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Date: Fri, 11 Jan 2019 13:46:22 -0800 (PST)
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Subject: 
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Xref: reader01.eternal-september.org comp.lang.ada:55265
Date: 2019-01-11T13:46:22-08:00
List-Id: <comp.lang.ada>

I am working my way through Barnes' excellent Ada book. This is a code samp=
le for deep comparison of linked lists from section 11.7:


type Cell is
  record
    Next: access Cell;
    Value: Integer;
  end record;

function "=3D" (L, R: access Cell) return Boolean is
begin
  if L =3D null or R =3D null then    -- universal =3D
    return L =3D R;                 -- universal =3D (Line A)
  elsif L.Value =3D R.Value then
    return L.Next =3D R.Next;       -- recurses OK (Line B)
  else
    return False;
  end if;
end "=3D";


I can't seem to wrap my head around why in Line A operator "=3D" of the uni=
versal_access type is called (because of the preference rule), on Line B, h=
owever, the user-defined operator "=3D" is called (which makes recursion po=
ssible in the first place), this time with no preference for operator "=3D"=
 of universal_access.

Both L and R, as well as L.Next and R.Next are of the same anonymous type "=
access Cell". Why the difference in "dispatching"? Does it have to do with =
L and R being access parameters? If so, what is the rule there?

I did my best to find anything in the AARM, especially section 4.5.2, but c=
ould not make any sense of it.

Cheers.