Re: Selecting Components

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sat, 07 Jan 2006 20:24:20 GMT, Jeffrey R. Carter wrote:

> I guess this is primarily aimed at the language lawyers who hang out here.
> 
> Thinking about how Ada selects components of things, especially of composite 
> objects, I see that almost every case uses dot notation. Package.Declaration, 
> Record.Component, and so on. The one exception seems to be components of arrays, 
> which use parentheses:
> 
> Array (Index)
> Array (Index_1, Index_2)
> Array (High .. Low)

Interesting, but I indeed have once used "." notation for array component
extraction in a small language I designed. The reason was parsing
simplicity.

> I suppose one argument for this is that it's how it's always been done since the 
> earliest days of FORTRAN. Another is that it allows functions to be replaced by 
> arrays and arrays by functions.
> 
> However, many languages, including Pascal, Ada's ancestor, use [] for array 
> indices, so the function/array equivalence may not be that strong.
> 
> What would be the consequences if a language were designed so that all 
> selections used dot notation, including arrays:
> 
> Array.Index
> Array.Index_1, Index_2

Not so. Multidimensional array's index is a tuple. So if "." should be the
name of indexing operation, then it is always:

   Array.<index>

For a tuple you have to define aggregates. Then it becomes:

   Array.(Index_1, Index_2)

> Array.Low .. High

This is a different case. Here "." isn't indexing, but projection, which
yields a subarray, maybe of lesser dimension or differently constrained:

   Array.<index-subset>

Of course ranges should be first class citizens for that.

-----------
In general, I think that Ada should make indices and sets of indices legal
types classes and, of course, expose indexing operations primitive. This
would ease a lot of problems, especially in design of container libraries.
Having this we could associate indexing with either "()", or "." or
whatever operator one wants.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de