Re: operation can be dispatching in only one type

["Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>]

On 11/24/2015 02:08 AM, Serge Robyns wrote:
> 
> I didn't know about the position, I honestly believed it had to be the 1st. I don't know where I read that.  Anyway I'm reading now the A-LRM 2012.  I'm using "Self" as a naming convention.

Dispatching is described in ARM 3.9.2(2/3). Note that a function call can
dispatch on its result type.

> I though that packaging and typing were quite independent and hence this is why I merged these two types in one package as they form a coherent assembly.  I also recall some "advertisement" about that "fact".  But it seems not to be that fully true.

What is called a "class" in most languages (other than Ada) involves a type
declaration (a set of values and a set of operations on those values),
encapsulation of the value and its operations, and information hiding of the
implementation of the values. In Ada, the pkg provides encapsulation and
information hiding, independent of the mechanisms for declaring types. To get
the equivalent of a "class" in Ada one must put the type and operation
declarations into a pkg. Pkgs, unlike "classes", are intended to be used for
other things as well.

Note that the Ada construct that most closely matches a "class" is a protected type.

> Like for example creating new specialized versions of the T_Account and per the rules you can only write "type New_T is new Old_T" for tagged types.

I'm not sure what you're saying here, but one can derive a new type from any
type, not just tagged types. This existed in Ada 83:

type George is new Integer [range Low .. High];

The Ada-95 designers built on that to provide type extension:

type George is new Martha with record ...

The only thing you *need* tagged types for in Ada is finalization. They also
provide Object.Operation notation, which is nice, and many types are declared
tagged solely to provide that (the containers are examples of this). Type
extension, combined with a container, also provides a kind of syntactic noise
that allows self-referential types without access types/values.

Other than those uses, with adequate thought before beginning coding, one can
implement anything more clearly without tagged types than with.

-- 
Jeff Carter
"Whatever it is, I'm against it."
Horse Feathers
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