Design Process: Lumiera Forward Iterator
========================================

[grid="all"]
`------------`-----------------------
*State*         _Final_
*Date*          _2009-11-01_
*Proposed by*   link:Ichthyostega[]
-------------------------------------

The situation focussed by this concept is when an API needs to expose a
sequence of results, values or objects, instead of just yielding a function
result value. As the naive solution of passing an pointer or array creates
coupling to internals, it was superseded by the GoF
https://en.wikipedia.org/wiki/Iterator[Iterator pattern]. Iteration can be
implemented by convention, polymorphically or by generic programming; we use
the latter approach.


Lumiera Forward Iterator concept
--------------------------------
.Definition
An Iterator is a self-contained token value, representing the promise to pull a
sequence of data

 - rather then deriving from an specific interface, anything behaving
   appropriately _is a Lumiera Forward Iterator._
 - the client finds a typedef at a suitable, nearby location. Objects of this
   type can be created, copied and compared.
 - any Lumiera forward iterator can be in _exhausted_ (invalid) state, which
   can be checked by +bool+ conversion.
 - especially, default constructed iterators are fixed to that state.
   Non-exhausted iterators may only be obtained by API call.
 - the exhausted state is final and can't be reset, meaning that any iterator
   is a disposable one-way-off object.
 - when an iterator is _not_ in the exhausted state, it may be _dereferenced_
   (+*i+), yielding the ``current'' value
 - moreover, iterators may be incremented (+++i+) until exhaustion.


Discussion
~~~~~~~~~~
The Lumiera Forward Iterator concept is a blend of the STL iterators and
iterator concepts found in Java, C#, Python and Ruby. The chosen syntax should
look familiar to C++ programmers and indeed is compatible to STL containers and
ranges. To the contrary, while a STL iterator can be thought off as being just
a disguised pointer, the semantics of Lumiera Forward Iterators is deliberately
reduced to a single, one-way-off forward iteration, they can't be reset,
manipulated by any arithmetic, and the result of assigning to an dereferenced
iterator is unspecified, as is the meaning of post-increment and stored copies
in general. You _should not think of an iterator as denoting a position_ --
just a one-way off promise to yield data.

Another notable difference to the STL iterators is the default ctor and the
+bool+ conversion. The latter allows using iterators painlessly within +for+
and +while+ loops; a default constructed iterator is equivalent to the STL
container's +end()+ value -- indeed any _container-like_ object exposing
Lumiera Forward Iteration is encouraged to provide such an +end()+-function,
additionally enabling iteration by +std::for_each+ (or Lumiera's even more
convenient
 +util::for_each()+).

Implementation notes
^^^^^^^^^^^^^^^^^^^^
*iter-adapter.hpp* provides some helper templates for building Lumiera Forward
 Iterators.

 - _IterAdapter_ is the most flexible variant, intended for use by custom
    facilities.
   An IterAdapter maintains an internal back-link to a facilitiy exposing an
   iteration control API, which is accessed through free functions as extension
   point. This iteration control API is similar to C#, allowing to advance to
   the next result and to check the current iteration state.
 - _RangeIter_ wraps two existing iterators -- usually obtained from +begin()+
    and +end()+ of an STL container
   embedded within the implementation. This allows for iterator chaining.
 - _PtrDerefIter_ works similar, but can be used on an STL container holding
    _pointers,_
   to be dereferenced automatically on access

Similar to the STL habits, Lumiera Forward Iterators should expose typedefs for
+pointer+, +reference+ and +value_type+. Additionally, they may be used for
resource management purposes by ``hiding'' a ref-counting facility, e.g.
allowing to keep a snapshot or restult set around until it can't be accessed
anymore.


Tasks
^^^^^
The concept was implemented both for unit test and to be used on the
_QueryResolver_ facility; thus it can be expected to show up on the session
interface, as the _PlacementIndex_ implements _QueryResolver_. QueryFocus also
relies on that interface for discovering session contents. Besides that, we
need more implementation experience.

Some existing iterators or collection-style interfaces should be retro-fitted.
See https://issues.lumiera.org/ticket/349[Ticket #349]. +
Moreover, we need to 
gain experience about mapping this concept down into a flat C-style API.



Alternatives
^^^^^^^^^^^^
. expose pointers or arrays
. inherit from an _Iterator_ ABC
. unfold the iteration control functions into the custom types
. define a selection of common container types to be allowed on APIs
. use _active iteration,_ i.e. pass a closure or callback


Rationale
~~~~~~~~~
APIs should be written such as not tie them to the current implementation.
Exposing iterators is known to create a strong incentive in this direction and
thus furthers the creation of clean APIs.

Especially in Steam-Layer we utilise already several iterator implementations,
but without an uniform concept, these remain just slightly disguised
implementation types of a specific container. Moreover, the STL defines various
and very elaborate iterator concepts. Ichthyo considers most of these an
overkill and an outdated aproach. Many modern programming languages build with
success on a very simple iterator concept, which allows once to pull a sequence
of values -- and nothing more.

Thus the idea is to formulate a concept in compliance with STL's forward
iterator -- but augmented by an stop-iteration test. This would give us basic
STL integration and look familiar to C++ and Java programmers without
compromising the clean APIs.





Comments
--------
//comments: append below

Now in use since more then a year, without turning up any serious problems.
The only _minor concern_ I can see is that this concept, as such, doesn't solve
the problem with exposing implementation details of the underlying container on the API.
Similar to STL Iterators, the actual implementation representation is only disguised
behind a 'typedef'. But, generally speaking, this is an inevitable consequence of the
``zero overhead'' abstraction. For the cases when an indirection (via VTable) is feasible,
I've created the 'IterSource' template, which sticks to this Lumiera Forward Iterator
concept, but provides an opaque frontend, allowing to decouple completely from the
actual implementation. Besides that, over time I've written several standard adapters
for the most common STL containers, plus Map, key and value extractors.

Ichthyostega:: 'Sa 16 Apr 2011 00:20:13 CEST' 

minor change: removed support for post-increment. It doesn't fit with the concept
and caused serious problems in practice. A correct implementation of post-increment
would require a ``deep copy'' of any underlying data structures.

Ichthyostega:: 'Sa 07 Jan 2012 21:49:09 CET' ~<prg@ichthyostega.de>~ 

NOTE: Lumiera Forward Iterators can be made to work together with the
      'range for loop', as introduced with C++11. The preferred solution
      is to define the necessary free functions `begin` and `end` for ADL.
      This is best done on a per implementation base. We consider a generic
      solution not worth the effort

This is to say, these two concepts can be made to work together well. While,
at a conceptual level, they are not really compatible, and build on a
different understanding: The standard for-loop assumes ``a container'',
while our Forward Iterator Concept deals with ``abstract data sources''.
The user needs to understand the fine points of that conceptual difference:

- if you apply the range-`for` construct on a container, you can iterate
  as often as you like. Even if the iterators of that container are
  implemented in compliance with the Lumiera Forward Iterator concept.
- but if you apply the range-`for` construct on _a given iterator_,
  you can do so only once. There is no way to reset that iterator,
  other than obtaining a new one.

See `71167be9c9aaa` for the typical bridge implementation

Ichthyostega:: 'Sa 04 Jul 2015 22:57:51 CEST' ~<prg@ichthyostega.de>~


//endof_comments:


Final
~~~~~
Accepted on developer meeting

    Do 14 Apr 2011 02:52:30 CEST Christian Thaeter


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