Re: using `use' : what is the best practice ?

[Anders Wirzenius <anders.wirzenius@netikka.fi>]

milouz <a.michelizza@gmail.com> writes:

> Hi guys,
> Sorry with my annoying questions, but it's not easy to program in Ada
> after 20 years programming in asm / C ;-)
>
> Maybe a not so anecdotic question : when using the 'use' clause ?
>
> My first thought is that using 'use' is not good because it masks the
> package tree.
> For example, if I have 2 packages with procedures sharing the same
> name, example `P1.Foo' and `P2.Foo', my code won't be very readable if
> I only write the short name (`Foo') :
>
>    with P1; use P1;
>    with P2; use P2;
>
>    procedure Main is
>       My_Val : Integer;
>    begin
>       Foo(My_Val);  -- P1 or P2 ?
>    ...
>
> But always avoiding 'use' give some rather obfuscated code, especially
> when using some specific arithmetic operator. For example, playing
> with Address arithmetic without 'use' :
>
>    with System;
>    with System.Storage_Elements;
>
>    procedure Main is
>       A : System.Address;
>       N : Integer;
>    begin
>       A := System.Storage_Elements.To_Address(16#10000#);
>       N := 4;
>  
> System.Storage_Elements."+"(System.Storage_Elements.Storage_Offset(N),
> A);
>    end Main;
>
> The same code with 'use' is more readable :
>
>    with System; use System;
>    with System.Storage_Elements; use System.Storage_Elements;
>
>    procedure Main is
>       A : Address;
>       N : Integer;
>    begin
>       A := To_Address(16#10000#);
>       N := 4;
>       A := A + Storage_Offset(N);
>    end Main;
>
> As I'm very new, I'd like to have your opinion about the way to use
> 'use'.
>
>
>

You may restrict the context of a 'use':

with Text_IO;
procedure Bl is
   A : String := "aaa";
begin
   Text_IO.Put_Line (A);
   declare
      use Text_IO;
      A : character := 'b';
   begin
      Put (A);
   end;
   Put_Line (A);
end;

Try to compile the code. The error message (Gnat) will be:

gnatmake bl.adb 
gcc-4.4 -c bl.adb
bl.adb:12:04: "Put_Line" is not visible
bl.adb:12:04: non-visible declaration at a-textio.ads:263
bl.adb:12:04: non-visible declaration at a-textio.ads:259
gnatmake: "bl.adb" compilation error

-- 
Anders