Ah. Thanks. Please disregard the final question on my last email. You just answered it.
On 7 March 2014 12:27, Jens Axel Søgaard <jensa...@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 <zenspi...@gmail.com>: > > > > On Mar 7, 2014, at 2:45, Daniel Carrera <dcarr...@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 > -- When an engineer says that something can't be done, it's a code phrase that means it's not fun to do.
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