Parent/child dependencies and internal library units

Parent/child dependencies and internal library units

[Victor Porton]

I noticed that child units depend on their parents.

As such making parent dependent on a child makes a dependency loop.

Before noting this feature of Ada language, I thought parent units could be 
implemented based on their child units, but that does not work, because 
making parents dependent on children is a circular dependency.

So, my question:

What is a good way to create "internal" or "implementation" units for a 
unit? Should I create a *.Internal.* hierarchy of packages?

-- 
Victor Porton - http://portonvictor.org

Re: Parent/child dependencies and internal library units

[mockturtle]

On Wednesday, July 23, 2014 6:20:27 PM UTC+2, Victor Porton wrote:
> I noticed that child units depend on their parents.
> 
> As such making parent dependent on a child makes a dependency loop.
> 
> Before noting this feature of Ada language, I thought parent units could be 
> implemented based on their child units, but that does not work, because 
> making parents dependent on children is a circular dependency.
> 
> So, my question:
> 
> What is a good way to create "internal" or "implementation" units for a 
> unit? Should I create a *.Internal.* hierarchy of packages?
> 


My personal solution to this kind of problem is to make the implementation units "private sibling" of the main unit.  Also note that the body of the parent (if I am not mistaken) can use a child.

Let me do an example (maybe not a perfect one, I am inventing it "on the spot").  Suppose you have a program (call it "calculator") that uses some kind of expressions.  In particular, you want to parse expressions and evaluate the result of parsing.  Say the parser needs a "symbol table" package to keep track of variables and stuff, and a "scanner" to split the input into tokens.

So, you could have a tree of packages like

package Calculator is ...       
   -- The "root" of the whole program
end Calculator;

package Calculator.Expressions  is 
   -- The "root" of the part that handles expressions
   -- Depending on your code, it could  be empty 
   -- and just provide a root for its subtree.  It is
   -- more likely that it will have a "Expression" type
   -- that represents the internal version of an expression
end Calculator.Expressions;

package Calculator.Expressions.Parsing is 
   --  This package exports resources (types, procedures...)
   --  for parsing expressions.
end Calculator.Expressions.Parsing;

private package Calculator.Expressions.Scanner is 
   --  Implements a scanner.  Note the "private" keyword
   --  None outside the Calculator.Expressions subtree can use this
end Calculator.Expressions.Scanner;

private package Calculator.Expressions.Symbol_Tables is 
   --  Implements the symbol table
end Calculator.Expressions.Symbol_Tables;

Re: Parent/child dependencies and internal library units

[Simon Wright]

Victor Porton <porton@narod.ru> writes:

> I noticed that child units depend on their parents.
>
> As such making parent dependent on a child makes a dependency loop.
>
> Before noting this feature of Ada language, I thought parent units could be 
> implemented based on their child units, but that does not work, because 
> making parents dependent on children is a circular dependency.
>
> So, my question:
>
> What is a good way to create "internal" or "implementation" units for a 
> unit? Should I create a *.Internal.* hierarchy of packages?

I dare say there are uses which could make this not work (possibly
involving initialisation of objects in specs via function calls, leading
to access-before-elaboration) but you can make a parent body depend on a
child spec.

If the implementation is meant to be hidden from users outside the
parent, consider using private child packages.

   package Hierarchy is
      procedure Tell;
   end Hierarchy;

   private package Hierarchy.Internal is
      procedure Tell;
   end Hierarchy.Internal;

   with Ada.Text_IO;
   package body Hierarchy.Internal is
      procedure Tell is
      begin
         Ada.Text_IO.Put_Line ("Hierarchy.Internal");
      end Tell;
   end Hierarchy.Internal;

   with Ada.Text_IO;
   with Hierarchy.Internal;
   package body Hierarchy is
      procedure Tell is
      begin
         Ada.Text_IO.Put_Line ("Hierarchy");
         Internal.Tell;
      end Tell;
   end Hierarchy;

   procedure Hierarchy.Main is
   begin
      Tell;  -- this is Hierarchy.Tell
   end Hierarchy.Main;

Re: Parent/child dependencies and internal library units

[Adam Beneschan]

On Wednesday, July 23, 2014 9:20:27 AM UTC-7, Victor Porton wrote:
> I noticed that child units depend on their parents. 
> 
> As such making parent dependent on a child makes a dependency loop.
> 
> Before noting this feature of Ada language, I thought parent units could be 
> implemented based on their child units, but that does not work, because 
> making parents dependent on children is a circular dependency.

When you talk about parent units being "implemented" based on their child units, are you talking about the *body* of a parent unit (which is where "implementation" normally lives)?  There's no problem with making the body of a parent unit depend on a child unit.  A child unit depends on its parent's *specification* but not on its body, so having the body of the parent depend on the child does not create a loop.

It might help to give a short example of what you want to write (even if it's not legal Ada because of a circular dependency).  As it is, I'm not clear on what you're trying to do.

                                -- Adam

Re: Parent/child dependencies and internal library units

[Stefan.Lucks]

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On Wed, 23 Jul 2014, Victor Porton wrote:

> What is a good way to create "internal" or "implementation" units for a
> unit? Should I create a *.Internal.* hierarchy of packages?

I am not sure if it is a good way, but I usually define a project-specific 
"super package", often just an empty package. Then I define both internal 
and external packages as children of the super package.


------  I  love  the  taste  of  Cryptanalysis  in  the morning!  ------
     <http://www.uni-weimar.de/cms/medien/mediensicherheit/home.html>
--Stefan.Lucks (at) uni-weimar.de, Bauhaus-Universität Weimar, Germany--

Re: Parent/child dependencies and internal library units

[Robert A Duff]

Victor Porton <porton@narod.ru> writes:

> I noticed that child units depend on their parents.
>
> As such making parent dependent on a child makes a dependency loop.
>
> Before noting this feature of Ada language, I thought parent units could be 
> implemented based on their child units, but that does not work, because 
> making parents dependent on children is a circular dependency.
>
> So, my question:
>
> What is a good way to create "internal" or "implementation" units for a 
> unit? Should I create a *.Internal.* hierarchy of packages?

Cyclic dependences are allowed if you say "limited with".
And as others have pointed out, a package body can "with"
its children.

- Bob