Re: Inferring array index type from array object

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

Dmitry A. Kazakov wrote:
> On Wed, 23 Jun 2010 00:30:23 -0700 (PDT), Maciej Sobczak wrote:
> 
>> Consider this:
>>
>> S : String := "Hello";
>>
>> It is possible to iterate over all indices of this array with this
>> construct:
>>
>> for I in S'Range loop ...
>>
>> I would like to declare I as a free variable instead and I would
>> expect some symmetry in the language by doing this:
>>
>> I : S'Range := S'First;
> 
> subtype Index_Span is Integer range S'Range;
> I : Index_Span := S'First;
> 
>> I have two questions:
>>
>> 1. What is the standard justification for this assymetry?
> 
> "We don't want to do anything [useful]." (:-))
> 
>> What exactly
>> makes S'Range "work" in a for loop?
> 
> It is hard wired, since ranges are not first class citizens.
> 
>> 2. Is it possible to declare the index variable without hardcoding the
>> index type (that is, to infer it from the array object)?
> 
> No, without improving the type system. E.g. introducing abstract index
> types, and abstract range types (or more general sets of index types), and
> abstract composite types like arrays.

I don't think such large language changes would be necessary. The 
expression S'Range gives the compiler all the information about the type 
and its constraints, so I see no reason why Ada could not be extended 
simply to allow S'Range in a variable declaration, as in the above 
quoted "I : S'Range". The declared variable "I" would have the same 
(sub)type as the loop counter in "for I in S'Range loop ...".

On the other hand, I haven't really felt the need for this extension in 
my programs. Whenever I declare an array type, either the index type has 
no meaning for the application, and I use a subtype of Integer (eg. the 
array just represents a sequence or set of objects), or I also define an 
index type (eg. an object identifier) that is associated with the array 
type. So I have not felt a need for using S'Range in a variabnle 
declaration.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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