Re: Implementing Memorize

["Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net>]

"Lutz Donnerhacke" <lutz@iks-jena.de> wrote in message
news:slrnbpch9n.oa.lutz@taranis.iks-jena.de...
> In order to implement a generic memorize, I tried the following
>   (see <slrnbpajhj.ns.lutz@taranis.iks-jena.de> for the context)
>
>    generic
>       with function Callback (n : Natural) return Natural;
>    function Action (n : Natural) return Natural;
>
>    function Action (n : Natural) return Natural is
>    begin
>       case n is
>          when 0 | 1  => return 1;
>          when others => return Callback(n-1) + Callback(n-2);
>       end case;
>    end Action;
>
>    function Fib_Direct is new Action (Fib_Direct); -- won't compile

I can see why this doesn't compile: you are using Fib_Direct to define
Fib_Direct. We could avoid this bit of circularity by declaring another
function to serve as the generic parameter, then implement this other
function as a simple call to the generic instantiation, as follows:

    function Fib_Indirect(n : Natural) return Natural;

    function Fib_Direct is new Action( Fib_Indirect );

    function Fib_Indirect(n : Natural) return Natural is
    begin
       return Fib_Direct(n);
    end Fib_Indirect;

Of course, for the particular problem you posted, I would not use generics
at all. This implementation works quite nicely:

    function Fib_Direct(n : Natural) return Natural is
    begin
       case n is
          when 0 | 1  => return 1;
          when others => return Fib_Direct(n-1) + Fib_Direct(n-2);
       end case;
    end Fib_Direct;