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From: shang@corp.mot.com (David L. Shang)
Subject: Re: What Should An Exception Handling Do? -- Clarification of rules
Reply-To: shang@corp.mot.com
Organization: MOTOROLA 
Date: Tue, 2 Apr 1996 15:47:22 GMT
Message-ID: <1996Apr2.154722.29777@schbbs.mot.com>
References: <vwjg2ao6hqp.fsf@osfb.aber.ac.uk>
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In article <vwjg2ao6hqp.fsf@osfb.aber.ac.uk> pcg@aber.ac.uk (Piercarlo Grandi)  
writes:
> Therefore exception handling reduces to simply providing a mechanism by
> which the *author* of a code segment provides a mechanism by which the
> *user* of that code segment can specify additional cases, as for example
> in:
> 
> 	float sqrt(float n)
> 	{
> 	  if
> 	     n > 0 -> ...;
> 	  [] n == 0 -> return 0;
> 	  [] n < 0 -> return sqrt_negative(n);
> 	  fi
> 	}
> 
> where the only difficulty is that sqrt_negative must be dynamically
> scoped instead of statically scoped, for it must be redefinable by the
> *user* of the procedure.
> 
> This again means that termination is the only possible outcome for this
> case too.
> 

Termination? When you execute:

	return sqrt_negative(n)

first,  you call "sqrt_negative" to get the result, then you do return
to terminate. It is not the case that you first terminate then you call
"sqrt_negative". Once your terminate, you can never go back.

If I write code like:

 	float sqrt(float n)
 	{
 	  if
 	     n > 0 -> ...;
 	  [] n == 0 -> return 0;
 	  [] n < 0 -> { n1= sqrt_negative(n); return n1; }
 	  fi
 	}

Is the program terminated at the point of calling "sqrt_negative"?
This embedded exception handler exactly provides a resumption
semantics, which, according to your previous analysis, is the
only case that exception handling makes any sense(?).


David Shang