J3/03-141

To:      J3
From:    David Muxworthy
Subject: TYPEALIAS description
Date:    12 March 2003

UK comment TC8 was a proposal to delete the TYPEALIAS facility on the
grounds that it added little benefit to the language and indeed its use
was potentially confusing as it appeared to offer a quick way of
changing variable types without at the same time changing corresponding
constants, operators, format codes, or any other related entities.

The Canadian Fortran Working Group had suggested this facility which
appeared as item 11 in the 1995 WG5 repository of requirements (N1144)
for Fortran 2000, but which was not selected for the language, possibly
on the grounds that it appears to promise more than it actually
delivers.

Nevertheless, should TYPEALIAS be retained in the language, it is
suggested that the basic description be simplified as shown below and
that the example of use given by the Canadian group should appear as an
additional example.

Edits suggested are:

61:2-3.  Replace "A type alias ...  new type." by "A <<type alias>> is
a pseudonym for an existing type and a fixed set of type parameter
values.  It may be used to declare entities; it is not a separate
type."

61:6. Delete (italicised) "declaration-".

{since <declaration-type-spec> differs from <type-spec> only in two
 ways:
 (1) it allows CLASS ... but that's not allowed in a type alias
     statement, so is irrelevant.
 (2) it requires the type parameter values to be specification
     expressions, but since in a type alias stmt they are required to
     be initialization expressions that too is irrelevant.
 So we should use the simpler BNF rule, viz <type-spec>.}

61:9. Delete C479.

61:10. Delete (italicised) "declaration-".

61:13. Delete (italicised) "declaration-".

61:13+.  Add new Note 4.59a:

"If variables a, b are declared as:

   TYPE(float) :: a, b

then the precision of these and corresponding variables, but not
literal constants or other related entities in a program, may be
changed by choice of one of, for example,

  TYPEALIAS :: float => REAL(KIND(1.0))
  TYPEALIAS :: float => REAL(KIND(1.0D0))
  TYPEALIAS :: float => TYPE(verylong)

in a single location in an appropriate scope."