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Date: 13 Dec 91 17:27:22 GMT
From: pattis@beaver.cs.washington.edu  (Richard Pattis)
Subject: Help with Exceptions for Generic Instantiation
Message-ID: <1991Dec13.172722.8432@beaver.cs.washington.edu>
List-Id: <comp.lang.ada>

I am trying to write various packages to access/advance-through a sequence
of characters (in a file, from a terminal, in an array, etc). I will then be
writing genric packages that are instantiated using this information; for
example, a lexical analyzer (or regular expression pattern matcher) taking
input from a file, a terminal, an array).

So far, one example is the following package specification

  PACKAGE File_Stream IS
    Open_Error         : EXCEPTION;
    Reset_Error        : EXCEPTION;
    No_More_Data_Error : EXCEPTION;
    Close_Error        : EXCEPTION;
  
    TYPE Stream IS LIMITED PRIVATE;
  
    PROCEDURE Open  (S : IN OUT Stream;  Info : IN STRING);
    PROCEDURE Reset (S : IN OUT Stream);
    PROCEDURE Get   (C : OUT CHARACTER; S : IN OUT Stream);
    PROCEDURE Close (S : IN OUT Stream);
  PRIVATE
    TYPE Stream IS NEW Text_IO.File_Type;
  END File_Stream;


A simple echo utility might then be written as follows:

  
  WITH Terminal_Utility, String_Utility;	-- My own I/O interfaces
  GENERIC
    Message : IN STRING;
    TYPE Stream IS LIMITED PRIVATE;
    WITH PROCEDURE Open  (S : IN OUT Stream;  Info : IN STRING);
    WITH PROCEDURE Reset (S : IN OUT Stream);
    WITH PROCEDURE Get   (C : OUT CHARACTER; S : IN OUT Stream);
    WITH PROCEDURE Close (S : IN OUT Stream);
  PROCEDURE Echo;
  

  PROCEDURE Echo IS
    PACKAGE SU  RENAMES  String_Utility;
    PACKAGE TTY RENAMES Terminal_Utility;
  
    S : Stream;
    C : CHARACTER;
  
  BEGIN
    Open(S, TTY.Prompt(Message));
    BEGIN
      LOOP
        Get(C, S);
        TTY.Announce(C);
      END LOOP;
    EXCEPTION
      WHEN OTHERS => Close(S);		-- No_More_Data_Error
    END;
    TTY.Announce(SU.CR);
  END Echo;


The problem is that real programs/packages (beyond the complexity of Echo)
will need to individually handle various exceptions by name, yet I cannot find
any way to instantiate generics using exceptions. Am I overlooking something
obvious or is there something subtle that will help solve my problem?

I realize that I could supply generic predicates to "replace" the exceptions,
but my question still stands - is there a reasonable way to use exceptions.


------------------------------------------------------------------------------
  Richard E. Pattis			"Programming languages are like
  Department of Computer Science	 pizzas - they come in only "too"
    and Engineering			 sizes: too big and too small."
  University of Washington
  Seattle, WA  98195			"When debugging, a novice programmer
  pattis@cs.washington.edu		 inserts corrective code; an
  voice: (206) 685-1218			 experienced programmer removes
  fax  : (206) 543-2969			 defective code."
-- 
------------------------------------------------------------------------------
  Richard E. Pattis			"Programming languages are like
  Department of Computer Science	 pizzas - they come in only "too"
    and Engineering			 sizes: too big and too small."