To: J3                                                     J3/24-148r1
From: Malcolm Cohen
Subject: Revised formal specifications for generic subprograms
Date: 2024-June-26
References: 23-244r1, 23-223r2, N2217


1. Introduction
===============

At its meeting Jun 12-16, 2023, WG5 decided to approve generic
procedures as described in N2217 for Fortran 202Y. Use cases for
this are set out in N2217. Formal requirements are in 23-223r2.
The previous formal specifications were in 23-244r1.

This is the formal specifications document, revised according to the
discussion arising from 24-140. The illustrative syntax section of 23-223r2
has been omitted, as a syntax paper is expected imminently.

Terminology: For the purpose of these specifications, the generic
procedures produced by this facility are termed "auto-generic", as it is
the compiler/processor that generates all of the anonymous specific
procedures.


2. Formal specifications
========================

Auto-Generic procedure definition and identification:

s01. An "auto-generic procedure" is defined by a "generic subprogram".

s02. A generic subprogram shall be a module subprogram or internal
     subprogram. It shall not have multiple entry points, that is, no
     ENTRY statement is allowed.

NOTE: Although N2217 envisaged generic external subprograms, external
      subprograms need to have global names to be referenced. That would be
      undesirable - putting them in a module is easier to understand and
      safer to use.

NOTE: In principle, this includes the possibility of being a separate
      module subprogram defined in a submodule, in which case there would
      be a generic module procedure interface in the ancestor module.

NOTE: This specification may be revisited if sufficiently compelling
      reasons for needing generic external subprograms are found, and that
      are not satisfied by other means such as submodules.

s03. A generic subprogram that is a module subprogram can have internal
     subprograms; those internal subprograms shall not themselves be
     independently generic.

s04. Every generic subprogram shall have a generic name.

s04a. That generic name may be the same as an existing generic name that is
      accessible in the containing scoping unit.

s05. The name of a generic subprogram shall be able to be added to the
     generic set of a generic identifier that is not a name,
     i.e. operator, assignment, input/output.

s06. There is no specification regarding the ability or inability (as now),
     for adding a generic name that is a traditional generic set to the
     generic set of another generic identifier.
     Consistency may suggest doing this.

Within a generic subprogram:

s07. The subprogram may have zero, one, or more generic dummy arguments.
     Even with no generic dummy argument, it is still a generic subprogram
     providing a generic procedure name and no specific procedure name.

s08. There shall be a syntax that specifies all valid kinds of an intrinsic
     type for a generic dummy argument. The dummy argument is a generic
     dummy argument even if a compiler supports only one kind for the type.

s09. There shall be a syntax that specifies all valid ranks for a generic
     dummy argument.

s10. There shall be a syntax that specifies a set of kind type parameter
     values (as constant expressions) for a generic dummy argument. This
     syntax shall be usable for parameterized derived types as well as for
     intrinsic types. The dummy argument is still generic even if it only
     evaluates to one kind (assuming the syntax is different from normal
     non-generic kind specification, which seems likely).

s11. There should be a hopefully-similar syntax for a list of ranks for a
     dummy argument to be generic over. Even if the list of ranks evaluates
     to a single valid rank, the dummy argument is still considered to be
     generic.

s12. Assuming that generic-by-type is feasible and we do it, syntax is
     needed to supply a list of type-specs similarly, which may be
     intrinsic types or user-defined types.

s13. A rank-generic dummy argument shall be a pointer, allocatable, or
     assumed-shape. It may have the CONTIGUOUS attribute, even if rank zero
     is included in the set of ranks.

s14. A generic dummy argument may be polymorphic, if and only if every
     type over which it is generic is an extensible type. None of these
     types may be an extension of another.

s15. The declarations within a generic subprogram shall be consistent with
     every generic combination.

s16. The executable code within a generic subprogram, other than that
     within an ad-hoc specialization construct, shall be consistent with
     every generic combination.

s17. Not only local variables (and the function result) of a generic
     subprogram may be declared to have the same attributes as a generic
     dummy argument, but also local variables (and function results) of
     nested scoping units viz. BLOCK constructs and internal subprograms.

s18. The executable code within a block of an ad-hoc specialization need
     only be consistent with the specified generic combination.

NOTE: This is exactly how our existing runtime ad-hoc polymorphism
      constructs work.

s19. Ad-hoc specialization shall be performed at compile time, that is,
     there will be no trace of the non-chosen specializations in the
     generated anonymous specific.

NOTE: This is aspirational, as there is no way at present to express it
      normatively.


3. Example
==========

        Module example1
            Interface Operator(.myop.)
                Procedure s ! All of the specific procedures of s.
            End Interface
        Contains
            Generic Subroutine s(a,b)
                Type(Real,Complex), Intent(InOut), Rank(*) :: a, b
                Type(Typeof(b)), Rank(Rank(b)) :: temp
                ...
                Select Type (b)
                Type Is (Real)
                    temp = temp * (1-b)
                Type Is (Complex)
                    ! Just this once, we want the conjugate.
                    temp = temp * (1-Conjg(b))
                End Select
                ...
            End Subroutine
        End Module

===END===