To: J3                                                     J3/23-182r1
From: Brad Richardson
Subject: Expanded Requirements
Date: 2023-June-13

#Reference:

Introduction:
-------------

This paper describes extensions of the REQUIREMENT and REQUIRES
template features proposed feature for F202Y.

The templates feature has introduced the concept of a REQUIREMENT,
which serves to encapsulate and reuse sets of declarations. In some
cases it would be desirable to also specify properties of certain
types or values which cannot be specified by a declaration. For example
it would be nice to specify the requirement that a value be a valid kind
for a given intrinsic type. For example something like,

  REQUIRES VALID_REAL_KIND(K)

One avenue for solving the above use cases would be to allow a constant
LOGICAL expression in place of REQUIREMENT in a REQUIRES statement. For
example VALID_REAL_KIND could be defined as:

  REQUIREMENT VALID_REAL_KIND(K)
    USE ISO_FORTRAN_ENV, ONLY: REAL_KINDS
    REQUIRES any(K == REAL_KINDS)
  END REQUIREMENT

There are some cases where there is no way to specify a logical
expression that establishes a property that you want to check. There may
also be some properties which the processor may need to understand
semantically in order to write certain templates. This will also
potentially serve a role as we allow for polymorphism with deferred
types. For example something like

  REQUIRES EXTENSIBLE(T)

may be needed in order to write a template like

  TEMPLATE ELEMENT_WRAPPER(T)
    REQUIRES EXTENSIBLE(T)
    TYPE :: WRAPPER
      CLASS(T), ALLOCATABLE :: ELEMENT
    END TYPE
  END TEMPLATE

Something like

  REQUIRES EXTENDS_FROM(T, U)

may be needed to write certain templates as well.