Re: Help needed for ada package

[Georg Bauhaus <bauhaus@futureapps.de>]

strictly_mk@hotmail.com wrote:
> is there a more appropriate type that
> wont upset the rest of the program?

Use one of Ada's formal types. That is, specify what
the generic needs as a type using the syntax for formal
types. Examples

generic
  type T is range <>;
package Pack ...

In this case the formal type T says, if you want to make
an instance of this generic package, you have to provide
a signed integer type as the (actual) type to be used in
the created instance, as in

  package Foo is new Pack (T => Integer);

Integer is a signed integer type, as are Natural, or
Long_Integer. When you instantiate Pack, you can choose the type
to be used. When you write the generic package, you only
specify some assumptions about the possible types.
In this case, "range <>" specifies "some signed integer type".

Given these possible assumptions about T, you can use
T in expressions, without knowing what T is going to be in
some instance of the generic package. Like T'First (the first of
the values in T's range). You can add values of T because
T denotes some integer type, no matter which one actually,
later.

Another example is 

generic
  type T is (<>);
package Pack2 ...

In this case formal type T says, if you want to make
an instance of this generic package, you have to provide
a discrete type as the (actual) type to be used in
the created instance. Discrete types are either integer
types or enumeration types. Just no real types.

So if you had, somewhere

  type Family is (Jack, Jakub, Florentina, Marge, Bart);

i.e. an enumeration type, you could say

  package Foo2 is new Pack2 (T => Family);

Inside Pack2 (the generic) you can use T just like you
use any enumeration type. For example, you can say T'First,
but you cannot add values, because this doesn't work for
enumeration types. When you write the package, you know you
can assume T is denoting an enumeration type, but not which one.
Because that is decided when someone, possibly you, instantiate
Pack2.

Last, when you have

generic
  type ID is new Long_Integer;
package PoP ...

the choice of types to be used when instantiating is pretty fixed.
Only types derived from Long_Integer (includes Long_Integer) or
subtypes thereof can be filled in when you write

  package My_PoP_Instance is new PoP(ID => ...);