Re: Parent/child dependencies and internal library units

[mockturtle <framefritti@gmail.com>]

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;