Re: Position of "use"

[anon@att.net]

Using a global "use_clause" can sometime cause the compiler to hide all 
routines of one or more libraries. It is better to place most use_clause 
in a local routine or within the declare section of a block_statement.

--
--  Example File: Test_2.adb
--
--  Showing where compiler could hide visibility access to packages.
--
with Ada.Text_IO ;  --  There are a number of routines that are 
with GNAT.IO ;      --  common in both packages so, placement of
                    --  the use_clause is critical, if used.

procedure body Test_2 is 

--  Both Ada.Text_IO and GNAT.IO packages must be access by using 
--  "selected_component" at this point

  procedure Test_2b is 
    use GNAT.IO ; --  Grants global visibility to package GNAT.IO. but
                  --  limits Ada.Text_IO to a "selected_component" 
                  --  access
    begin
      null ;
    end ;

  --  Allow all following routines will have visibility to 
  --  Ada.Text_IO could also cause visibility problems with other 
  --  packages.
  use  Ada.Text_IO ; 

  procedure Test_2a is 
      use GNAT.IO ; --  Will cause the compiler to hide packages, 
                    --  so in this example any reference must be by 
                    --  "selected_component" for both Ada.Text_IO
                    --  and GNAT.IO packages
    begin
      null ;
    end ;

begin
  null ;
end ;


For Ada compilers, where you can have more than one compilation_unit 
within one file (RM 10.1 -- 10.1.1), it allows all preceding 
compilation_unit to have access the same library with only one 
"with_clause" without sharing global visibility to that package.

Note: GNAT limits the compilation_unit to one per file.  But GNAT still 
allows this type of program design. Putting the "with_clause" in the 
specification file (.ads) allows access for the corresponding body file 
(.adb) in some cases without having the "with_clause" in the body file.


--
--  Example of a multiple compilation_unit File: Test_Lib.ada
--
--  Showing different access/visibility to package Text_IO

--
--  compilation_unit 1 : Access to Text_IO is forbidden in this 
--                       package
package Test_Init is
--  ...
end Test_Init ;
--
with Text_IO ;  -- Access is Allowed
--  use Text_IO ;  --  Visibility would be global to all
--                 --  specification and body units if this 
--                 --  statement is used.

--
--  compilation_unit 2 : Limits visibility to Text_IO to a 
--                       "selected_component", unless the 
--                       global "use_clause" is used.
--
--              example :  Text_IO.Col 
--
package Test_55 is
  procedure Init ;
--  ...
end Test_55 ;
--
--
--  compilation_unit 3 : Visibility of package depend upon 
--                       placement of the "use_clause"
--
package body Test_55 is

--  use Text_IO ;  -- Visibility is global within this package body

  procedure Init is
      use Text_IO ;  --  Visibility is local to procedure
    begin
      Set_Col ( 5 ) ; 
      Set_Line ( 2 ) ; 
    end Init ;

  procedure Error is
    begin
      declare
        use Text_IO ;  --  Visibility is local to this block statement
      begin
       Put ( "Error" ) ;
      end ;
    end Error ;
--    ...
end Test_55 ;
--

--
--  compilation_unit 4 : Limits visibility to Text_IO to a 
--                       "selected_component", unless the 
--                       global "use_clause" is used.
--
--              example :  Text_IO.New_Line ;
--
package Test_Cal is
--    ...
end Test_Cal ;



In <lp86r0$ssl$1@speranza.aioe.org>, Victor Porton <porton@narod.ru> writes:
>What is the difference between
>
>with X; use X;
>package Y is
>end Y;
>
>and
>
>with X;
>package Y is
>   use X;
>end Y;
>
>?
>
>-- 
>Victor Porton - http://portonvictor.org