From: James A. Crippen
Subject: Deftype question
Date: 
Message-ID: <m3n0u0evl0.fsf@kappa.unlambda.com>
Why can't I say the following?

(deftype boolean ()
  (member nil t))

In CMUCL 18d I get the following cryptic (as usual) warning:

In: DEFTYPE BOOLEAN
  T
Warning: This is not a (VALUES &OPTIONAL LIST &REST T):
  T

Is what I'm doing wrong?  Or is CMUCL just being weird?  I stared at
the CLHS entry for DEFTYPE for a while and couldn't see anything
obviously wrong with (member nil t).

The point is I want to restrict values to only T and NIL.  Nothing
else.  I could do this in every place I want only these values by
explicitly checking for (eq t) and (eq nil) but I'd rather put the
burden on the type checker.

'james

-- 
James A. Crippen <·····@unlambda.com> ,-./-.  Anchorage, Alaska,
Lambda Unlimited: Recursion 'R' Us   |  |/  | USA, 61.20939N, -149.767W
Y = \f.(\x.f(xx)) (\x.f(xx))         |  |\  | Earth, Sol System,
Y(F) = F(Y(F))                        \_,-_/  Milky Way.
From: Christophe Rhodes
Subject: Re: Deftype question
Date: 
Message-ID: <sq660og9o3.fsf@lambda.jcn.srcf.net>
·····@unlambda.com (James A. Crippen) writes:

> Why can't I say the following?
> 
> (deftype boolean ()
>   (member nil t))
> 
> In CMUCL 18d I get the following cryptic (as usual) warning:
> 
> In: DEFTYPE BOOLEAN
>   T
> Warning: This is not a (VALUES &OPTIONAL LIST &REST T):
>   T

You want:

(deftype boolean ()
  '(member nil t))

The body of a DEFTYPE is evaluated as an implicit progn and returns a
type specifier. So in your example, CMUCL is quite rightly trying to
_execute_ the call to MEMBER, and failing because T isn't a
list. Amusingly, if you write it the other way round:

(deftype boolean ()
  (member t nil))

Then, since the call to member returns NIL, your new BOOLEAN type will
be the empty type. :-)

Christophe
-- 
Jesus College, Cambridge, CB5 8BL                           +44 1223 510 299
http://www-jcsu.jesus.cam.ac.uk/~csr21/                  (defun pling-dollar 
(str schar arg) (first (last +))) (make-dispatch-macro-character #\! t)
(set-dispatch-macro-character #\! #\$ #'pling-dollar)
From: James A. Crippen
Subject: Re: Deftype question
Date: 
Message-ID: <m3elfceula.fsf@kappa.unlambda.com>
Christophe Rhodes <·····@cam.ac.uk> writes:

> ·····@unlambda.com (James A. Crippen) writes:
> 
> > Why can't I say the following?
> > 
> > (deftype boolean ()
> >   (member nil t))
> > 
> > In CMUCL 18d I get the following cryptic (as usual) warning:
> > 
> > In: DEFTYPE BOOLEAN
> >   T
> > Warning: This is not a (VALUES &OPTIONAL LIST &REST T):
> >   T
> 
> You want:
> 
> (deftype boolean ()
>   '(member nil t))

I just figured that out.  That's what I get for asking too soon.

'james

-- 
James A. Crippen <·····@unlambda.com> ,-./-.  Anchorage, Alaska,
Lambda Unlimited: Recursion 'R' Us   |  |/  | USA, 61.20939N, -149.767W
Y = \f.(\x.f(xx)) (\x.f(xx))         |  |\  | Earth, Sol System,
Y(F) = F(Y(F))                        \_,-_/  Milky Way.
From: Frode Vatvedt Fjeld
Subject: Re: Deftype question
Date: 
Message-ID: <2h4rg8kgnk.fsf@vserver.cs.uit.no>
·····@unlambda.com (James A. Crippen) writes:

> Why can't I say the following?
>
> (deftype boolean ()
>   (member nil t))

Also, are you aware that the boolean type is built into Common Lisp?

-- 
Frode Vatvedt Fjeld
From: James A. Crippen
Subject: Re: Deftype question
Date: 
Message-ID: <m37kl4e1d2.fsf@kappa.unlambda.com>
Frode Vatvedt Fjeld <······@acm.org> writes:

> ·····@unlambda.com (James A. Crippen) writes:
> 
> > Why can't I say the following?
> >
> > (deftype boolean ()
> >   (member nil t))
> 
> Also, are you aware that the boolean type is built into Common Lisp?

I am now.  I must have missed it when looking.  I don't usually worry
about types much...

'james

-- 
James A. Crippen <·····@unlambda.com> ,-./-.  Anchorage, Alaska,
Lambda Unlimited: Recursion 'R' Us   |  |/  | USA, 61.20939N, -149.767W
Y = \f.(\x.f(xx)) (\x.f(xx))         |  |\  | Earth, Sol System,
Y(F) = F(Y(F))                        \_,-_/  Milky Way.
comp.lang.lisp / 2002 / jun