Modular programming

So far, our examples have been simple standalone subprograms. Ada is helpful in that regard, since it allows arbitrary declarations in a declarative part. We were thus able to declare our types and variables in the bodies of main procedures.

However, it is easy to see that this is not going to scale up for real-world applications. We need a better way to structure our programs into modular and distinct units.

Ada encourages the separation of programs into multiple packages and sub-packages, providing many tools to a programmer on a quest for a perfectly organized code-base.

Packages

Here is an example of a package declaration in Ada:

    
    
    
        
package Week is

   Mon : constant String := "Monday";
   Tue : constant String := "Tuesday";
   Wed : constant String := "Wednesday";
   Thu : constant String := "Thursday";
   Fri : constant String := "Friday";
   Sat : constant String := "Saturday";
   Sun : constant String := "Sunday";

end Week;

And here is how you use it:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Week;
--  References the Week package, and
--  adds a dependency from Main to Week

procedure Main is
begin
   Put_Line ("First day of the week is "
             & Week.Mon);
end Main;

Packages let you make your code modular, separating your programs into semantically significant units. Additionally the separation of a package's specification from its body (which we will see below) can reduce compilation time.

While the with clause indicates a dependency, you can see in the example above that you still need to prefix the referencing of entities from the Week package by the name of the package. (If we had included a use Week clause, then such a prefix would not have been necessary.)

Accessing entities from a package uses the dot notation, A.B, which is the same notation as the one used to access record fields.

A with clause can only appear in the prelude of a compilation unit (i.e., before the reserved word, such as procedure, that marks the beginning of the unit). It is not allowed anywhere else. This rule is only needed for methodological reasons: the person reading your code should be able to see immediately which units the code depends on.

In other languages

Packages look similar to, but are semantically very different from, header files in C/C++.

  • The first and most important distinction is that packages are a language-level mechanism. This is in contrast to a #include'd header file, which is a functionality of the C preprocessor.

  • An immediate consequence is that the with construct is a semantic inclusion mechanism, not a text inclusion mechanism. Hence, when you with a package, you are saying to the compiler "I'm depending on this semantic unit", and not "include this bunch of text in place here".

  • The effect of a package thus does not vary depending on where it has been withed from. Contrast this with C/C++, where the meaning of the included text depends on the context in which the #include appears.

    This allows compilation/recompilation to be more efficient. It also allows tools like IDEs to have correct information about the semantics of a program. In turn, this allows better tooling in general, and code that is more analyzable, even by humans.

An important benefit of Ada with clauses when compared to #include is that it is stateless. The order of with and use clauses does not matter, and can be changed without side effects.

In the GNAT toolchain

The Ada language standard does not mandate any particular relationship between source files and packages; for example, in theory you can put all your code in one file, or use your own file naming conventions. In practice, however, an implementation will have specific rules. With GNAT, each top-level compilation unit needs to go into a separate file. In the example above, the Week package will be in an .ads file (for Ada specification), and the Main procedure will be in an .adb file (for Ada body).

Using a package

As we have seen above, the with clause indicates a dependency on another package. However, every reference to an entity coming from the Week package had to be prefixed by the full name of the package. It is possible to make every entity of a package visible directly in the current scope, using the use clause.

In fact, we have been using the use clause since almost the beginning of this tutorial.

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
--                ^ Make every entity of the
--                  Ada.Text_IO package
--                  directly visible.
with Week;

procedure Main is
   use Week;
   --  Make every entity of the Week
   --  package directly visible.
begin
   Put_Line ("First day of the week is " & Mon);
end Main;

As you can see in the example above:

  • Put_Line is a subprogram that comes from the Ada.Text_IO package. We can reference it directly because we have used the package at the top of the Main unit.

  • Unlike with clauses, a use clause can be placed either in the prelude, or in any declarative region. In the latter case the use clause will have an effect in its containing lexical scope.

Package body

In the simple example above, the Week package only has declarations and no body. That's not a mistake: in a package specification, which is what is illustrated above, you cannot declare bodies. Those have to be in the package body.

    
    
    
        
package Operations is

   --  Declaration
   function Increment_By
     (I    : Integer;
      Incr : Integer := 0) return Integer;

   function Get_Increment_Value return Integer;

end Operations;

    
        
package body Operations is

   Last_Increment : Integer := 1;

   function Increment_By
     (I    : Integer;
      Incr : Integer := 0) return Integer is
   begin
      if Incr /= 0 then
         Last_Increment := Incr;
      end if;

      return I + Last_Increment;
   end Increment_By;

   function Get_Increment_Value return Integer is
   begin
      return Last_Increment;
   end Get_Increment_Value;

end Operations;

Here we can see that the body of the Increment_By function has to be declared in the body. Coincidentally, introducing a body allows us to put the Last_Increment variable in the body, and make them inaccessible to the user of the Operations package, providing a first form of encapsulation.

