X3J3/96-143
TO: X3J3
FROM: R. Baker Kearfott
SUBJECT:  Specifying default precision (Minor Technical Enhancement 12)

JOR item 35 deals with specifying a default precision for real
data types in Fortran 2000.

Argument in favor: It makes it easier for users to transport
codes, especially legacy codes that do not use type parameters
on each occurrence of constants.

Argument against:  It supports an older paradigm, namely single
versus double precision, and constants that are specified
without type parameters.  Some feel that this paradigm
should not be encouraged.

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SYNTAX:
As far as I can see, the syntax proposed in the JOR item suffices.
The change does not affect any current implementation, but proposes
a small amount of additional syntax.
Namely,


  IMPLICIT (REAL,KIND=SELECTED_REAL_KIND(10,30)) (DEFAULT)

declares the 'default real' kind to be a processor-dependent
type which allows 10 decimal digits and an exponent range of at
least 30. Each real variable and real literal constant appearing
in a program unit containing the above declaration will be of
that type.


  default declaration statement    is

              IMPLICIT (type,  kind-selector ) (DEFAULT)

      type    is      INTEGER

        or      REAL

        or      DOUBLE PRECISION

        or      COMPLEX

        or      LOGICAL

        or      INTERVAL

      kind-selector   is  ( [ KIND = ] scalar-int-initialization-expr )

Note: For example, a large-format default integer type can be
      specified as default for use in I/O statements.

Note: The intrinsically defined relational operators yield a type of
      "default logical."  Does allowing the user to change the default
      cause more work to be required to implement the intrinsic
      relational operators?  LOGICAL is included for regularity.
      A possible straw vote:  Include logical / exclude logical

Note: The interval data type may possibly be optional.  If so, the
      optionality would be reflected in the final edits to this
      item.

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EDITS:

In 5.3 after R543 (p. 63, line 40):
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  or

              IMPLICIT (type,  kind-selector ) (DEFAULT)

      type    is      INTEGER

        or      REAL

        or      DOUBLE PRECISION

        or      COMPLEX

        or      LOGICAL

        or      INTERVAL

      kind-selector   is  ( [ KIND = ] scalar-int-initialization-expr )

 Constraint:  If IMPLICIT (type, kind-selector) (DEFAULT) appears,
              it shall appear before any other implicit statements
              and before any other occurrences of symbolic variable
              names or literal constants.

Immediately above note 5.24 (page 64):
_____________________________________

If the IMPLICIT (type, kind-selector) (DEFAULT) construction
appears, then the default kind of variables and literal constants
of type type shall be that specified by kind-selector.