<div dir="ltr">Ah. Thanks. Please disregard the final question on my last email. You just answered it.</div><div class="gmail_extra"> <div class="gmail_quote">On 7 March 2014 12:27, Jens Axel Søgaard <<a href="mailto:jensaxel@soegaard.net" target="_blank">jensaxel@soegaard.net</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">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 <<a href="mailto:zenspider@gmail.com">zenspider@gmail.com</a>>: > > On Mar 7, 2014, at 2:45, Daniel Carrera <<a href="mailto:dcarrera@gmail.com">dcarrera@gmail.com</a>> 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: > <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> -- -- Jens Axel Søgaard </blockquote></div> <div> </div>-- When an engineer says that something can't be done, it's a code phrase that means it's not fun to do. </div>