J3/04-121

Date:    10 Dec. 2003
To:      J3
From:    Dan Nagle
Subject: Finding Logical and Character Kinds


Despite years of thought and some discussions, no way has been
discovered to define a SELECTED_LOG_KIND function, probably
because true is true and false is false and that is that.  But a
programmer may want to portably set logical kind.  Also,
character kinds are set via names but kinds not specified
in the standard have no guaranteed to be portable name.
No way other than blind search for names is possible
under Fortran 2003.

Number:

Title: LOGICAL AND CHARACTER Kinds Available

Submitted By: J3

Status: For Consideration

References:

Basic Functionality: Provide a way to find all character and logical
                     kinds a processor supports to allow a program
                     or an automatic code generator to make
                     a fully informed choice of kinds.

Rationale:  A program may want to use the smallest logical kind
            available, say for a randomly addressed array not updated
            frequently but sharing cache with work arrays.  A program
            may have distinct preferences for character set.  But
            there's no way at present for a program to discover what
            these kind values are.

            I know of no suggestion for a SELECTED_LOG_KIND, and
            SELECTED_CHAR_KIND relies on names (character strings)
            making a search impossible.  Furthermore, the number
            of character sets with standard-specified names is small.
            A compiler desiring to interoperate with MS .NET,
            for example, may want to use Unicode-16.  But what's
            the name for SELECTED_CHAR_KIND?  "UTF-16", "Unicode_16"?
            "ISO_something or other"?

Estimated Impact: I estimate the impact of these features is low,
                  as compilers know what kinds they support and,
                  in the case of character kinds, by what name they
                  select them.

Detailed Specification: Arrays be added to ISO_FORTRAN_ENV defined
                        to include the kind values of supported logical
                        kinds and the names recognized by SELECTED_CHAR_KIND.
                        The array of logical kinds should be defined
                        so that kinds occur in order of increasing size.
                        These declarations might appear as follows
                        in user-written code:

                        integer, parameter, dimension( ???) :: &
                                 logical_kinds = ...

                        character( len= *), parameter, dimension( ??? ) :: &
                                 character_kinds = ...

                        Examples:

                        ! choose smallest logical kind
                        logical( kind= logical_kinds( 1)), dimension( large) &
                                                           :: mask

                        ! print all character kinds
                        write( output_unit, *) character_kinds

History: