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From: "Gene" <gene.ressler@gmail.com>
Newsgroups: comp.lang.ada
Subject: Surprise in array concatenation
Date: 31 Aug 2005 20:16:43 -0700
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Date: 2005-08-31T20:16:43-07:00
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Seldom am I surprised by Ada semantics, but today they got me.

type Int_List_Type is array(Integer range <>) of Integer;

procedure Main is

  function F(a : Int_List_Type) return Integer is
    i : Integer;
  begin
    if a'Length <= 1 then
      return a(1);
    else
      -- calculate i s.t. 1 <= i <= a'Last
      -- with some algorithm; details irrelevant
      ...
      return F(a(1..i - 1) & a(i + 1, a'Last));
    end if;
  end Calculate;

  a : Int_List(1..100);

begin
  -- yada yada; fill a with values
  Put(Calculate(F(a)));
end;

The gotcha is that when (and only when) i is determined to be zero, the
concatenation in the recursive call to F produces a starting index of 2
rather than 1.  Of course this causes the base case test to raise an
exception in the next recursive call.  This is in accordance with the
ALRM and GNAT implements it perfectly.

Yes I know how to recode this to use a'First instead of 1 as needed.

Nonetheless the rule seems silly:  When a leading zero-length array is
catenated to another array the result takes on the starting index of
the _second_ operand.  This doesn't make sense to me.  Why not use the
starting index of the zero-length array?

Gene