To: J3                                                      08-254r1
From: Toon Moene
Subject: The new TYPE_IS_EXTENSIBLE intrinsic.
Date: 2008 August 13

References: 08-254

Rationale:

A library designer, who does not know what types the user will
impose upon his interface, might want improved control
over user input, which may come from a source external
to the program, when using unlimited polymorphic objects together
with SAME_TYPE_AS() or EXTENDS_TYPE_OF().

The dynamic type of an unlimited polymorphic object can belong to
one of the following three classes:

 (1) an intrinsic type
 (2) an extensible type
 (3) a bind(c) or sequence type

As it stands, the language allows him to disambiguate between (1)
and (2)+(3) by using SELECT TYPE with all possible intrinsic types
and kinds; it is not possible to disambiguate (2) and (3).

Therefore, it is suggested to add an intrinsic TYPE_IS_EXTENSIBLE()
to the language which allows to do just that.

Edits:

[322 after the TRIM entry]

"TYPE_IS_EXTENSIBLE       (A)      I   True if and only if dynamic
                                       type of A is extensible."

[393 after 13.7.169]

"13.7.169+ <<TYPE_IS_EXTENSIBLE(A)>>

<<Description.>> True if and only if the dynamic type of A is extensible.

<<Class.>> Inquiry function

<<Argument.>> A may be of any type. It may be a scalar or array.

<<Result Characteristics.>> Default logical scalar.

<<Result Value.>> The result is true if and only if A is unlimited
                  polymorphic and an object of extensible type."