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From: "Nick Roberts" <nickroberts@adaos.worldonline.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: Whether to raise exception or End_of_List function
Date: Thu, 22 Nov 2001 03:53:15 -0000
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Date: 2001-11-22T03:53:15+00:00
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"Preben Randhol" <randhol+abuse@pvv.org> wrote in message
news:slrn9vntpg.7nm.randhol+abuse@kiuk0156.chembio.ntnu.no...
> ...
> As I also read
> that exceptions can be a monstrosity if over used.

And sometimes a piece of code can become a monstrosity (in terms of a
proliferation of if-then-elses or whatever) if exceptions are underused.
Indeed, sometimes the neophyte attempting to avoid using the humble 'goto'
can end up in a similar mess.

When should one use exceptions? Exceptionally. The clue is in the name ;-)

--
Best wishes,
Nick Roberts



Let me provide a bit of an example. Supposing we are reading a sequence of
data items from a device. Version A:

   procedure Read_Data is
   begin
      if Status_Bit(Port(1047)) /= 1 then
         Put(Current_Error,"Could not read altitude first byte");
      else
         Read(Port(1050),Alt_Bytes(1));
         if Status_Bit(Port(1047)) /= 1 then
            Put(Current_Error,"Could not read altitude second byte");
         else
            Read(Port(1050),Alt_Bytes(2));
            if Status_Bit(Port(1047)) /= 1 then
               ...

and so on until you run off the right margin!

Version B:

   Read_Port_Error: exception;

   procedure Read_Byte (Byte: out Interfaces.Unsigned_8) is
   begin
      if Status_Bit(Port(1047)) /= 1 then
         raise Read_Port_Error;
      end if;
      Read(Port(1050),Byte);
   end;

   procedue Read_Data is
   begin
      Read_Byte(Alt_Bytes(1));
      Read_Byte(Alt_Bytes(2));
      ...

Much better, eh? If more detailed diagnostics are required, one can always
use Ada.Exceptions and provide extra information in the Message parameter
when calling the Raise_Exception procedure.