[racket] first vs car ; last vs cdr ; empty? vs null?

good point, thanks.


On Fri, Mar 7, 2014 at 1:18 PM, Jens Axel Søgaard <jensaxel at soegaard.net>wrote:

> Error messages use the name reported by object-name.
>
> 2014-03-07 13:11 GMT+01:00 Laurent <laurent.orseau at gmail.com>:
> > Is this really a need? I really wouldn't bother to have
> >> (object-name empty?)
> > 'null?
> >
> > (I have no strong opinion on the subject though, I'm just wondering if
> there
> > is a pratical different too)
> >
> > Laurent
> >
> >
> >
> > On Fri, Mar 7, 2014 at 12:27 PM, Jens Axel Søgaard <
> jensaxel at soegaard.net>
> > wrote:
> >>
> >> The (define (empty? l) (null? l)) is needed due to object-name.
> >>
> >> > (object-name null?)
> >> 'null?
> >> > (object-name empty?)
> >> 'empty?
> >> > (define my-empty? empty?)
> >> > (object-name my-empty?)
> >> 'empty?
> >>
> >> /Jens Axel
> >>
> >>
> >> 2014-03-07 12:19 GMT+01:00 Ryan Davis <zenspider at gmail.com>:
> >> >
> >> > On Mar 7, 2014, at 2:45, Daniel Carrera <dcarrera at gmail.com> wrote:
> >> >
> >> >> Hello,
> >> >>
> >> >> Is there any difference between `first` and `car`, or between `last`
> >> >> and `cdr`, or between `empty? and null?` ?
> >> >>
> >> >> I had assumed that these were just synonyms, added by Racket because
> >> >> they might be more memorable to a student. But apparently Racket
> doesn't
> >> >> think they are equal:
> >> >>
> >> >> -> (equal? first car)
> >> >> #f
> >> >> -> (equal? last cdr)
> >> >> #f
> >> >> -> (equal? empty? null?)
> >> >> #f
> >> >>
> >> >>
> >> >> I suppose that they could be separate functions that happen to do the
> >> >> same thing, but if so, my next question would be why they aren't just
> >> >> aliases. As in:
> >> >>
> >> >> -> (define myfirst car)
> >> >> -> (equal? myfirst car)
> >> >
> >> > For many/most things in racket, you can bring up the definition for
> >> > something inside of DrRacket:
> >> >
> >> > (define (first x)
> >> >   (if (and (pair? x) (list? x))
> >> >     (car x)
> >> >     (raise-argument-error 'first "(and/c list? (not/c empty?))" x)))
> >> >
> >> > I couldn't for car, so I'm assuming it is considered a primitive.
> >> >
> >> > last and cdr aren't synonymous:
> >> >
> >> > (define (last l)
> >> >   (if (and (pair? l) (list? l))
> >> >     (let loop ([l l] [x (cdr l)])
> >> >       (if (pair? x)
> >> >         (loop x (cdr x))
> >> >         (car l)))
> >> >     (raise-argument-error 'last "(and/c list? (not/c empty?))" l)))
> >> >
> >> >
> >> > (define (empty? l) (null? l))
> >> >
> >> > null? seems to be a primitive as well. Not sure why they're not direct
> >> > synonyms in this case.
> >> >
> >> >
> >> > ____________________
> >> >   Racket Users list:
> >> >   http://lists.racket-lang.org/users
> >>
> >>
> >>
> >> --
> >> --
> >> Jens Axel Søgaard
> >>
> >> ____________________
> >>   Racket Users list:
> >>   http://lists.racket-lang.org/users
> >
> >
>
>
>
> --
> --
> Jens Axel Søgaard
>
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