Re: Help with Exceptions for Generic Instantiation

[terminus.umd.edu!brad@umd5.umd.edu (Brad Balfour)]

ae@sei.cmu.edu (Arthur Evans) replies:

>pattis@cs.washington.edu (Richard Pattis) asks about parameterizing a
>generic with an exception.

>Well, if you are missing something it's because it isn't there.  Ada 83
>has no mechanism to do what you want.  Ada 9X will, but that doesn't
>help much right now.

>Art Evans

Well... Not necessarily.
Draft 3.1 of the Ada 9X Mapping Specification from August 1991 does not
have exceptions as generic formal parameters. And, (IMHO) it doesn't look likel
y
that this will change unless people demand it.

However, the Mapping/Revision Team has proposed another mechanism that might
work in this case: Generic formal package parameters.

I have attempted to work out Rich's example with the new (as of v3.1) syntax.

    -- new --
  Generic  -- must be generic to be passed as a generic formal parameter
    -- rest is same as before --
  PACKAGE File_Stream IS
    ...
  END File_Stream;

A simple echo utility might then be written as follows (modified for 9X v3.1):

  WITH Terminal_Utility, String_Utility;        -- His own I/O interfaces
  GENERIC
    Message : IN STRING;
    with package A_Stream is new File_Stream(<>) ;
        -- any instantiation of File_Stream will match this parameter
  PROCEDURE Echo;


  PROCEDURE Echo IS
    PACKAGE SU  RENAMES  String_Utility;
    PACKAGE TTY RENAMES Terminal_Utility;

    S : A_Stream.Stream;
    C : CHARACTER;

  BEGIN
    Open(S, TTY.Prompt(Message));
    BEGIN
      LOOP
        A_Stream.Get(C, S);
        TTY.Announce(C);
      END LOOP;
    EXCEPTION
      WHEN A_Stream.No_More_Data_Error =>  -- now the exception name is visible
            Close(S);
    END;
    TTY.Announce(SU.CR);
  END Echo;

Hope this helps.

------
Brad Balfour
EVB Software Engineering, Inc.
1-800-877-1815
brad@terminus.umd.edu