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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.scheme
Subject: (case key (() ...) ...) interpretation.
Message-ID: <5538@goanna.cs.rmit.oz.au>
Date: 3 May 91 08:02:41 GMT
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 29

I haven't seen the final version of the Scheme standard, only Draft 4.
D4 says of "case" that

	Each <clause> shall have the form
	((<datum1> ...) <expression1> <expression2> ...)
	where each <datum> is an external representation of some object.

The impression that I get from this is that
	(() expr expr...)
is a legal case clause, whose body will never be evaluated, because it
contains no <datum>.

However, several Scheme implementations do the CL thing of allowing the
selector to be a non-list.  (For example, the TI PC Scheme manual says
that the selector is "a non-pair, or a list of non-pairs (not evaluated),
including the optional keyword _else_".  I have come across another
Scheme with the same extension, where a selector of () is interpreted
as if it were (()), so that
	(() expr expr...)
will be selected if the key evaluates to ().

This came up when writing a macro to generate 'case' forms, where it
was rather convenient to take D4 literally.  The existing variation in
Scheme systems means that I have to rewrite the macro anyway, but what
_should_ (() expr expr...) do as a case clause?

-- 
Bad things happen periodically, and they're going to happen to somebody.
Why not you?					-- John Allen Paulos.