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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Help with Exceptions!
Date: 1996/05/10
Message-ID: <Dr7J1p.18G@world.std.com>#1/1
X-Deja-AN: 154182045
references: <4mmimq$s4r@hatathli.csulb.edu>
 <4mqio5$a8b@news.sanders.lockheed.com> <4mt0um$dgu@hearst.cac.psu.edu>
 <JSA.96May10151348@organon.com>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-05-10T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <JSA.96May10151348@organon.com>,
Jon S Anthony <jsa@organon.com> wrote:
>This is a nice trick which allows you to approximate Eiffel style
>exception capabilities.

I don't like Eiffel's exception capabilities.  The "retry" feature is
just a loop implemented with a goto (i.e. "retry" is really a backward
goto).  I much prefer the Ada style, where you explicitly code a loop
statement, and put a block-statement-with-exception-handler inside that.

Eiffel has some nice features for asserting things about loops (loop
invariants, and loop variants -- the latter helps prove that the loop
terminates in a finite number of steps).  But the retry construct seems
to by-pass all that -- how does one prove that a retry will ever
terminate?

Of course, the example below is *intentionally* an infinite loop, which
is fine.  If the user never types the right answer, it loops forever (or
recurs forever, until stack is exhausted, unless the compiler is clever
enough to turn the tail recursion into a jump).  (Is it "recurs" or
"recurses"?.  ;-) )

Whether or not this is user-friendly is a different question.

>> function Response (Prompt : String) return integer is
>> 
>>    line : ...
>>    Size : ...
>> 
>> begin
>>    Put (Prompt);
>>    Get_Line (...;
>>    return ...
>> exception
>>    when others =>
>>       Put_Line (...;  -- error message
>>       return Response(Prompt);
>> end Response;

- Bob