied to correct
this defect in the standard.

EDITS:

{to 10-007}

[314:6-7] 12.8.1, C1290, delete ", and shall not ... expression".
{Delete erroneous constraint.}

[314:7+] Insert new constraint
  "C1290a In the <specification-expr> that specifies a type parameter
          value of the result of an elemental function, an object
          designator with a dummy argument of the function as the base
          object shall appear only as the subject of a specification
          inquiry, and that specification enquiry shall not depend on
          a property that is deferred."

SUBMITTED BY: Malcolm Cohen

HISTORY: 10-nnn   m193 Submitted

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