This works because entities declared in the body are only visible in the body.

This example shows how Last_Increment is used indirectly:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Operations;

procedure Main is
   use Operations;

   I : Integer := 0;
   R : Integer;

   procedure Display_Update_Values is
      Incr : constant Integer := Get_Increment_Value;
   begin
      Put_Line (Integer'Image (I)
                & " incremented by "
                & Integer'Image (Incr)
                & " is "
                & Integer'Image (R));
      I := R;
   end Display_Update_Values;
begin
   R := Increment_By (I);
   Display_Update_Values;
   R := Increment_By (I);
   Display_Update_Values;

   R := Increment_By (I, 5);
   Display_Update_Values;
   R := Increment_By (I);
   Display_Update_Values;

   R := Increment_By (I, 10);
   Display_Update_Values;
   R := Increment_By (I);
   Display_Update_Values;
end Main;

Child packages

Packages can be used to create hierarchies. We achieve this by using child packages, which extend the functionality of their parent package. One example of a child package that we've been using so far is the Ada.Text_IO package. Here, the parent package is called Ada, while the child package is called Text_IO. In the previous examples, we've been using the Put_Line procedure from the Text_IO child package.

Important

Ada also supports nested packages. However, since they can be more complicated to use, the recommendation is to use child packages instead. Nested packages will be covered in the advanced course.

Let's begin our discussion on child packages by taking our previous Week package:

    
    
    
        
package Week is

   Mon : constant String := "Monday";
   Tue : constant String := "Tuesday";
   Wed : constant String := "Wednesday";
   Thu : constant String := "Thursday";
   Fri : constant String := "Friday";
   Sat : constant String := "Saturday";
   Sun : constant String := "Sunday";

end Week;

If we want to create a child package for Week, we may write:

    
    
    
        
package Week.Child is

   function Get_First_Of_Week return String;

end Week.Child;

Here, Week is the parent package and Child is the child package. This is the corresponding package body of Week.Child:

    
    
    
        
package body Week.Child is

   function Get_First_Of_Week return String is
   begin
      return Mon;
   end Get_First_Of_Week;

end Week.Child;

In the implementation of the Get_First_Of_Week function, we can use the Mon string directly, even though it was declared in the parent package Week. We don't write with Week here because all elements from the specification of the Week package — such as Mon, Tue and so on — are visible in the child package Week.Child.

Now that we've completed the implementation of the Week.Child package, we can use elements from this child package in a subprogram by simply writing with Week.Child. Similarly, if we want to use these elements directly, we write use Week.Child in addition. For example:

    
    
    
        
with Ada.Text_IO; use Ada.Text_IO;
with Week.Child;  use Week.Child;

procedure Main is
begin
   Put_Line ("First day of the week is "
             & Get_First_Of_Week);
end Main;

Child of a child package

So far, we've seen a two-level package hierarchy. But the hierarchy that we can potentially create isn't limited to that. For instance, we could extend the hierarchy of the previous source-code example by declaring a Week.Child.Grandchild package. In this case, Week.Child would be the parent of the Grandchild package. Let's consider this implementation:

    
    
    
        
package Week.Child.Grandchild is

   function Get_Second_Of_Week return String;

end Week.Child.Grandchild;

    
        
package body Week.Child.Grandchild is

   function Get_Second_Of_Week return String is
   begin
      return Tue;
   end Get_Second_Of_Week;

end Week.Child.Grandchild;

We can use this new Grandchild package in our test application in the same way as before: we can reuse the previous test application and adapt the with and use, and the function call. This is the updated code:

    
    
    
        
with Ada.Text_IO;           use Ada.Text_IO;
with Week.Child.Grandchild; use Week.Child.Grandchild;

procedure Main is
begin
   Put_Line ("Second day of the week is "
             & Get_Second_Of_Week);
end Main;

Again, this isn't the limit for the package hierarchy. We could continue to extend the hierarchy of the previous example by implementing a Week.Child.Grandchild.Grand_grandchild package.

