Selecting Components

Selecting Components

[Jeffrey R. Carter]

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)

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
Array.Low .. High

?

-- 
Jeff Carter
"Spam! Spam! Spam! Spam! Spam! Spam! Spam! Spam!"
Monty Python's Flying Circus
53

Re: Selecting Components

[(see below)]

On 7/1/06 20:24, in article
UrVvf.339$Hd4.149@newsread1.news.pas.earthlink.net, "Jeffrey R. Carter"
<spam@spam.com> wrote:

> 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
> Array.Low .. High

The consequence would be ambiguous arithmetic expressions.

Consider:

Array.Index + 1

is this (Array.Index)+1 or Array.(Index+1)?
-- 
Bill Findlay
<surname><forename> chez blueyonder.co.uk

Re: Selecting Components

[Jeffrey R. Carter]

(see below) wrote:
> 
> Consider:
> 
> Array.Index + 1
> 
> is this (Array.Index)+1 or Array.(Index+1)?

Certainly that would have to be defined.

-- 
Jeff Carter
"I soiled my armor, I was so scared."
Monty Python & the Holy Grail
71

Re: Selecting Components

[Dmitry A. Kazakov]

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

Re: Selecting Components

[Keith Thompson]

"(see below)" <yaldnif.b@blueyonder.co.uk> writes:
> On 7/1/06 20:24, in article
> UrVvf.339$Hd4.149@newsread1.news.pas.earthlink.net, "Jeffrey R. Carter"
> <spam@spam.com> wrote:
>
>> 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
>> Array.Low .. High
>
> The consequence would be ambiguous arithmetic expressions.
>
> Consider:
>
> Array.Index + 1
>
> is this (Array.Index)+1 or Array.(Index+1)?

Presumably it would be (Array.Index) + 1; there's no reason for "."
to have a different precedence for arrays than for records.

There's a visual ambiguity: does Foo.Bar mean the Bar'th element of
the array Foo, or the component Bar of the record Foo?  This is of
course resolved by the type of Foo (unless Foo is an overloaded
function), but what's easy for the compiler may be difficult for a
human reader.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
We must do something.  This is something.  Therefore, we must do this.

Re: Selecting Components

[Georg Bauhaus]

On Sat, 2006-01-07 at 20:24 +0000, Jeffrey R. Carter wrote:


> 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
> Array.Low .. High

Interesting.
Is "Index_1, Index_2" and expression, or an identifier
as in Pack.Op, Rec.Comp, etc.?

with Index;
...
declare
  Index: ...
begin
  for k in a'range loop
    Result := Result + a.Index.k;
  end loop;
end;

For the record, in Eiffel you'd write array @ expr,
Haskell uses array !! expr.
Eiffel uses '.' for object.feature, not as an operator.
Haskell uses '.' for function composition, too, if surrounded
by white space.

Re: Selecting Components

[Jeffrey R. Carter]

Georg Bauhaus wrote:
> 
> Interesting.
> Is "Index_1, Index_2" and expression, or an identifier
> as in Pack.Op, Rec.Comp, etc.?

Array.Index_1, Index_2 would be equivalent to Ada's Array (Index_1, Index_2), 
where "Array" represents a 2D array object.

-- 
Jeff Carter
"All citizens will be required to change their underwear
every half hour. Underwear will be worn on the outside,
so we can check."
Bananas
29

Re: Selecting Components

[Stephen Leake]

"Jeffrey R. Carter" <spam@spam.com> writes:

> 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
> Array.Low .. High

Mass confusion.

No new software is written.

All trains stop running.

No food is delivered.

All die.

Oh, the embarrassment :).

-- 
-- Stephe

Re: Selecting Components

[Jeffrey R. Carter]

Stephen Leake wrote:


> Mass confusion.
> 
> No new software is written.
> 
> All trains stop running.
> 
> No food is delivered.
> 
> All die.
> 
> Oh, the embarrassment :).

You say that as if it were a bad thing.

-- 
Jeff Carter
"Run away! Run away!"
Monty Python and the Holy Grail
58