Split Program Structure module into chapters

courses/fundamentals_of_ada/130_program_structure/01-introduction.rst

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+==============
+Introduction
+==============
+
+--------------
+Introduction
+--------------
+
+* Moving to "bigger" issues of overall program composition
+* How to compose programs out of program units
+* How to control object lifetimes
+* How to define subsystems
+

courses/fundamentals_of_ada/130_program_structure/02-building_a_system.rst

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+===================
+Building a System
+===================
+
+-------------------
+What Is a System?
+-------------------
+
+* Also called Application or Program or ...
+* Collection of :dfn:`library units`
+
+   - Which are a collection of packages or subprograms
+
+----------------------
+Library Units Review
+----------------------
+
+* Those units not nested within another program unit
+* Candidates
+
+   - Subprograms
+   - Packages
+   - Generic Units
+   - Generic Instantiations
+   - Renamings
+
+* Dependencies between library units via :ada:`with` clauses
+
+   - What happens when two units need to depend on each other?
+

courses/fundamentals_of_ada/130_program_structure/03-circular_dependencies.rst

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+=========================
+Circular Dependencies
+=========================
+
+------------------------------
+Handling Cyclic Dependencies
+------------------------------
+
+* Elaboration must be linear
+* Package declarations cannot depend on each other
+
+   - No linear order is possible
+
+* Which package elaborates first?
+
+.. image:: cyclic_dependencies.png
+   :width: 50%
+   :align: center
+
+--------------------------------------
+Body-Level Cross Dependencies Are OK
+--------------------------------------
+
+* The bodies only depend on other packages' declarations
+* The declarations are already elaborated by the time the bodies are elaborated
+
+.. image:: mutual_dependencies.svg
+   :width: 70%
+   :align: center
+
+--------------------------
+Resulting Design Problem
+--------------------------
+
+* Good design dictates that conceptually distinct types appear in distinct package declarations
+
+   - Separation of concerns
+   - High level of *cohesion*
+
+* Not possible if they depend on each other
+* One solution is to combine them in one package, even though conceptually distinct
+
+   - Poor software engineering
+   - May be only choice, depending on language version
+
+     - Best choice would be to implement both parts in a new package
+
+-------------------------------------------
+Circular Dependency in Package Declaration
+-------------------------------------------
+
+.. code:: Ada
+
+   with Department; --  Circular dependency
+   package Personnel is
+     type Employee is private;
+     procedure Assign (This : in Employee;
+                        To : in out Department.Section);
+   private
+     type Employee is record
+       Assigned_To : Department.Section;
+     end record;
+   end Personnel;
+
+   with Personnel; --  Circular dependency
+   package Department is
+     type Section is private;
+     procedure Choose_Manager (This : in out Section;
+                                Who : in Personnel.Employee);
+   [...]
+   end Department;
+
+------------------------
+`limited with` Clauses
+------------------------
+
+* Solve the cyclic declaration dependency problem
+
+   - Controlled cycles are now permitted
+
+* Provide a :dfn:`limited view` of the specified package
+
+   - Only type names are visible (including in nested packages)
+   - Types are viewed as :dfn:`incomplete types`
+
+* Normal view
+
+   .. code:: Ada
+
+      package Personnel is
+        type Employee is private;
+        procedure Assign ...
+      private
+        type Employee is ...
+      end Personnel;
+
+* Implied limited view
+
+   .. code:: Ada
+
+      package Personnel is
+        type Employee;
+      end Personnel;
+
+.. container:: speakernote
+
+   Note that the names of nested packages are of course visible, otherwise we could not reference the names of types declared within them.
+
+..
+  language_version 2005
+
+------------------------
+Using Incomplete Types
+------------------------
+
+* A type is :dfn:`incomplete` when its representation is completely unknown
+
+   - Address can still be manipulated through an :ada:`access`
+   - Can be a formal parameter or function result's type
+
+      + Subprogram's completion needs the complete type
+      + Actual parameter needs the complete type
+
+   - Can be a generic formal type parameters
+   - If :ada:`tagged`, may also use `'Class`
+
+.. code:: Ada
+
+   type T;
+
+* Can be declared in a **private** part of a package
+
+  - And completed in its body
+  - Used to implement opaque pointers
+
+* Thus typically involves some advanced features
+
+--------------------------------------
+Legal Package Declaration Dependency
+--------------------------------------
+
+.. code:: Ada
+
+   with Department;
+   package Personnel is
+     type Employee is private;
+     procedure Assign (This : in Employee;
+                        To : in out Department.Section);
+   private
+     type Employee is record
+       Assigned_To : Department.Section;
+     end record;
+   end Personnel;
+
+   limited with Personnel;
+   package Department is
+     type Section is private;
+     procedure Choose_Manager (This : in out Section;
+                                Who : in Personnel.Employee);
+   private
+     type Section is record
+       Manager : access Personnel.Employee;
+     end record;
+   end Department;
+
+..
+  language_version 2005
+
+----------------------------------------
+Full `with` Clause on the Package Body
+----------------------------------------
+
+* Even though declaration has a :ada:`limited with` clause
+* Typically necessary since body does the work
+
+   - Dereferencing, etc.
+
+* Usual semantics from then on
+
+   .. code:: Ada
+
+      limited with Personnel;
+      package Department is
+      ...
+      end Department;
+
+      with Personnel; -- normal view in body
+      package body Department is
+      ...
+      end Department;
+
+..
+  language_version 2005
+