Multiple children

So far, we've seen a single child package of a parent package. However, a parent package can also have multiple children. We could extend the example above and implement a Week.Child_2 package. For example:

    
    
    
        
package Week.Child_2 is

   function Get_Last_Of_Week return String;

end Week.Child_2;

Here, Week is still the parent package of the Child package, but it's also the parent of the Child_2 package. In the same way, Child_2 is obviously one of the child packages of Week.

This is the corresponding package body of Week.Child_2:

    
    
    
        
package body Week.Child_2 is

   function Get_Last_Of_Week return String is
   begin
      return Sun;
   end Get_Last_Of_Week;

end Week.Child_2;

We can now reference both children in our test application:

    
    
    
        
with Ada.Text_IO;  use Ada.Text_IO;
with Week.Child;   use Week.Child;
with Week.Child_2; use Week.Child_2;

procedure Main is
begin
   Put_Line ("First day of the week is "
             & Get_First_Of_Week);
   Put_Line ("Last day of the week is "
             & Get_Last_Of_Week);
end Main;

Visibility

In the previous section, we've seen that elements declared in a parent package specification are visible in the child package. This is, however, not the case for elements declared in the package body of a parent package.

Let's consider the package Book and its child Additional_Operations:

    
    
    
        
package Book is

   Title : constant String :=
     "Visible for my children";

   function Get_Title return String;

   function Get_Author return String;

end Book;

    
        
package Book.Additional_Operations is

   function Get_Extended_Title return String;

   function Get_Extended_Author return String;

end Book.Additional_Operations;

This is the body of both packages:

    
    
    
        
package body Book is

   Author : constant String :=
     "Author not visible for my children";

   function Get_Title return String is
   begin
      return Title;
   end Get_Title;

   function Get_Author return String is
   begin
      return Author;
   end Get_Author;

end Book;

    
        
package body Book.Additional_Operations is

   function Get_Extended_Title return String is
   begin
      return "Book Title: " & Title;
   end Get_Extended_Title;

   function Get_Extended_Author return String is
   begin
      --  "Author" string declared in the body
      --  of the Book package is not visible
      --  here. Therefore, we cannot write:
      --
      --  return "Book Author: " & Author;

      return "Book Author: Unknown";
   end Get_Extended_Author;

end Book.Additional_Operations;

In the implementation of the Get_Extended_Title, we're using the Title constant from the parent package Book. However, as indicated in the comments of the Get_Extended_Author function, the Author string — which we declared in the body of the Book package — isn't visible in the Book.Additional_Operations package. Therefore, we cannot use it to implement the Get_Extended_Author function.

We can, however, use the Get_Author function from Book in the implementation of the Get_Extended_Author function to retrieve this string. Likewise, we can use this strategy to implement the Get_Extended_Title function. This is the adapted code:

    
    
    
        
package body Book.Additional_Operations is

   function Get_Extended_Title return String is
   begin
      return "Book Title: " & Get_Title;
   end Get_Extended_Title;

   function Get_Extended_Author return String is
   begin
      return "Book Author: " & Get_Author;
   end Get_Extended_Author;

end Book.Additional_Operations;

This is a simple test application for the packages above:

    
    
    
        
with Ada.Text_IO;                use Ada.Text_IO;
with Book.Additional_Operations; use Book.Additional_Operations;

procedure Main is
begin
   Put_Line (Get_Extended_Title);
   Put_Line (Get_Extended_Author);
end Main;

By declaring elements in the body of a package, we can implement encapsulation in Ada. Those elements will only be visible in the package body, but nowhere else. This isn't, however, the only way to achieve encapsulation in Ada: we'll discuss other approaches in the Privacy chapter.

Renaming

Previously, we've mentioned that subprograms can be renamed. We can rename packages, too. Again, we use the renames keyword for that. The following example renames the Ada.Text_IO package as TIO:

    
    
    
        
with Ada.Text_IO;

procedure Main is
   package TIO renames Ada.Text_IO;
begin
   TIO.Put_Line ("Hello");
end Main;

We can use renaming to improve the readability of our code by using shorter package names. In the example above, we write TIO.Put_Line instead of the longer version (Ada.Text_IO.Put_Line). This approach is especially useful when we don't use packages and want to avoid that the code becomes too verbose.

Note we can also rename subprograms and objects inside packages. For instance, we could have just renamed the Put_Line procedure in the source-code example above:

    
    
    
        
with Ada.Text_IO;

procedure Main is
   procedure Say (Something : String)
     renames Ada.Text_IO.Put_Line;
begin
   Say ("Hello");
end Main;