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Scheme Digest #276                             17 Jan 90  02:32:34 EST

Today's Topics:

        (none)
        () as an expression (3 messages)

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Message-ID: <502@qusunitb.queensu.CA>
Date: 15 Jan 90 16:11:36 GMT
From: Prakash Panangaden
 <cs.utexas.edu!jarvis.csri.toronto.edu!qucis!prakash@yale-zoo.arpa>
To: scheme@mc.lcs.mit.edu
Subject: Re: (none)

Is there anyone who knows where to get PC schemes in Canada?
Thanks

------------------------------

Message-ID: <1990Jan16.193134.10491@sun.soe.clarkson.edu>
Date: 16 Jan 90 19:31:34 GMT
From: Jason Coughlin <image.soe.clarkson.edu!jk0@uunet.uu.net>
To: scheme@mc.lcs.mit.edu
Subject: () as an expression

The Revised^3 Report says that () is an illegal expression.  It must
be quoted.  However, MIT-Scheme and PC-Scheme both allow it, ie:

MIT-SCHEME => ()
()

MIT-SCHEME =>

Why is () an invalid expression?  It seems to me that it is a constant.

(eq? #t #t) => #t
(eq? #f #f) => #t
(eq? '() '()) => #t

now why isn't () considered a constant, when it really IS a constant?
--
Jason Coughlin ( jk0@sun.soe.clarkson.edu , jk0@clutx )
"Every jumbled pile of person has a thinking part that wonders what the
part that isn't thinking isn't thinking of." - They Might Be Giants

------------------------------

Message-ID: <5413@tekcrl.LABS.TEK.COM>
Date: 16 Jan 90 20:55:13 GMT
From: Ken Dickey <zephyr.ens.tek.com!tekcrl!tekchips!kend@uunet.uu.net>
To: scheme@MC.lcs.mit.edu
Subject: Re: () as an expression

In article <1990Jan16.193134.10491@sun.soe.clarkson.edu>
 jk0@sun.soe.clarkson.edu (Jason Coughlin) writes:
>The Revised^3 Report says that () is an illegal expression.  It must
>be quoted.  However, MIT-Scheme and PC-Scheme both allow it, ie:
..
>Why is () an invalid expression?  It seems to me that it is a constant.
>
>(eq? #t #t) => #t
>(eq? #f #f) => #t
>(eq? '() '()) => #t
>
>now why isn't () considered a constant, when it really IS a constant?

The syntax () is considered a combination (a.k.a. procedure call) and
as such must have at lease one subexpression.  So, the empty list is
valid, but the empty combination is an error.

Note also that #f and '() may be distinct in newer Scheme
implementations [the value of (eq? '() #f) is currently unspecified].

There are other non-R^3RS behaviors allowed by various versions of
Scheme implementations, particularly those implementations done before
R^3RS was issued.

-Ken Dickey

------------------------------

Message-ID: <1990Jan16.231311.18316@sun.soe.clarkson.edu>
Date: 16 Jan 90 23:13:11 GMT
From: Jason Coughlin
 <snorkelwacker!usc!zaphod.mps.ohio-state.edu!rpi!image.soe.clarkson.edu!jk0@blo
 om-beacon.mit.edu>
To: scheme@mc.lcs.mit.edu
Subject: Re: () as an expression

 From article <5413@tekcrl.LABS.TEK.COM>, by kend@tekchips.LABS.TEK.COM (Ken
 Dickey):
> Note also that #f and '() may be distinct in newer Scheme
> implementations [the value of (eq? '() #f) is currently unspecified].

This leads to another point: I think we need a definitive answer on
whether #f == ().  In my Scheme, #f and () are two different entities.
I like this because in my mind, #f != ().  #f is boolean, () is an empty
list.

So what say you about:
(BOOLEAN? '())
(NULL? '())
(EQ? '() #f)
--
Jason Coughlin ( jk0@sun.soe.clarkson.edu , jk0@clutx )
"Every jumbled pile of person has a thinking part that wonders what the
part that isn't thinking isn't thinking of." - They Might Be Giants

------------------------------

End of Scheme Digest
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