To: J3                                                     J3/19-201
From: Nathan John Sircombe & Malcolm Cohen
Subject: Draft design for SIMPLE procedures
Date: 2019-August-05

Introduction
============

18-273r1 introduces the concept of SIMPLE procedures as an extension of
PURE procedures. PURE procedures operate without writing to global
variables beyond their arguments, SIMPLE procedures would be further
restricted from referencing global variables beyond their arguments.

Requirements:
=============
Introduction of a new procedure prefix for subprograms which do not
reference or update global variables beyond their arguments.
 - A new prefix-spec, 'SIMPLE', added to the standard.
 - Requirements and constraints, closely following those for PURE,
   added to the standard.

Specifications:
===============
 - A SIMPLE procedure is PURE.
 - A SIMPLE procedure is not permitted to reference or define any variable
   outside its arguments and local variables.
 - A subprogram is specified to be SIMPLE if it has a SIMPLE prefix in its
   initial SUBROUTINE or FUNCTION statement.
 - A SIMPLE procedure cannot be IMPURE.
 - A SIMPLE procedure may be ELEMENTAL.
 - A SIMPLE procedure can access derived type definitions and named
   constants from outside its scope, but is not permitted to reference
   any variables except local variables or dummy arguments.
 - Similarly, a specification inquiry that is not a constant expression
   shall not depend on a variable other than a local variable or dummy
   argument. (See p157 of 18-007r1 for the definition of specification
   inquiry.) This allows, e.g. KIND(X) where X is a variable accessed
   by use or host association, but not LEN(Y) unless Y has constant
   length.

Other specifications follow from those of a PURE procedure, with the
additional constraint that all internal subprograms in a SIMPLE subprogram
shall be SIMPLE and all procedure references in a SIMPLE subprogram shall
be SIMPLE.


Syntax:
=======

prefix-spec <<is>> ! existing rules plus
            <<or>> SIMPLE

Thus the SIMPLE prefix-spec may appear in any position in the prefix of a
SUBROUTINE or FUNCTION statement.

Constraint: If SIMPLE appears, IMPURE shall not appear.

(Specifying PURE is redundant but permitted.)

Constraints: as described in the specs for a SIMPLE procedure.

Edits
=====

To follow.