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From: fst@doc.ic.ac.uk (Frank Taylor)
Newsgroups: comp.lang.functional
Subject: Re: Implementing Error Values
Message-ID: <FST.90Nov23132540@achilles.doc.ic.ac.uk>
Date: 23 Nov 90 13:25:40 GMT
References: <86438@lll-winken.LLNL.GOV>
Sender: news@doc.ic.ac.uk
Organization: DoC, Imperial College, London SW7, UK.
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In-reply-to: miller@lll-crg.llnl.gov's message of 20 Nov 90 16:46:43 GMT

In article <86438@lll-winken.LLNL.GOV> miller@lll-crg.llnl.gov (Patrick Miller) writes:

>   I'm looking for pointers to papers about implementing error values in
>   functional languages (or other styles).  By error values, I mean that
>   the value of a calculation is replaced by a placeholder denoting the
>   error.  This allows some form of error handling without invoking the
>   concept of global state.

From what I remember, John Backus' FP has just such an idea. It has a
value called bottom which is returned when there has been an error.

>   Example: (plus (div a 0) (plus b c))	;;; [ (a/0) + (b+c) ]

In FP would be:
	{func +@[/@[1 %0] +@[2 3]]}
where the args to func are equivalent a, b and c.

>	    The value of (div a 0) is ERROR.
>	    Therefore the value of (plus (div a 0) (plus b c))
>	    is also ERROR.

Calling:
	func:<1 2 3>

gives back:
	?
Which is bottom --- cute!

I think it says that any function whose arguments contain a ? will
return a ?.

>   What I'm especially looking for is info on implementations of these
>   values.

I pulled an implementation of FP by Andy Valencia from the Sources
Archive here at Imperial. You may find useful stuff there.

>   Pat Miller				     Professional Student
Frank Taylor, another Professional Student

Major Disclaimer: The author disclaims all responsibility for any loss
or damage incurred by use of the software or knowledge given above. In
other words I don't know that much about FP :-)