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

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