To: J3                                                     J3/23-199
From: Malcolm Cohen
Subject: Default kinds
Date: 2023-June-14


1. Introduction

Virtually all compilers have options to control what kinds the default
kinds actually are, e.g. -r8 to change default Real kind to binary64.
This proposal suggests that this could be done within the program unit,
so that the essential information, about what the default kinds are,
appears in the program source.


2. Proposal

That a new statement be added to specify what the default kinds are.
Things like EQUIVALENCE and COMMON would be reworded in terms of single
and double precision for reals, etc., thus decoupling the default kind
from storage association rules.

The default kind would be used for literal constants with no kind-param,
as well as those entities declared with a type-spec with no kind-selector.
Any explicit kind specification overrides the default, just as it does
today.

Such a statement would appear in the program unit in the same place as
the IMPLICIT statement. (Or maybe between USE and IMPLICIT?)


3. Illustrative syntax

Nothing here is set in mud, let alone concrete.

    PROGRAM example
        DEFAULT(REAL=SELECTED_REAL_KIND(30))
        REAL :: x = 1.23456789012345678901234567890
        PRINT '(F0.29)',x
    END PROGRAM

That would be expected to print a reasonably accurate approximation to the
literal constant.

===END===