Re: Design problem: generic package vs tagged subtype

[Marek Janukowicz <marek@janukowicz.net>]

Simon Wright wrote:
>> For Model I use a tagged type wrapped into generic package:
>> generic
>>    Tbl_Name : String;
>>    type Attribute_Type is (<>);
>>    type Attribute_Type_Array is array (Attribute_Type range <>) of
> 
> Does this have to be an unconstrained array? If it wasn't, you wouldn't
> need to worry about whether Attribute_Types covered the range of
> Attribute_Type.

The only reason it was an unconstrained array is that my Ada skills are 
extremely poor and I didn't know it would work this way :) I'm a bit lost in 
this subject in general: could anyone tell me the exact difference between:
- array (Attribute_Type)
- array (Attribute_Type range <>)
- array (Attribute_Type'Range)


>> Attribute_Type_Type;
>>    Attribute_Types : Attribute_Type_Array;
>> package Dar.Models.Base is
>>             
>>    type Model is abstract tagged limited record
> 
>    type Model is new Abstract_Base with ...
> 
>> ...
>>
>> For associations I also use a generic package:
>> generic
>>    with package Source_Model is new Dar.Models.Base(<>);
>>    with package Target_Model is new Dar.Models.Base(<>);
>>    Foreign_Key : String;
>> package Dar.Models.Associations is
>>    function Find
>> ...
> 
> ... then the association can be between objects of type
> Abstract_Base_Class'Class (though I don't see how you'd express the
> actual relationship, not clear how Foreign_Key enters the picture).

That's how I really have it implemented, didn't want to go that much into 
the details. Generally, I need to figure out what's really my problem here 
and then ask again :)

>> 2. I have a type Attribute_Type_Type used to specify types of attributes
>> (Integer, Date, String etc.). I then have a variant record:
>>   type Attribute_Value( Attr_Type : Attribute_Type_Type ) is
>>       record
>>          case Attr_Type is
>>             when Attr_Date =>
>>                Date_Value : APQ_Date;
>>             when Attr_Time =>
>>                Time_Value : APQ_Time;
>>             ...
>> Is there any way to use builtin types here instead of Attr_Date,
>> Attr_Integer, etc? Note that I need to specify type of each attribute in
>> one array.
> 
> I don't believe so.
> 
> As a side note I really dislike the name Attribute_Type_Type. It seems
> to me that what you've called Attribute_Type is much more like a Name
> than a Type.

Yeah, you might be right on that. I still need to find out what a good 
naming convention would be. I wouldn't like to use:

type Attribute_Name_Type is (Id, Username, ...)

but if I call the type Attribute_Name I'll have problems naming variables of 
that type (because Attribute_Name seems to be the best name for them as 
well). 

That's one of the things I dislike in Ada - everything (types, variables, 
packages, operations) follows exactly the same naming convention and doesn't 
use any special character/whatever to distinguish between them, so you run 
out of good names in no time...

-- 
Marek Janukowicz

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