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Subject: anonymous records as tuples
From: Stephen Leake <stephen_leake@stephe-leake.org>
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Xref: reader02.eternal-september.org comp.lang.ada:50939
Date: 2018-03-12T10:32:44-07:00
List-Id: <comp.lang.ada>

I recently had to change a function that returned a single integer into a p=
rocedure that returns two. So my code when from something like:

Next_Shared_Token :=3D Next_Shared_Token - Tree.Count_Terminals (Index);

to:

declare
   Real : Integer;
   Virtual : Integer;
begin
   Tree.Count_Terminals (Index, Real, Virtual);
   if Virtual =3D 0 then
      Next_Shared_Token :=3D Next_Shared_Token - Real;
   end if;
end;

This bothers me mainly because the variables Real, Virtual are not initiali=
zed, and not declared constant, when clearly they should be.

Some other languages handle a similar situation with tuples. For example, i=
n Lua (https://www.lua.org/manual/5.3/manual.html#3.33) this would be:

Real, Virtual =3D Tree.Count_Terminals (Tree_Index);

We could do something similar in Ada with an "anonymous record":

declare
   constant record
     Real : Integer;
     Virtual : Integer;
   end record :=3D Tree.Count_Terminals (Index);
begin=20
   if Virtual =3D 0 then
      Next_Shared_Token :=3D Next_Shared_Token - Real;
   end if;
end;

Perhaps the 'constant' should appear on each component instead; then some c=
ould be not constant.

Count_Terminals would be declared to return an anonymous record:

function Count_Terminals (Tree : in out Tree_Type; Index : in Index_Type)
return record=20
  Real : Integer;
  Virtual : Integer;
end record;

The body of Count_Terminals would return a record aggregate for the result.

Allowing anonymous records in other contexts could easily get messy. For ex=
ample, there could be a larger record containing Real, Virtual:

type Record_1 is record
   A : Float;
   Real : Integer;
   Virtual : Integer;
   B : Float;
end record;

Then an aggregate for that could be:

(A =3D> 1.0,=20
 record Real, Virtual end record =3D> Tree.Count_Terminals (I),=20
 B =3D> 2.0)

Alternately, the function result could be implicitly deconstructed:

(1.0, Count_Terminals (I), 2.0)

Or we could require Record_1 to contain an anonymous record:

type Record_1 is record
   A : Float;
   record
      Real : Integer;
      Virtual : Integer;
   end record;
   B : Float;
end record;

I have no idea how complicated this would be for compiler implementors.

I searched the Ada Issues database (http://ada-auth.org/search-ai12s.html) =
for "anonymous record"; it occurs once (http://www.ada-auth.org/sresult.cgi=
?Search=3D23&Index=3D21&File=3D292&Seq=3D1), where Bob Duff complains that =
Ada has anonymous arrays but not anonymous records.

"tuples" appears many times, mostly in discussions, never in an actual prop=
osal.

-- Stephe