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Date: Mon, 3 Jun 2013 18:09:01 -0700 (PDT)
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Subject: Possible for Ada 2020: "Cursors" for Arrays
From: Shark8 <onewingedshark@gmail.com>
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Xref: number.nntp.dca.giganews.com comp.lang.ada:181792
Date: 2013-06-03T18:09:01-07:00
List-Id: <comp.lang.ada>

In a few threads a few people have expressed a desire to be able to initial=
ize Arrays based on some formula usually including the indices. In order to=
 do this we need some sort of "cursor"-type/attribute.

So, let us propose a cursor-attribute so that some array type, T1, defined =
as
  type T1 is Array( 1..10 ) of Integer;
could have a variable initialized as follows:
  V1 : T1:=3D (others =3D> T1'Cursor'Index)
resulting in V1 getting (1,2,3,4,5,6,7,8,9,10);

This doesn't scale to unconstrained arrays, however because given
  type T2 is Array( Positive Range <>) of Integer
  V2 : T2 :=3D (others =3D> others =3D> T1'Cursor'Index)
becomes ambiguous/indeterminate [lacking any constraints to determine the c=
orrect size] -- this could be ameliorated by requiring explicit bounds and =
throwing a compiler error.

Taking a page from the 'Length attribute we could make the attribute take a=
n optional parameter indicating which of the indices we are referring to.

  -- completely forced example.
  Type CBT is Array(1..8,'A'..'H') of Piece;

  -- Transforms (1,A) to 1, (1,B) to 9, [...], (8,H) to 64
  Chess_Board_Transform : CBT :=3D=20
     (others =3D> 8*(Character'Pos(CBT'Cursor(2))-Character'Pos'A') + CBT'C=
ursor(1) );

Note that because we know the types for the indices we should be able to us=
e the cursor as that type for that index... This would also be a good place=
 for implementation of a 'Type attribute, as matching the 'cursor' to that =
particular index would entail interrogating the previously known definition=
 for the proper type anyway.

My question: does this look reasonable, or is it just superfluous?