Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!comp.vuw.ac.nz!cc-server4.massey.ac.nz!massey!E.Ireland
From: E.Ireland@@massey.ac.nz (Evan Ireland)
Newsgroups: comp.lang.scheme
Subject: Re: evaluating () should be an error
Message-ID: <515sis-d@@massey.ac.nz>
Date: 24 Mar 91 22:14:02 GMT
References: <1991Mar24.064144.4256@daffy.cs.wisc.edu> <JAFFER.91Mar24154354@zohar.ai.mit.edu>
Reply-To: E.Ireland@@massey.ac.nz
Organization: Information Sciences, Massey University, New Zealand
Lines: 33

>Scheme allows lots of sloppy and incompatible coding practices.  For
>instance, using the values of define, set!, set-car!, string-set!,
>etc.  These functions represent far worse compatablility problems than
>'().  Some implementations return the new value and some the old value
>of the object changed.

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.  The print routine would print nothing for an
Unspecified value.

    => (+ 3 4)
    7
    => (set! x (+ 3 4))
    => x
    7

The only way to check for an Unspecified value would be to do a trick

    => (set! *unspecified* (set! x 1))

And then you could check (using equal?) if a value was Unspecified.
Of course I never did this!  Clearly this discourages the use of
procedures such as "set!" in a non-portable manner, since there is
only one Unspecified value, and "equal?" will always return #t.

Anyway, this seemed a reasonable idea at the time, though perhaps
taking the word of the report too literally!  I wonder if anyone else
has implemented something similar.
_______________________________________________________________________________

E.Ireland@massey.ac.nz          Evan Ireland, School of Information Sciences,
  +64 63 69099 x8541          Massey University, Palmerston North, New Zealand.