Why these "Drop" parameters? (was: variable lenght strings)

["Björn Persson" <spam-away@nowhere.nil>]

Martin Dowie wrote:

> Being added in Ada2005, see Ada.Text_IO.Unbounded_IO in
> http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00301.TXT?rev=1.12

Reading that file I see these declarations:

procedure Set_Bounded_String
    (Target :    out Bounded_String;
     Source : in     String;
     Drop   : in     Truncation := Error);

function Bounded_Slice
    (Source : in Bounded_String;
     Low    : in Positive;
     High   : in Natural;
     Drop   : in Truncation := Error)
        return Bounded_String;

procedure Bounded_Slice
    (Source : in     Bounded_String;
     Target :    out Bounded_String;
     Low    : in     Positive;
     High   : in     Natural;
     Drop   : in     Truncation := Error);

procedure Set_Unbounded_String
    (Target :    out Unbounded_String;
     Source : in     String;
     Drop   : in     Truncation := Error);

function Unbounded_Slice
    (Source : in Unbounded_String;
     Low    : in Positive;
     High   : in Natural;
     Drop   : in Truncation := Error)
        return Unbounded_String;

procedure Unbounded_Slice
    (Source : in     Unbounded_String;
     Target :    out Unbounded_String;
     Low    : in     Positive;
     High   : in     Natural;
     Drop   : in     Truncation := Error);

For Set_Bounded_String it is quite obvious that Drop specifies what 
should happen if Source is longer than the maximum length of Target, but 
can anyone explain what all the other "Drop" parameters are for?

In the case of Bounded_Slice, shouldn't a slice of Source always fit in 
Target? Aren't they the same type, with the same max length?

Unbounded strings are supposed to be unbounded. Their only limit should 
be the available memory, no? So, are the "Drop" parameters there meant 
as a way to avoid Storage_Error?

-- 
Björn Persson                              PGP key A88682FD
                    omb jor ers @sv ge.
                    r o.b n.p son eri nu