Re: Building blocks (Was: Design By Contract)

[nobody@REPLAY.COM (Anonymous)]

<5v34m5$pl9$1@trumpet.uni-mannheim.de>
<wihnDCAGPDI0IwlD@treetop.demon.co.uk> <34215E3D.77AE@gsfc.nasa.gov>
<3421E190.49CC@chimu.com> <EGrz21.GoG@world.std.com>
<3423BE13.9C3852A4@munich.netsurf.de>

On Wed, 24 Sep 1997 23:55:02 -0700, Alan E & Carmel J Brain
<aebrain@dynamite.com.au> wrote:
>[...]
> If you want to resume, or retry, or whatever, you need to confine the
> exception to it's own block. A typical example looks something like:
> 
> (Declarations, Constants omitted for simplicity)
> 
> UnTransmitted := true;
> ReTryCounter := 0;
> 
> while UnTransmitted loop
>     
>   if ReTryCounter > MaxRetries then
>      raise LineContinuouslyBusy; end if; -- Line Too busy
>                                          -- Report this upwards
>   Transmission_Attempt_Block :
>   begin
> 
>     Ethernet.TryToTransmit;
>     UnTransmitted := false; -- Successful Transmission
> 
>   exception
>     when Ethernet.LINE_BUSY =>
> 	ReTryCounter := ReTryCounter+1; -- Try again
>                                         -- Note that this 'handles'
>                                         -- the exception
>     when Ethernet.NACK =>
>         Untransmitted := false;         -- Ignore Negative Ack.
>                                         -- Again, exception handled.
>     when others => raise;
> 	-- something unexpected and Nasty, Propagate it out!
>         
>   end Transmission_Attempt_Block;
> 
> end loop;
> 

This doesn't seem typical to me. It differs from typical in

1. Using mixed-case identifiers instead of underline-separated
identifiers (ReTryCounter instead of Re_Try_Counter).

2. Using while instead of loop/exit (ref. Ichbiah, Barnes, & Firth, "Ada
Launch," 1980 Dec 10, videotape, on why while was included in the
language).

3. The design of Ethernet is faulty, since it uses exceptions for
non-exceptional circumstances (line busy, NACK received).

4. Unusual formatting of the if.

If we have to use Ethernet as is, I'd say the following is more typical

Retry_Counter := 0;

Send : loop
   if Retry_Counter > Max_Retries then
      raise Line_Continuously_Busy;
   end if; -- Line too busy; report this upwards

   Try_One : begin
      Ethernet.Try_To_Transmit;

      exit Send; -- Successful transmission
   exception -- Try_One
   when Ethernet.Line_Busy =>
      Retry_Counter := Retry_Counter + 1; -- Try again
   when Ethernet.Nack =>
      exit Send; -- Accept NACK as success
   end Try_One;
end loop Send;

Jeff Carter  PGP:1024/440FBE21
My real e-mail address: ( carter @ innocon . com )
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