To: J3                                                     J3/18-123
From: Tom Clune
Subject:  Distilled issues for container use case
Date: 2018-February-13

Reference:  18-110r1

The use-case paper for containers (18-110r1) is a bit rambling.  This
paper omits the justification and list of use cases, and summarizes
the various aspects of developing containers that could be improved
through new language Features.

1) Generic programming

   Generic programming would significantly reduce the development
   effort for containers for varying types of contained entities.  If
   something akin to C++ standard templates were used, then some
   mechanism for template _specialization_ would also be required for
   important corner cases.  In particular, enabling containers of
   deferred length strings would likely require judicious use of some
   sort of specialization.

2) Accessors

   Succinct notation for getting/setting elements of containers would
   be very beneficial.  Fortran arrays allow expressions such as:

   A(i,j) = 5

   and it would be desirable with a container V to do something like

   V(k) = 5

3) Improved semantics for iterators

   Exploiting Fortran's existing loop construct (DO WHILE ... END DO)
   can be made to work adequately for simple cases with an explicit
   call to the iterator incrementor at the end of the loop.  But if
   the user ever uses CYCLE, it is all too easy to forget to include
   an explicit increment there as well which can lead to
   subtle/confusing problems.  It is desirable to have a mechanism
   that guarantees the increment happens for each iteration.

   Note that such a feature/generalization has potential applications
   beyond just containers.

4) Containers of entities with POINTER attribute

   Fortran does not provide a mechanism to robustly order entities
   with the POINTER attribute.  This prevents any implementation of
   containers of such entities that rely on such ordering.  While
   Vectors of such entities are implementable, Sets are not, nor are
   Maps with keys that have the pointer attribute.


`