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From: matthias@leto.rice.edu (Matthias Felleisen)
Newsgroups: comp.lang.scheme
Subject: Re: evaluating () should be an error
Message-ID: <1991Mar25.154155.9453@rice.edu>
Date: 25 Mar 91 15:41:55 GMT
References: <1991Mar24.064144.4256@daffy.cs.wisc.edu> <JAFFER.91Mar24154354@zohar.ai.mit.edu> <515sis-d@@massey.ac.nz>
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In article <515sis-d@@massey.ac.nz> E.Ireland@@massey.ac.nz writes:
>The implementation I mentioned in my last message kept to the word of
>the language report (3rd version) and returned a special Unspecified
>value in these cases. ... I wonder if anyone else has implemented
>something similar. 
Chez Scheme (3.9l) does that, too. 

A useful alternative is to have the programmer specify what the value
of a side-effect expression is. At Rice and Indiana, we have used
  (sigma (x ...) body),
which is like lambda except that upon application it assigns the
arguments to the lexical bound variables x ... (in parallel) and then
evaluates body returning its value. If the values are already known,
we use 
  (set! ((x ex) ...) body), 
which does the assignments and then evaluates the body. By specifying
the value of such forms and functions explicitly, no compatibility
problems can arise: they are a part of the program. 

-- Matthias Felleisen