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Date: Fri, 07 Dec 2007 11:21:11 +0100
From: Georg Bauhaus <rm.tsoh+bauhaus@maps.futureapps.de>
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Subject: Re: Exceptions
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Date: 2007-12-07T11:21:15+01:00
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Dmitry A. Kazakov wrote:
> On Thu, 06 Dec 2007 15:24:44 -0600, tmoran@acm.org wrote:
> 
>>> Is there any way to force programmers to catch exception that
>>> your function throws?
> 
>> Do you mean force them to write code like:
>> begin
>>   y := func(x);
>> exception
>>   when funcs_important_exception=>null; -- ignore unimportant exception
>> end;


> When "a way to force" reads "exceptions contract", then the answer is no,
> there is no way, alas.
> 

Nevertheless, the function can be declared in a package next
to an exception declaration. This should, I think, alarm users
of the package that the subprograms might raise an occurence.
It should also point to who is responsible for catching.
The exception's name can also be chosen to be more specific than
Constraint_Error.


Eiffel's way is to have names for pre- and post-conditions (etc.).
The model is instructive, I think.


   feature {ANY} -- visible to any client

	func (x: INTEGER): INTEGER
		require
			even: x \\ 2 = 0
			sizeable: x.abs > 2
		do
			Result := raises_if_odd (x)  -- o.K.
			Result := at_random(Result)  -- might raise
		ensure
			-- Result is another integer provided some
			-- random number is good;
			-- otherwise, bad things might happen.
		end

   feature {NONE} -- fully private

	raises_if_odd(x: INTEGER): INTEGER
			-- assume x has been tested
		do
			Result := 42 // (x \\ 2 - 1)
		end

	g: RANDOM
	at_random(x: INTEGER): INTEGER
			-- try to produce another integer from `x`
			-- Forces a dangerous operation for this demo
		do
			g.forth
			Result := x // g.item  -- deliberate danger!
		end